Patient-circumference-adapted dose regulation in body computed tomography. A practical and flexible formula.
OBJECTIVE
To illustrate that the attenuation formula based on monochromatic radiation in homogeneous objects may be used for dose regulation in body computed tomography (CT) based on patient circumference and using a simple cloth measuring tape.


METHODS
Based on the attenuation formula for monochromatic radiation the following Microsoft Excel equation was derived: mAs(x) = mAs(n)*EXP((0.693/ HVT)*(O(x)-O(n))/PI()), where mAs(x) (milliampere second) in a patient with circumference O(x) is calculated based on the nominal mAs(n) set for a reference patient with the circumference O(n) with regard to indication, scan protocol, and available CT scanner. The HVT = half-value thickness (object thickness change in cm affecting mAs setting by a factor of 2) resulting in the least mAs difference compared with published studies investigating the mAs needed for constant image noise in abdominal CT phantoms at 80-140 kVp was evaluated. Clinically recommended HVT values were applied to 20 patients undergoing abdominal CT using 130 effective mAs and 94 cm circumference as nominal settings, and an HVT of 9 cm.


RESULTS
The object-sized dependent mAs for constant image noise at 80-140 kVp in 10-47 cm diameter abdominal phantoms (31-148 cm in circumference) differed, with few exceptions, by no more than 10% from those obtained with our formula using an HVT of 3.2-3.8 cm. An HVT of 9 cm in the patient study resulted in the same image noise-patient circumference relation as a phantom study using a "clinically adapted mAs" resulting in an acceptable noise according to diagnostic requirements. Clinical experiences recommend an HVT of about 8 cm for abdominal CT and 12 cm in thoracic CT. Changing the kVp from 120 to 80, 100, or 140 requires a mAs change roughly by factors of 4, 2, and 0.6, respectively, for constant image noise.


CONCLUSIONS
Until fully automatic automatic exposure control systems have been introduced, applying the formula in a computer program provides the radiologist with an easy, quick, flexible, and practical instrument for reasonably good patient-sized adjusted exposure levels in clinical practice.
Nyman
U
U
Department of Diagnostic Radiology, Lasarettet Trelleborg, Sweden. ulf.nyman@skane.se
Ahl
T L
TL
Kristiansson
M
M
Nilsson
L
L
Wettemark
S
S
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001837:Body Weights and Measures / Q000379:methods*; D003287:Contrast Media / Q000008:administration & dosage; D003973:Diatrizoate / Q000008:administration & dosage; D006801:Humans; D007455:Iodine / Q000008:administration & dosage; D016014:Linear Models; D008432:Mathematical Computing; D008875:Middle Aged; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D011856:Radiographic Image Enhancement / Q000379:methods; D011860:Radiography, Abdominal / Q000379:methods; D014057:Tomography, X-Ray Computed / Q000295:instrumentation* / Q000379:methods*
D003287:Contrast Media; D003973:Diatrizoate; D007455:Iodine