The development of CT scanner technology in the past 10 years has brought a range of advances, including slip ring scanning, fast (sub-second) rotation times ...
The British Journal of Radiology, 74 (2001), 1088–1090
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2001 The British Institute of Radiology
Commentary
Real-time CT and CT fluoroscopy N KEAT, MSc, MIPEM ImPACT, Bence Jones Offices, Perimeter Road, St George’s Hospital, London SW17 0QT, UK
The development of CT scanner technology in the past 10 years has brought a range of advances, including slip ring scanning, fast (sub-second) rotation times and greater computing power. These advances have been combined to allow rapid reconstruction of CT scanner data that can be updated continuously to provide crosssectional images in near real-time. CT scanning is conventionally prescriptive, with scan sequences planned in advance. Real-time scanning expands the role of CT to an interactive imaging technique. Applications of real-time CT lie in two main areas: timing and monitoring of helical scan sequences, and interventional procedures such as tissue biopsy and the drainage of fluid-filled lesions. In this paper, the use of real-time CT in interventional procedures is referred to as CT fluoroscopy. With the advantages of this new technology comes concern over doses to both patient and equipment operator, as well as the need for careful control over its use. This commentary examines the technology, applications and dosimetry of real-time CT scanning and draws out some of the important points in each of these areas.
Technology In 1993, Toshiba Medical Systems Co. Ltd (Tokyo, Japan) began the development of realtime capabilities for their Xpress/SX CT scanner [1]. This scanner had a gantry rotation time of 1 s, and the Aspire CI real-time package was capable of reconstructing and displaying three images per second. Each image displayed on-screen represented the object in the scan field of view in the preceding second. This did not match the almost instantaneous display provided by conventional planar fluoroscopy units, but was rapid enough to provide feedback on what was currently in the scan plane and allow the scanner operator to react to that information. Received 31 October 2000 and in revised form 31 May 2001, accepted 11 June 2001. ImPACT is the UK’s CT scanner evaluation group, funded by the Medical Devices Agency. 1088
Subsequently, all the medical CT scanner manufacturers have developed real-time scanning systems. These have generally been for their topof-the-range single slice scanners, but real-time CT capabilities are also being developed for multislice scanners, with the potential to display more than one adjacent real-time image simultaneously. Despite minor differences, the manufacturers’ systems have a lot in common. Scanning is generally performed at 120 kV with tube current in the range 30–90 mA and a scan time of 1 s and below (down to 0.5 s for the fastest system). Images are displayed at a rate of between 3 s21 and 12 s21. In general, a scanner with a fast tube rotation time will provide better feedback to the operator than a scanner with a more rapid image display rate and a slower tube rotation time. For interventional procedures, images are displayed on a monitor in the scanner room. Exposure is controlled with a foot switch, and couch movement is achieved either with a joystick mounted at the side of the patient table or using a freely floating couch top that can be moved by hand. ‘‘Last image hold’’ retains the final image in a sequence on the monitor after scanning stops. Reconstruction times for real-time CT scanning seem very fast compared with the 1–2 s per standard image for a modern single slice CT scanner. Rates of three images per second and above are achieved using a number of simplifications in the reconstruction process. The real-time image is generally reconstructed on a 2566256 pixel matrix, rather than the more usual 5126512 pixel matrix for conventional CT. Other image calculations, such as beam hardening corrections, may also be omitted. Image display rates are also helped by the fact that, in order to display the nth image in a sequence, only the projection data not present in the nth21 image needs to be back projected. As display rate is increased, the number of calculations required to produce each image decreases. All CT fluoroscopy systems on the market have an alarm timer, similar to those in conventional planar fluoroscopy. There is no standard time limit for these alarms, but a 100 s limit is pre-set on a number of models. The British Journal of Radiology, December 2001
Commentary: Real-time CT and CT fluoroscopy
Real-time CT and CT fluoroscopy are often sold as separate packages, so purchase of a system with real-time capabilities does not necessarily imply CT fluoroscopy facilities will also be included. Upgrade options are available for most modern single slice scanners, usually consisting of exposure foot pedal, table top control mechanism and in-room monitor, and sometimes including hardware to enable rapid image reconstruction.
Applications The ability to visualize cross-sectional CT images in real-time has been applied to two main tasks: timing of beginning and end points for helical scan sequences, and interventional procedures (CT fluoroscopy).
Scan timing The time between injection of CT contrast medium and its appearance in the organ of interest varies from patient to patient. Producing optimal contrast studies requires images to be acquired at the correct vascular phase. Realtime CT can be used to aid this process by continuously monitoring a region of interest, such as the aorta, and starting the conventional helical run once the mean CT number in this region reaches a pre-set threshold [2]. This process is commonly known as bolus tracking and has been shown to be effective in increasing contrast enhancement [3, 4]. Real-time CT scanning can also be employed to offer a continuous transaxial view during helical scanning. This allows the operator to stop acquisition before the prescribed end point, for example if the patient moves significantly or if the entire organ of interest has been imaged before the end of the scan run. For a helical run planned from a scan projection radiograph the latter situation should not occur, so the usefulness of real-time helical scanning is generally limited to observing images rather than reacting to them.
CT fluoroscopy The use of the term ‘‘CT fluoroscopy’’ varies, but in this article it describes application of realtime CT scanning in interventional procedures. Images from the scan room monitor enable the operator to guide a needle to a specific site within the body. Silverman et al [5] described two main approaches to this. The first uses continuous imaging and a needle holding device, similar to sponge forceps, to keep the operator’s hands out of the primary radiation beam. The second employs intermittent imaging and manual needle manipulation. Of these, the first can result in The British Journal of Radiology, December 2001
higher doses to patient and operator but provides a less tactile method for needle placement. The second is closer to existing methods for nonreal-time CT biopsy but has the advantage of shorter periods between scanning and adjustment of needle position, which improves the interactivity of the procedure. The main reported applications of CT fluoroscopy [5, 6] are tissue biopsy and drainage of fluid-filled lesions, as well as a variety of other procedures such as ethanol ablation of tumours, placement of catheters and guidance of sacroiliac injections.
Dosimetry The dosimetry concepts in conventional and real-time CT are broadly similar but bring different issues to the fore. For most real-time applications, scanning takes place in one scan position, meaning that local patient skin doses have the potential to reach levels where deterministic radiation effects are seen. In CT fluoroscopy the equipment operator can receive considerable doses. This is a departure from the usual situation in CT scanning, during which the equipment operator is located in a separate control suite and receives negligible radiation dose. Published values of screening times for CT fluoroscopy procedures [5–8] range from 3 s to 660 s, with mean screening times in these studies of 79–165 s. This wide range reflects differing techniques, levels of operator experience and workloads studied, as well as the degree of complexity of individual procedures.
Dose to patient A recent ImPACT ‘‘blue cover’’ report [9] examined the dose from abdominal CT fluoroscopy procedures using typical exposure parameters (120 kV, 50 mA, 1 s rotation time, 10 mm slice thickness) for a range of scanners. Skin dose rate and effective dose rate were estimated to be approximately 4–5 mGy s21 and 60 mSv s21, respectively. For the previously discussed studies, this would result in mean skin doses of 0.4– 0.8 Gy. The study with a maximum scan time of 660 s would result in a skin dose of approximately 3 Gy. This is greater than the threshold level for transient erythema (2 Gy) and in the region of that for temporary epilation (3 Gy). Mean effective patient doses would be between 4.7 mSv and 9.9 mSv for these studies. This compares with an effective dose of 11.7 mSv for a standard abdomen scan using the ‘‘European Guidelines on Quality Criteria for Computed Tomography’’ [10]. The above values are examples of the magnitude of patient doses. When studying individual 1089
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patient doses, it is important to use actual exposure parameters and scanning time in dose estimations as there is potential for wide variation from patient to patient.
Dose to operator In addition to operator position relative to the scan plane, the variables relevant to staff dose are the same as those influencing patient dose (i.e. kV, mA, slice width, total scan time). Minimizing operator dose can be achieved by keeping as far from the scan plane as possible. Estimates of dose rates to the operator from CT fluoroscopy drop very rapidly with distance from the scan plane. ImPACT measurements on a Toshiba Asteion (Toshiba Medical Systems Co. Ltd, Tokyo, Japan), with a 32 cm CT dose index phantom to provide scatter were 4 mGy s21 in the scan plane, 9 mGy s21 at the body trunk and 2 mGy s21 to the eyes (operator positioned 40 cm from the scan plane). These measurements are obviously sensitive to operator position, but correspond well with those of Ozaki [11]. It is difficult to estimate dose per procedure to staff in the scan room, as operator technique is a key factor. However, careful monitoring of doses to hands, trunk and eyes is needed to ensure occupational dose limits are observed. Use of lead aprons by the operator is strongly recommended; Ozaki noted that they reduced dose rate to the trunk by a factor of 14. Consideration should also be given to the use of thyroid shields and lead glasses.
Conclusions Real-time CT and CT fluoroscopy are useful techniques offering a number of potential benefits, including more consistent timing of contrast studies and improved diagnostic accuracy of biopsies. As with other interventional radiology techniques, doses to both patient and equipment operator from CT fluoroscopy have the potential to reach high levels and should be monitored carefully. In particular, an intermittent imaging technique is preferred to continuous imaging using
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a needle holder owing to reduced radiation doses and improved needle manipulation. The availability of CT fluoroscopy is still limited, but as its use becomes more widespread it is likely to become a routine aid to a growing range of procedures. Biopsy guidance is the most obvious application of this technology, but any technique requiring real-time three-dimensional guidance of a probe through the body could potentially benefit.
References 1. Katada K, Kato R, Anno H, Ogura Y, Koga S, Ida Y, et al. Guidance with real-time CT fluoroscopy: early clinical experience. Radiology 1996;200:851–6. 2. Anno H, Katada K, Kato R, Ogura Y, Koga S. Scan timing control in contrast helical CT studies using the real-time reconstruction technique. In: Iinuma K, editor. Toshiba Medical Review Aspire CI Special Edition. Tokyo: Medical Systems Division, Toshiba Corporation, 1996:43–50. 3. Dinkel HP, Fieger M, Knupffer J, Moll R, Schindler G. Optimizing liver contrast in helical liver CT: value of a real-time bolus-triggering technique. Eur Radiol 1998;8:1608–12. 4. Kirchner J, Kickuth R, Laufer U, Noack M, Liermann D. Optimized enhancement in helical CT: experiences with a real-time bolus tracking system in 628 patients. Clin Radiol 2000;55:368–73. 5. Silverman SG, Tuncali K, Adams DF, Nawfel RD, Zou KH, Judy PF. CT fluoroscopy-guided abdominal interventions: techniques, results and radiation exposure. Radiology 1999;212:673–81. 6. Daly B, Templeton PA. Real-time CT fluoroscopy: evolution of an interventional tool. Radiology 1999; 211:309–15. 7. Daly B, Krebs TL, Wong-You-Cheong JJ, Wang SS. Percutaneous abdominal and pelvic interventional procedures using CT fluoroscopy guidance. AJR 1999;173:637–44. 8. Meyer CA, White CS, Wu J, Futterer SF, Templeman PA. Real-time CT fluoroscopy: usefulness in thoracic drainage. AJR 1998;171:1097–101. 9. Keat N. Real time CT and CT fluoroscopy. Norwich: HMSO, 2000. 10. European Commission. European guidelines on quality criteria for computed tomography, EUR 16262 EN. Luxembourg: EC, 1999. 11. Ozaki M. Development of a real-time reconstruction system for CT fluorography. In: Iinuma K, editor. Toshiba Medical Review Aspire CI Special Edition. Tokyo: Medical Systems Division, Toshiba Corporation, 1996:57–62.
The British Journal of Radiology, December 2001
