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Version History: Version 1.0 December 2007

Fundamentals of PET/CT Introduction This course is intended for imaging professionals and radiation technologists as an introduction to the components, theory, use, and benefits of PET/CT technology PET/CT is designed to provide imaging professionals and radiation technologists with a basic understanding of the components, theory, use, and benefits of PET/CT technology. It begins with a discussion of the theory and development of PET and CT scanners and the steps that led to merging the technologies to produce PET/CT. Other topics include a brief look at some of the PET/CT systems currently available; the image acquisition process, including whole body scans, heart and brain studies, and radiation therapy planning; and image formation and reconstruction. It concludes with a section on quality control procedures necessary for maintaining and operating PET/CT equipment. By the end of this course, the student should be able to: •

Describe key points of Coincidence Theory for PET imaging;

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Identify specific qualities of 2D and 3D imaging;

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Recognize several methods of Reconstruction and specific information regarding each;

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Describe the key components of computed tomography including slip ring technology;

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Describe the history and development of PET/CT and its clinical applications;

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Identify clinically used PET scintillators and the criteria for PET crystals;

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Describe acquisition protocols for whole body scans, heart and brain studies, and radiation therapy planning; and

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Identify critical steps, both process and purpose, for PET/CT equipment quality control.

1 Fundamental of PET/CT

Table of Contents Fundamentals of PET/CT

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PET Theory

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Acquisition Modes

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Image Formation and Reconstruction

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OS-EM, AW-OSEM, RAMLA

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CT Overview

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CT Detector Materials

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CT Computer Systems

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PET/CT Overview

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PET/CT Systems

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Acquisitions

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PET/CT and Radiation Therapy Planning

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Quality Control

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Fundamental of PET/CT

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Fundamentals of PET/CT Pet Overview PET Definition and Use The best way to grasp an in-depth understanding of the PET/CT technology is to first break down this diagnostic tool into the two components that comprise it. First we’ll start with PET. Positron Emission Tomography or PET is an in vivo imaging technique used to visualize and quantify biochemistry and physiological functions of the human body. Established as a modality in the 1950s, it was primarily utilized in academia and research. With the development of commercial cyclotrons, the improvement in instruments and computer software and hardware, and the reimbursement of PET procedures, PET imaging has become a valuable clinical tool to study organ physiology, especially of the brain and heart. Additionally, oncology also utilizes PET to diagnose and manage various forms of cancer. How does PET differ from a general X-ray? X-rays are used to create images of internal anatomy and structure such as bone. PET provides a “functional” image of metabolic processes. In the case of an oncology patient, PET shows a hypermetabolic activity associated with tumors. Likewise, for a heart patient, PET can help distinguish viable and non-viable cardiac tissue.

PET Theory Coincidence Detection So how does the PET technology actually work? The technology itself relies on several important physics theories, first of which we call “Coincidence Detection.” Let’s take a look. During the PET imaging process, PET cameras

3 Fundamental of PET/CT

detect gamma rays emitted as a result of radioisotope decay. In clinical applications, a radioisotope, which is typically produced in a cyclotron, is injected into the patient’s vascular system. This isotope contains excess protons. During its decay process, the isotope emits a positron, which is a positively charged electron. After traveling a short distance in the patient’s tissue, this positron then encounters a free electron. The two briefly combine and then "annihilate" each other. When the annihilation occurs, the mass of these particles is converted into two high-energy gamma rays of 511 keV each. The gamma rays are emitted 180 degrees opposite of each other. Two separate detectors, arranged on opposing sides of the positron-emitting object, must individually detect each of the dual 511 keV gamma rays. If the two gamma rays are detected by the scanner within a prescribed timing window, (known as the coincidence timing window), the system uses two single photon events to define a positron annihilation event, sometimes referred to as a prompt coincidence event.


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Gamma rays detected within the timing window are used to define a line of response (LOR) connecting the pair of detected photons and identifying the location of the positron-emitting radiopharmaceutical. When a coincidence event is identified, it’s added to the appropriate element in the sinogram that most approximates its LOR. Only coincidence events contribute to the final clinical PET image to be read by the physician, whereas other single gamma ray events are rejected by PET scanner electronics. Random Events and Poisson Statistics In this section we’ll explore the random events and poison statistics, that also play a crucial role in the PET imaging process. As we explained in the previous section, PET imaging relies on the simultaneous detection of two gamma rays. Because the coincidence-timing test is the criterion used to define a positron annihilation event, the system will use any two single events that the timing coincidence circuit detects to define a coincidence LOR. But, in addition to "true events," a PET scanner also acquires random events and scatter coincidences. Both random events and scatter coincidences degrade image quality. Random events occur when unrelated random pulses from independent annihilation events are close enough to be recorded within the coincidencetiming window. If the random fraction is more than 15 to 20 percent of the total counts, random correction is imperative. How do we determine random correction? First, we assume that for every "true" event, we also have the same number of "unrelated" events in the same time window and in any delayed time window. As a result, the number of random events can be measured using an additional delayed time window.


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The formula we use is as follows: (Trues = Prompts - Randoms for every line of response.) Poisson statistics dictate that the rate of random coincidence added to the image is proportional to the total coincidence-timing window for both detectors. The wider the coincidence-timing window is, the larger the chance that unrelated photons will be detected.

In order to keep the fraction of counts that arise from random coincidence as small as possible, PET systems limit the width of the coincidence-timing window to the smallest time required to capture most of the true coincidence events. Scatter and Attenuation Correction Random events aren’t the only components that can impact image quality and therefore require adjustments. Attenuation is another important factor that must be considered when working with PET technology. So what is attenuation? When radiation from a beam is lost because of either scatter caused by Compton or Rayleigh interactions or by absorption caused by photoelectric interaction, the result is called attenuation. In general, failure to correct for attenuation will invalidate reconstruction algorithms, distort emitter distribution, and make quantification impossible. Also, under certain conditions, lung lesions may not be visible in uncorrected images.


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First, let’s examine the amount of attenuation that must be accounted for. Since the density of the human body is about the same as water, about half the radiation is lost every 7.2 centimeters or 2 and 7/8 inches. OBVIOUSLY, the cross-section of a patient’s body is much greater than 2 and 7/8 inches, so many of the photons are scattered in the patient before reaching the PET detectors.

While scatter events are from single annihilation points, one or both of the emitted photons are scattered in the patient, so they distort the inferred line of response away from its true position. Scatter will add a diffuse pattern to the volume surrounding the localized PET radioisotope in the body. In addition, scatter makes LOR’s (line of response) "fuzzy" and puts counts where they shouldn’t be, resulting in incorrect quantification. It also causes artifacts in emission images, creates bias, and reduces contrast.


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If you fail to compensate for scatter coincidence, you’ll have additional blurring of the true emission image and an incorrectly high representation of the PET radioisotope in the surrounding volume.

Acquisition Modes PET offers two methods to acquire data – two-dimensional (2D) and threedimensional (3D). Let’s take a look at 2D first. With 2D imaging, lead septa are extended into the field of view (FOV) in order to prevent some of the gamma rays from reaching the crystals. As a result, less information is collected. With septa inserted, 2D imaging restricts the acceptance level of LOR’s in the axial direction. The sensitivity profile is clipped, producing a more trapezoidal shape, thus reducing sensitivity. Septa reduce the effect of scatter and randoms, but they also increase the need for either a higher dose or longer imaging time in order to obtain accurate count statistics. Longer imaging time is most commonly used since increasing the dose also increases the amount of scatter and random events. With 3D imaging, more events are measured so this type of acquisition mode requires that the system be much faster than what’s required for 2D. The increased power of the system is necessary in order to utilize the additional information. The fast detectors required for 3D acquisition modes gather more information and are also able to deal with the increased amount of scatter correction. Fast detectors are a combination of the scintillation crystal’s light decay time properties and fast electronics. Fast electronics include photo-multiplier tubes that are coupled to the crystal and coincidence processing circuitry. Among the advantages of 3D imaging are the additional counts and higher sensitivity with lower dose requirements and a shorter scan time. It also provides high resolution in all imaging planes (i.e. sagittal, and coronal, and transaxial imaging planes), which


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we’ll discuss further, later in this course.

Image Formation and Reconstruction Now let’s examine how PET images are formed and reconstructed. To obtain the unique functional images that PET offers, raw data must be reconstructed. Thanks to a variety of technological advancements, PET image quality has significantly improved. In addition, the criterion for determining a good PET image is totally subjective to the user. Multiple factors influence which iterative reconstruction algorithm (IR) to employ in PET imaging. Reconstruction options include choice of filter, attenuation and blur, weighting, comparison by ratio and difference, and the number of iterations. While sharper filters and larger numbers of iterations offer higher resolution images, they also increase noise. Smoother filters reduce the noise level, but then the image resolution isn’t as high. Let’s look at a few reconstruction pitfalls of which you should be aware. First, algorithm tuning is delicate. Every change requires validation on large data sample acquired on multiple scanners with multiple detection tasks. Second, no matter how accurate the reconstruction algorithm, no other component can replace good emission data, which can only be obtained with enough statistics, no patient motion, an empty patient bladder, no patient contamination, no dose infiltrations, etc. Furthermore, high quality system corrections, including frequent normalization, attenuation correction, and scatter correction, also are desirable. Finally, NEVER compromise emission and transmission statistics, especially on heavy patients. Statistical reconstruction removes only inconsistent noise, which doesn’t account for all the noise. Also, don’t forget that respiratory motion decreases standard uptake values or SUVs inside the lungs


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Analytical Reconstruction (FBP – Filtered Back Projection) When we break down the various technological components of PET and discuss reconstruction, we can’t skip filtered back projection (FBP). What’s FBP? By processing each individual projection with a filter prior to back projection, FBP improves overall quality. While it’s not as accurate as iterative, this method is fast and uses less computing power. Furthermore, despite the fact that more elegant iterative and analytical techniques have been devised, FBP is still widely used in commercial instrumentation because of "history" and its ease of use and implementation. FBP is employed for general PET. So, it’s used for reference images. FBP can help you to determine positioning, detect patient movement, etc. It’s effective and produces the "correct result" for high statistical data. FBP will also generate an image to tell you "where you are." Of course, FBP isn’t without its downsides and limitations. Primarily, FBP ignores noise, which gives all projections equal weight. So when highly attenuated lines of response are noisy and produce streak artifacts, the result is a bias in volume of interest (VOIs) or correlated noise. Furthermore, FBP noise is controlled through linear filtering in sinogram space, which is sub-optimal.

Iterative Reconstruction (IR and ML-EM) Now we’re ready for what we call IR or iterative reconstruction. It’s, of course, helpful if you remember that the word iterative means “repeat.” The basic concept of IR is to compare the numerical projection data and the measured projection data in a feedback loop. The feedback from this comparison is


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used to adjust the image estimate and to minimize the difference between these two data sets. As the process is repeated or iterated, the derived changes to the image estimate become small and the estimate converges on the solution image. The number of iterations required to reach convergence is dependent upon which IR algorithm is utilized. Optimal resolution and contrast are used as a measure of convergence. Rapid convergence is a necessary prerequisite for practical clinical implementation.

Iterative reconstruction may take longer, but it is the more accurate method; however, we do have an alternative, which is called Maximum Likelihood – Expectation Maximization or ML-EM. For the EM iterative method, we start by guessing what the pixel count density will be in the reconstructed patient image. Typically, it’s one everywhere. We then numerically forward project this data into a trial projection data set. The trial projection is compared to the measured data. The initial guess for the patient image data is then adjusted for the magnitude of the discrepancy. Each voxel in the image is corrected by the average discrepancy over all projection pixels that back-project into the voxel. Starting each time with the new adjusted image, we repeat this process again for a specified number of times or until the average discrepancy is acceptably small. In EM methods, the power is the set of forward projection weights that control how the image data contributes to the trial projection data.


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OS-EM, AW-OSEM, RAMLA In this final section of the first chapter, we’ll explore some of the other reconstruction methods that are commonly used for PET. The most important point for you to recognize is that different PET programs utilize different methods for reconstructing image data. Having a general grasp of each one can prove quite helpful.

We’ll start with Ordered-Subset Expectation Maximization or OS-EM. PET typically utilizes this technique to reconstruct images. OSEM employs an EM method; however, it uses only a subset of the projection data in the forwardprojection step. The calculation of an update to the image involves only a fraction of the complete data set and takes only that fraction of the time the full EM algorithm needs to finish one update. Our next option is Attenuation Weighted – Ordered-Subset Expectation Maximization, also known by the much shorter acronym, AW-OSEM. This technique is simply an extension of the OS-EM technique described above, with the added capability of weighting LORs that pass through more highly attenuated areas, thereby addressing artifacts that have been detected in the more commonly found OSEM algorithm. Finally, we have RAMLA or Row Action Maximum Likelihood Algorithm. This 3D iterative image reconstruction algorithm substitutes voxels with spherically` symmetric volume elements, which are called blobs, and places them on a uniform 3D grid. Blobs’ advantage is their ability to define


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their amplitude and shape, thus impacting the reconstruction image quality. Attenuation correction, which is incorporated in the algorithm, is based upon a singles transmission scan, followed by image segmentation. RAMLA provides a small gain in image quality; on the other hand, it requires up to 25 minutes to reconstruct the image volume.

CT Overview Definition and Use In the last chapter, we delved into the PET half of PET/CT imaging. Now we’re ready to tackle computed tomography or CT. Developed as a viable imaging modality in the early 1970s, the CT scanner is a rotating, 360-degree detection system. Because the CT scanner requires a significant amount of core data to generate a usable image resolution, the CT scanner itself wasn’t actually created until a relatively cheap digital computer was developed. As we look back on the history of the CT, we can actually trace its roots to the X-ray. After all, when an X-ray was discovered, people were able to view the art of the human body in a noninvasive manner; however, X-rays left medical professionals hungry for still more imaging information. Why? Because with X-rays, the anatomic structures were superimposed, and soft tissue could not be differentiated. CT, on the other hand, allows us to view tomographic anatomy and density differences. CT is divided into a series of generations by manufacturers that produce them. As of the fall of 2003, when this course was developed, four recognized generations of CT scanners exist. The classification is based on configuration of the X-ray source or tube and the detectors’ geometry. Finally, you should understand that CT images are frequently used for attenuation correction of PET emission data because of the essentially noiseless image data that a CT produces.


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CT Scanning Components We’re now ready to explore the scanning components of the CT. First, the X-ray ingredients. The CT’s X-ray tube produces an Xray beam that’s collimated into a fan shape. An electrical current in the milli Amperage (mA) range produces an electron beam in the tube. This beam is then subjected to high voltage in the killi Volt (kV) range, which accelerates it toward the anode. Please note, a high heat unit is used to perform spiral scanning. The generator (red oval, shown in image on right) located in the rotating part of the gantry produces the needed high voltage and applies it to the X-ray tube so that it can generate Xray output. How do we measure the X-ray? Detectors make it possible. The X-ray tube and the detectors rotate around the human body during examination. The X-rays traveling through the body are attenuated, depending on the tissue and patient thickness. The detector captures the attenuated invisible X-rays and converts them into visible light, which is then transformed into electric signals in the photo diode. This information is then processed into images using complex calculations.


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CT Detector Materials Now that we’ve covered the X-ray components, let’s take a closer look at those detector materials that we briefly touched upon in the last section. CT’s can have two basic types of detector materials, which you’ll recall capture the attenuated invisible X-rays and convert them into visible light. The two types of detectors are direct converters, also known as xenon gas detectors, and scintillation detectors. Ordinarily solid-state detectors in the CT are made of cadmium tungstate (CdWO4) materials based on rare earths. To perform at the optimal level, the CT detector needs to meet certain requirements: First, it has to offer high quantum absorption or efficiency. The quantum efficiency needs to be close to 100 percent up to a 150 kV value. Second, the detector must offer minimal radiation drift so that healing times are in the seconds to minutes range. Third, the decay factor, which is the drop in the light emission intensity after the X-ray is turned off, should be below a value of 10 microsecs. Optimal detectors also present minimal afterglow and high luminous or scintillation efficiency. In other words, the relationship of converted light yield to absorbed X-rays is high.

This chart illustrates some of the detector material options available today. You can compare and contrast the general properties and advantages of each one. Siemens, for example, has developed GdOC (Gadolinium Oxide Ceramic) into Lightning Ultrafast Ceramic or (UFC) and manufactures it themselves.


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UFC presents outstanding image quality and achieves high quantum efficiency without losses. The high scintillation efficiency of UFC detectors allows for problem-free, shoulder to pelvis spiral exams. The UFC material also is environmentally friendly and easy to prepare. Slip Ring Technology This section focuses on the crucial slip ring technology that can be found in today’s newest type of CT, the spiral CT. But first, to better understand the advantages of the spiral CT, we’ll start with the older CT system. In conventional, sequence CT, the tube detectors rotate 360-degrees in one direction, stop, and then rotate 360-degrees in the other direction, allowing the low and high tension cables to unwind themselves. This process of the tube and detector system working in conjunction with each other occurs during each rotation so that each rotation results in one slice. The table moves to the next slice position and the process is repeated. The data is then transmitted via cables to the image processor. This process presents a few important issues. First, images can only be reconstructed where they are scanned, meaning no retrospective slice overlap. So, if the patient breathes differently, misregistration results because of the different levels of respiration. The "old" technology also presents an interscan delay that’s too long and requires larger amounts of contrast. On the other hand, with the new spiral scan technology, the slip ring allows for continuous tube rotation, radiation, data acquisitions, and table feed. The X-ray exposure, table movement, and data acquisition occur simultaneously for the entire volume acquisition. Each acquisition provides a complete volumetric data set.


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The slip ring assembly transmits data from the rotating gantry to the stationary gantry. Other advantages of the new spiral CT include shorter exam time, decreased contrast volume, increased small lesion detectability, meaning no misregistration, and overall improved quality. Next … the CT’s computer systems.

CT Computer Systems Now that we’ve broken down and studied both the PET and CT technologies, we can examine the PET/CT itself, starting with the history and development of this advanced diagnostic tool. Not surprisingly, the history of the PET/CT has its roots in the PET-side of the technology. In 1995, Dr. David Townsend, who had spent the previous decade advancing PET technology, proposed the development of the PET/CT scanner. The instrument was developed at the University of Pittsburgh and installed in a PET center in May 1998. The first PET/CT initially was used to evaluate a small group of patients as a pilot study, and as the technology proved itself as an effective diagnostic tool, it was slowly implemented throughout hospitals and diagnostic centers. The first PET/CT scanner was mass-produced by CPS Innovations, a joint venture between Siemens Medical Systems and CTI. The National Cancer Institute facilitated funding. Since the first PET/CT was developed, thousands of patients have been scanned, and the data has shown the value of the PET/CT’s improved diagnostic accuracy. As we explore the historical development of the PET/CT, we should look at the design goals for this tool. First, the PET component should have the highest possible performance in sensitivity and spatial resolution. Second, the CT should provide anatomical correlation for the functional information. It also must provide clinical diagnostic-quality CT images and the means for attenuation correction of the PET data. The patient handling system (PHS) should allow for highly accurate co-registration, and the software interface has to be integrated and streamlined. The original PET/CT prototype combined a single slice CT scanner with a half-ring BGO


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PET scanner (BGO being a scintillation crystal which will be discussed later). The co-scan range of the prototype was 100 cm.

PET/CT Overview Benefits of PET/CT As the last section demonstrated, the development of the PET/CT was funded and supported by both medical and commercial research. Why? Because early on, researchers foresaw the potential benefits of combining these two advanced imaging modalities. PET and CT allow imaging technologists to acquire both anatomical and functional information during a single exam. CT brings to the table its anatomic data, including detailed depictions of internal structures and the size, shape, and location of abnormal masses. Likewise, the PET scan delivers metabolic data, such as changes from abnormal cell growth. As a result, the co-registered images provide a more detailed and comprehensive view of body organs and tissues. While the PET/CT presents an array of diagnostic advantages for medical professionals, the instrument also offers patients several pluses. The PET/CT technology reduces scanning time with the exam generally taking about 30 minutes or less, (versus approximately an hour for dedicated PET scanners). In addition, patients experience non-invasive quality of the test and the convenience of having to obtain a single exam versus two separate exams.


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PET/CT also offers whole body imaging and the best possible co-registered alignment. Furthermore, the PET/CT improves the medical and surgical patient management in a significant number of cases. Finally, tumor detection and localization are enhanced with the PET/CT because the technology increases the ability to evaluate recurrence. This imaging tool is particularly valuable for the detection and treatment of melanoma, lymphoma, and the cancers of the head, neck, lung, colon and breast. The PET/CT helps medical professionals identify abnormal physiologic uptake, so that the therapy response can be assessed. PET Crystals Now let’s review the all-important scintillation crystals, which are used as radiation detectors in the PET scanner. When selecting a PET/CT scanner, you have several different crystal options from which to choose. As you evaluate the options, you should consider the main four desired properties that are crucial for PET imaging. The properties are: high-density, rapid signal time decay, high light output, and excellent resolution. GSO (Gadolinium Oxyortho Silicate), LSO (Lutetium Oxyortho Silicate), BGO (Bismuth Germanate) and Thallium-activated Sodium Iodide are the most common PET scintillation crystals. This chart allows us to compare and contrast the attributes of each crystal option.

As we mentioned earlier in this module, BGO has served the PET community since the 70s, and was used in the original PET/CT prototype. While BGO is very


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dense, it presents a much lower light output than LSO, GSO, and sodium iodide. BGO presents a relatively slow light decay time of 300 nsec, which is slower than the other crystals. LSO presents greater density, a rapid decay time, and a higher light output than BGO or GSO. LSO is the closest to sodium iodide in terms of its light output. But, unlike sodium iodide, LSO isn’t brittle, won’t crumble easily, is non-hygroscopic, and is significantly more dense for stopping 511 keV photons. Most importantly it offers a very short decay time, which is considered especially crucial, as the ability of a crystal to scintillate and emit a large quantity of light quickly, goes directly to the ability to have a short coincidence time window, (which ensures minimum image noise).

PET/CT Systems Overview The goal of diagnostic imaging, to improve patient care, has led to the consistent reinvention and improvement of technology, with the dual-modality PET/CT scanner being one of the most recent and promising developments. The positive results from the original PET/CT prototype developed by Dr. Townsend and his team (in 1998) stimulated medical and commercial interest to continue the production of hybrid equipment capable of performing both types of exams simultaneously. In 2000, the first commercial PET/CT unit was introduced. Three companies that currently manufacture hybrid PET/CT systems are GE Medical Systems, Phillips Medical Systems, and Siemens Medical Solutions. Different companies offer different crystals with their systems. Here we see a list of some of the systems available commercially. As you’ll note from this list, different companies offer different crystals with their systems. GE utilizes the BGO crystal for its Discovery ST and LS models, while Siemens offers the LSO in its Biograph line. Phillips sells both the GSO and sodium iodide systems.


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GE Medical Systems GE Medical Systems has developed two PET/CT instruments. The first, the Discovery LS, was installed in the University Hospital in Zurich in March, 2001. It combined a GE Lightspeed multi-slice CT scanner with the GE Advance NXi PET scanner. The newer Discovery ST model offers a four-slice CT, a 70 cm bore with short tunnel length and flexibility for radiation therapy, and a choice of 2D and 3D imaging. Both instruments meet PET/CT DICOM standards--the Discovery ST offers full fidelity DICOM PET and DICOM CT. The workstation houses the CT console, has LightSpeed’s unique user interface, and uses eNTEGRA for image fusion and review. General Electric also offers the option for “in the field” upgrades of existing GE PET Advance NXi or a LightSpeed Plus to the new Discovery LS.

Phillips Medical Systems Phillips Medical Systems PET/CT system, the Gemini, combines the ALLEGRO GSO PIXELAR PET scanner with the Phillips MX8000D multislice scanner. The CT scanner also covers the range of advanced applications such as CT Angiography, prospective gated cardiac imaging, perfusion exams, whole-body coverage, and


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vascular studies. The Gemini has an open design that relieves patient claustrophobia and allows continual patient access. The imaging table allows for the addition of a flat table top to the patient pallet for radiation therapy planning. It can also be lowered to accommodate wheelchair patients. The PET and CT components may be separated and used as stand alone systems. The Gemini’s software system, Syntegra, provides an integrated display, review, registration, and communication environment from a single workstation. Syntegra’s toolkit incorporates optimized, proprietary algorithms specific for certain clinical applications.

Siemens Medical Solutions Siemen’s PET/CT is the Biograph line which includes the Biograph BGO (BGO Crystals) the Biograph LSO (LSO Crystals) and the Biograph Sensation 16 (LSO Crystals). The Biograph BGO combines a Siemens ECAT EXACT HR+, a full-ring, 3D PET scanner that utilizes BGO crystals with the SOMATOM Emotion Duo-- a dual row, multi-slice spiral CT scanner. The Biograph LSO merges the SOMATOM Emotion Duo with a scanner that utilizes LSO crystals. The Biograph Sensation 16 merges their ECAT ACCEL PET scanner with the SOMATOM® Sensation 16 CT scanner. The Sensation 16 is designed for increased speed and improved image quality. PET images may be corrected to a full field of view and a PET/CT coscan range of 182 cm when scanning in the feet first position. This model’s electronics reduces acquisition time by half while gathering the same amount of data. Siemen’s multislice CT scanners employ a patented spiral reconstruction algorithm called SureView, and come standard with Turbo Recon and CARE dose –which allows for dosage adaptation for high and low attenuation views. The CT can also be used for stand alone procedures. The patient handling system is a floor-mounted, cantilevered bed on rails that will support patient weight up to 450 pounds. It is also designed to accommodate a flat radiation therapy planning pallet. The third component of the Biograph system is the Syngo software, a Windows based operating system that provides multi-modality viewing capabilities,


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interactive image manipulation, and output to any Windows or DICOM compatible printer.

Acquisitions PET/CT Exam/Patient Prep This chapter focuses on PET/CT acquisitions—actually working with the patients to obtain quality PET/CT images. Let’s begin with the patient’s preparation prior to the exam, a critical step for imaging success. As you probably know, protocols vary depending on the facility, equipment, and specific needs. When scheduling a patient for a PETCT exam, consider the other aspects of the patient’s care and medical treatment. For example, is the patient going to have chemotherapy, radiation therapy, or surgery prior to testing? Is the patient diabetic? Glucose levels should be checked carefully, especially in diabetic patients. Excessively elevated glucose levels can alter tracer uptake and affect image quality. Other general considerations include nothing by mouth or NPO for the four hours prior to FDG injection. You also need to maintain current medications and check for pain medication needs. In addition, patients should also remove jewelry, partials or anything that has metal before being placed in the gantry. Furthermore, the patient should void his or her bladder prior to scanning.

Clinical Imaging Procedure Now let’s walk through the actual clinical imaging procedure. First, gather a thorough medical history, making sure you rule out pregnancy, breastfeeding, etc. Next, check the patient’s physical statistics, including height, weight, pulse, blood pressure, and for patients with diabetes, the glucose level. When the patient is ready for an FDG injection, place the IV in an upper extremity vein and inject the proper FDG dosage. Depending on the referring physician and the patient, an oral CT contrast agent may also be administered. Allow the patient to rest comfortably. Imaging should begin no sooner than 40 minutes postinjection. First register the patient information into the computer data system. Mandatory information generally includes the patient’s name, ID, date of birth, and sex. Patient weight will be needed for SUV calculation. You will also need to indicate patient orientation, i.e., head first, supine. When you’re ready to begin the actual imaging process, proper patient placement is critical. You want to isocenter the patient into the gantry. To


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accomplish this goal, place the patient median within the center of the gantry. This way the patient can be centered top to bottom and side to side. In most cases, the patient’s arms should be positioned above the head. Make the patient as comfortable as possible on the patient handling system. Take all the appropriate measures to avoid motion artifacts.

PET/CT Oncology/Whole Body Imaging Now that we’ve reviewed the patient aspects of testing, we’re ready to focus on the technical protocols. Preparing the CT topogram is the first step in any acquisition protocol. The topogram is a planning image, because it defines the combined imaging range. When determining the proper technique and parameters for the CT image, consider the patient’s body habitus, age, any radiation protection issues, such as if the patient is in her childbearing years, etc. The typical CT parameters are about 130 mAs, 130 kVp, 5mm slice width, and a table feed of 8mm per rotation with a rotation time of 800 ms. While these parameters are generally built-in as defaults in the equipment, the technologist is encouraged to assess patient habitus and adjust imaging parameters, if needed, to ensure optimal image quality. Generally, the CT scan is acquired from the base of the brain to the mid-thigh superior to inferior. The CT scan acquires one complete volume of data in around 90 seconds, in one continuous acquisition. After the CT scan, the patient handling system moves the patient into the PET portion of the gantry. The PET scan is acquired inferior to superior or toe to head to ensure minimal bladder activity; it’s also acquired one section at a time. The emission data are collected covering the same axial examination range as the CT. The size of the axial field of view varies, depending upon the type of machine used.

PET Reconstruction Moving on to this next section, we now come to the topic of PET reconstruction. As soon as all CT data are processed and the first PET bed position data is acquired and submitted to the processing queue, PET data reconstruction automatically begins. As each consecutive bed position finishes acquiring, the data are automatically processed while the next bed position begins its own acquisition process. As the PET exam ends and the patient is removed from the gantry, the last bed position data are processed. Most systems allow PET data to be reconstructed using attenuation correction or simultaneous non-attenuation correction, if


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desired. In most cases, the entire PET/CT study is completely processed minutes after the exam ends. As a result, technologists benefit from the automatic data processing, which alleviates additional burden to their workflow. The quick turnaround time from scanning, processing, and image review improves overall clinical efficiency for the patient, the attending physician, the Radiologist or the Nuclear Medicine physician. The CT transmission data are used for attenuation and scatter correction of the PET emission data. In order for the CT to accurately process attenuation correction, the CT data must first be interpolated to the PET resolution. For whole body scans, iterative image reconstruction ensures good image quality. The study is reconstructed iteratively, utilizing the entire 128X128 matrix, an 8-mm Gaussian filter, and a post-reconstruction filter of 5.45 mm. The typical whole body iterative reconstruction parameters are generally as follows:

8 subsets, 2 iterations FORE + AWOSEM algorithm 6 mm Gaussian smooth filtering

When everything has been acquired and data from the last bed position have been processed, it can be sent for physician review. If the physician desires sharper or smoother data, most systems allow you to bring up raw data from a specific patient study, and then adjust the settings and reprocess the images.

Brain Protocol Neurologists can also benefit from PET and PET/CT technologies. While most people are aware that PET imaging is a standard protocol for patients suffering from seizures and brain tumors, recently the use of PET in the diagnosis and treatment of Alzheimer’s Disease has garnered a great deal of attention. Why is PET proving to be so useful for this


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mysterious and debilitating disease? Patients with Alzheimer’s typically experience metabolic changes prior to morphologic changes, and PET, of course, is designed to detect metabolic changes and abnormalities. While PET’s applications for Alzheimer’s are still in the investigation stages, approval will hopefully come through soon. PET is also used for brain scanning for epilepsy. In the US, epilepsy is the only neurology indication reimbursed by Medicare. PET and PET/CT have had a high rate of success finding dementia, brain tumor grading, Huntington's Disease Parkinson's Disease, and Cerebrovascular Disease. Neurologists also utilize PET to determine the neuralgic manifestations of diseases such as systemic lupus erythematosus. Let’s review the specific protocol for neurology cases. While the parameters and procedures differ per facility, specific diagnosis, and isotope, we can review some general procedures that typically apply across the board to both neurology and non-neurology studies. You’ll recall that this general protocol includes nothing by mouth for at least four hours prior to FDG injection. You also should review the patient’s pertinent medical history, weight, height, blood pressure and other general statistics, and, of course, discuss the procedure with the patient. Current medication should be maintained, and pain medication needs to be reviewed. In addition, be sure to check the patient’s blood glucose level. Glucose levels greater than 120 mg/dl can degrade image quality. Finally, the patient should be encouraged to void his or her bladder to ensure patient comfort during acquisition. Brain imaging is very similar to whole body imaging. Typically, only 1 to 2 bed positions are required. Images are reconstructed iteratively, utilizing 16 subsets and six iterations.

Heart Protocol Cardiology is another specialty that is attracted to the excellent diagnostic capabilities of the PET/CT. As with neurology, parameters and procedures differ per facility, specific diagnosis, and isotope, but we can review some of the general protocol that should be followed. The cardiology patient should be instructed to take nothing by mouth for four hours prior to FDG injection. Once again, you should review the patient’s pertinent medical history, weight, height, blood pressure, and other general statistics, and, of course, discuss the procedure


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Current medication should be maintained, and pain medication needs should be reviewed. In addition, be sure to check the patient’s blood glucose level, because when cardiology patients test at a level greater than 115 mg/dl, the nuclear medicine physician determines whether the diabetic protocol should be utilized. Next, you should obtain venous access and properly place the ECG leads and blood pressure cuff so that the patient’s heart can be monitored throughout the exam. The proper dose of FDG should be administered via the IV bolus injection. Be sure to record the dosage in the patient’s log and do your best to make the patient comfortable. After all, patient comfort is essential. Imaging should begin no sooner than 40 minutes post injection. The topogram will indicate the exact patient position for the cardiac scan. Acquisition is typically comprised of either one or two bed position. The difference often depends on the size of the patient’s heart. For example, patients with cardiomegaly, or an enlarged heart, usually require two beds. Image reconstruction for heart patients is accomplished in the same manner as it is with whole body image reconstruction. Image analysis requires that the data first be sent to a review station capable of reorienting the acquired short axis (transverse) data into corresponding horizontal long axis and vertical long axis slices, also known as standard cardiac orientation. The optional Siemens Esoft software that we discussed in module four automatically performs the required reorientation. In addition, optional image quantification packages are available.

PET/CT and Radiation Therapy Planning Oncology is the third specialty that regularly utilizes the PET/CT. In fact, PET/CT has proven especially beneficial for radiation therapy planning, also known as RTP. In 1998, Medicare announced that it would cover PET for lung cancer patients. With its ability to detect metabolic changes, PET is a valuable tool for cancer diagnosis, cancer staging, and therapy response evaluation; however, PET isn’t perfect. Its greatest drawback is its poor spatial resolution. CT images, on the


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other hand, excel at showing spatial context. Thus, the combination of these two technologies conquers the individual limitations of each and presents physicians with a powerful tool to better detect and locate abnormalities. What are some of the specific advantages and applications of the PET/CT in diagnosing cancer? First, the PET/CT helps determine target volume. Its ability to detect functional changes is crucial for cancer patients who, during cancer’s earliest, and most treatable phase, typically experience functional changes prior to structural ones. For many types of cancer, PET also provides better tumor detection compared to anatomical imaging, and it can quantify basic tumor biological properties that are useful for determining biological target volumes. Furthermore, when radiation is part of the oncology patient’s treatment protocol, the PET/CT helps the RTP team determine the optimal dosage and targeted location of radiation that a patient should receive. Finally, all of these properties can be followed during and after therapy to assess treatment response. The PET/CT’s combined imaging also offers physicians an opportunity at the outset of chemotherapy to see if the drug is working. If not, the protocol can be changed, allowing the patient to avoid a long, ineffective treatment regimen that can have many adverse side effects. Now let’s take a closer look at the radiation therapy pallet, which can be found on the patient handling system. Since the Biograph contains no septa, the inner diameter of the PET gantry can be opened to 70 cm, the same diameter of the CT gantry.


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This modification allows for RTP compatibility. In other words, the patient position can be aligned to match the radiation therapy table. Because the PHS pallet is curved and radiation therapy planning requires it to be flat, a removable pallet is used on the table. In addition, positioning lasers external to the gantry can be incorporated for more precise tumor localization. In addition to the RTP pallet, immobilization devices play an important role in patient setup for radiotherapy. They help to hold the patient in a fixed position so that the treatment beam hits the same exact spot for the entire treatment and for each successive treatment. Most devices have markings so that the exact position can be replicated. The different types of immobilization devices include breast boards, neck/head boards, foam cradles, hip fixations, and more.

Quality Control QC Overview Last, but certainly not least, in the final chapter of this course, we’ll review the extremely important steps that you can take to ensure quality control or QC, over all aspects of the PET/CT. To ensure that the efficiency and accuracy of the machines are optimized, the PET/CT equipment requires specific care. Each aspect of the operation needs to be checked on a regular basis. All computer components should be rebooted daily. Each gantry has specific daily quality control procedures. For CT, the technologist performs a check-up, calibration, quality checks of densities, settings, and rotation speeds. Daily PET QC measures a known quantity of a radioactive source & compares the results to the previous measurements. Additionally, normalization should be performed at least monthly.


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Anytime the gantries are separated for service and then reconnected, performing the Gantry X/Y offset calibration is crucial. This step ensures the proper coregistration of the PET image to the CT image.

CT Quality Control Processes Let’s start review the QC processes for the CT portion of the PET/CT. First, the computer boot-up procedure ensures that the system resources, including memory, are clean and that both the CT and PET systems are communicating. This QC checkup also warms up the anode and improves image quality Second, a daily QC calibration warm up is performed. When a 60-minute or more break in patient testing occurs, this step helps to warm-up the anode. Finally, daily QC check, which confirms the densities of known material, such as water, air and metal is conducted. This check also secures the different kV settings, mA settings, rotation speeds and pixel noise, also known as standard deviations.

PET Quality Control – Daily Daily quality control testing ensures that the integrity of the PET scanner is maintained. These audits also utilize the results in processing the patient data that’s acquired every day. Begin by testing the daily integrity of the scanner. Daily visual inspection of the QC sinogram can reveal block detectors that are not functioning optimally and those that might adversely impact patient acquisitions. Another important component to the PET QC checks is the Daily Two-Bed test scan, which serves two main purposes. First, it mimics "real" patient acquisition to ensure that both CT and PET gantries are functioning together properly. Second, if any artifacts in the daily QC sinogram will impact the field of view or FOV for patient acquisitions, this test will highlight the problem. Before conducting the Daily Two-Bed test, however, you must ensure that the scanner operation and the system communication are working properly. Finally, we need to archive patient data. Why? The data storage disk must be cleared periodically to optimize system performance. Of course, while we need to clear the storage disk, we also need to be able to retrieve data for review at a later time; therefore, archiving prior to clearing the disk is necessary. Data can be archived to the Optical Disk or MOD, a CD, or a PACS system depending upon the archiving system available at your diagnostic center.


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It is crucial that you DO NOT delete the following patients from the local database: Quality Assurance Patient, Service Test, Reference Images, or Temp PET. Every day, you should shut down and restart all systems.

PET QC – Weekly/Monthly In these last two sections we’ll review the PET’s monthly quality control testing, which is primarily comprised of PET Normalization. Normalization, in general, is a maintenance procedure whereby differences in the efficiency of the scanner detectors are measured and accounted for. You should perform the following three processes monthly . Begin with the Crystal Efficiency Calculation, which calculates how efficient each crystal is at detecting a known amount of radioactivity. Immediately following the Crystal Efficiency procedure, the second part of monthly normalization is the ECF Calibration, also frequently referred to as the ECAT Calibration Factor. This test gives you the ability to calculate calibration factors for standard uptake values (SUV’s) by calibrating the PET scanner to a known source of radioactivity. ECAT Calibration also determines plane efficiencies for the system. Please note: you should never repeat an ECF calibration without first conducting a Crystal Efficiency. The third aspect of monthly PET normalization is obtaining a Standard QC. This sets the standard to which the daily quality control protocol refers and compares the detector system integrity.

Gantry (XY) Offset Calibration Now we’re ready to review the less common quality control measures and service procedures. In addition to the three PET Normalization processes discussed in the last section, performing a Field of View Offset Calibration, also known as a Gantry Offset, is recommended after service involving gantry separation. Mobile units have to perform this task after every move to a new location so they may perform this calibration more frequently. The CT Constancy Test, which is technically comprised of multiple tests beginning with the Lightmarker test. The Lightmarker determines the deviation of the laser position to the image slice plane. As its name indicates, the Slice Thickness test determines the real slice thickness for the system, while the Homogeneity test establishes the CT values for all kV settings. Not nearly as “loud” as it sounds, the Noise test decides the pixel noise for all kV settings. Meanwhile, the appropriately named MTF test determines the


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modulation transmission function or MTF for the system. Finally, the Patient Table Position test verifies the accuracy of the table's reported position to within one mm. Now let’s review the Application Specific Integrated Circuit (ASIC) Bucket Setup for the PET. During this check, a Ge68 line source is used and several adjustments are performed. For example, the constant fraction discriminators are modified and the x-y position of the position profiles is adjusted. In addition, the time alignment for front-end electronics is revised and the detector blocks are setup. In other words, crystal boundaries are defined and photo multiplier tube or PMT gains are adjusted. Finally, we come to our last two remaining QC checks – PET Sensitivity and PET Uniformity. PET Sensitivity defines the rate at which coincidence events are detected in the presence of radioactive sources at activity levels where count rate losses are negligible. PET Uniformity describes the ability to measure the same activity independent of location within the imaging field of view or FOV.

Please return to your course player to take the final quiz.


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