Originally designed to produce cross- sectional images of the brain. CT
REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC.,
1999) ...
RAD 114 RADIATION PROTECTION
SPECIAL TOPICS IN PROTECTION: PATIENT DOSE IN COMPUTED TOMOGRAPHY
COMPUTED TOMOGRAPHY An x-ray modality developed in the early ’70’s by Sir Godfrey Hounsfield. Originally designed to produce crosssectional images of the brain.
CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
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CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
SPIRAL (HELICAL) CT A refinement that allows continuous patient scanning. The x-ray tube within the gantry revolves around the patient at the same time that the table is moving into the bore of the gantry.
CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
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HELICAL (SPIRAL) CT To convert the helical scan data into slices that can be individually viewed, i d th the computer t performs f what is called interpolation. This process fills in the gaps created by the helical scan.
CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
ADVANTAGES • • • •
RAPID DATA ACQUISITION SINGLE BREATH-HOLD IMAGING LARGE VOLUME COVERAGE 3D AND MULTIPLANAR RECONSTRUCTION
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CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
CT REGISTRY REVIEW PROGRAM™ (MEDICAL IMAGING CONSULTANTS, INC., 1999)
RADIATION DOSE DEPENDS ON • • • • • •
ENERGY OF THE BEAM COLLIMATION MAS FILTRATION SCAN PITCH LEAD SHIELDING?
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ENERGY OF THE BEAM As with radiography, the energy of the beam controls penetrability and scatter production production. In CT CT, the kVp is usually kept in the range of 120 to 130.
COLLIMATION Two sets of collimators are used. One is positioned in front of the primary i b beam ((pre-patient). ti t) The Th other is positioned in front of the detector array (post-patient).
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MAS The mAs is varied in order to perform each scan. More mAs is used to scan larger volumes of tissue or compensate for thinner scan slices li (ti (tight ht collimation). lli ti ) B Because CT is a computer assisted technology, the effects of changes in mAs do not appear the same way as with film-screen radiography. Instead, optimization in signal-to-noise (s/n) is sought.
FILTRATION Filtration serves two purposes: skin dose reduction and reduction in certain kinds of artifacts that result from the effects of “beam hardening.”
PITCH The pitch is the way in which the continuous spiral of the scan is stretched t t h d outt or compressed. d It is i defined as the ratio of the scan table increment (BI) per 360 degrees of x-ray tube rotation to the slice thickness (SW):
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PITCH
P
BI SW
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CT DOSE INDEX (CTDI) For a single slice, an ion chamber can be used to measure dose dose. That dose becomes what is called the CT dose index (CTDI) and can be used to perform calibration checks for quality control purposes.
MULTIPLE SLICE AVERAGE DOSE (MSAD) When dose calculations are taken from multiple scans (slices) then the dose is an average based on measurements taken from the center of those slices.
CTDI and MSAD As long as the slices are contiguous (one next to the other) the CTDI and MSAD are the same.
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CTDI and MSAD With changes in pitch, MSAD increases or decreases and is equal to: CTDI x INVERSE OF PITCH
PROBLEM Assume a CTDI of 1000 mrem. If spiral CT images are obtained using a table increment (BI) of 10mm per tube rotation and a slice thickness (SW) of 2.5 mm, what is the MSAD?
ANSWER • • • •
CTDI = 1000 MREM PITCH = 10 mm ÷ 2.5 mm = 4 MSAD = CTDI x INVERSE OF PITCH MSAD = 1000 x (1/4) = 250 mrem
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REPRESENTATIVE DOSES (EFFECTIVE DOSE EQUIVALENTS) • • • •
CT HEAD ≈ 200 MREM (7 MREM) CT CHEST ≈ 700 MREM (5 MREM) CT PELVIS ≈ 900 MREM (160 MREM) CT ABDOMEN ≈ 1000 MREM (130 MREM)
RISK vs. BENEFIT: QUESTIONS PRACTIONERS SHOULD ASK • CAN A LOWER DOSE ALTERNATIVE PROVIDE AN EQUAL BENEFIT TO CT? • WILL CT PROVIDE A BENEFIT THAT EXCEEDS THAT OF LOWER DOSE ALTERNATIVES? • CAN A NON-IONIZING DIAGNOSTIC ALTERNATIVE PROVIDE AND EQUAL OR INCREASED BENEFIT OVER CT?
WHAT TECHNOLOGISTS CAN DO TO LOWER DOSE • USE ONLY OPTIMAL MAS VALUES • CHILDREN REQUIRE LESS MAS THAN ADULTS • USE SLICE THICKNESSES THAT ARE OPTIMAL (THIN SLICES REQUIRE HIGHER MAS) • SET PITCH VALUES THAT ARE CONSISTENT WITH GOOD QUALITY (DOSE IS HIGHER WITH LOWER PITCH • PROVIDE WRAP-AROUND SHIELDING ESPECIALLY WHEN SCANNING CLOSE TO REPRODUCTIVE ORGANS(?)
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