Wednesday Case of the Day Physics Authors: Lingyun Chen Ph.D., Lifeng Yu, Ph.D., Shuai Leng, Ph.D., Cynthia McCollough, Ph.D. Department of Radiology, Mayo Clinic, Rochester MN History: Two patients underwent abdomen & pelvis CT scan, patient 1 had CTDIvol = 47.5 mGy, patient 2 had CTDIvol = 17.14 mGy. According to ACR practice guideline, the diagnostic reference level (DRL) of adult abdomen CT is 25 mGy. Patient 1: lateral width measured across the mid liver level is 45 cm. Scanning protocol: 140 kV, 64x0.6 mm, reference mAs 180, effective mAs 483. Patient 2: lateral width measured across the mid liver level is 33 cm. Scanning protocol: 120 kV, 64x0.6 mm, reference mAs 240, effective mAs 224. Which of the following statement about these exams is true ? (a) exam of patient 1 is inappropriate, because CTDIvol of this exam is much higher than ACR DRL; exam of patient 2 is appropriate. (b) exam of patient 1 is appropriate; exam of patient 2 is inappropriate, because CTDIvol of this exam is below ACR DRL. (c) both exams are inappropriate. (d) both exams are appropriate.

Diagnosis: The correct answer is (d), both exams are appropriate Diagnosis: The correct answer is (d), both exams are appropriate. Scanning technique should be adjusted based on the patient size in order to achieve acceptable diagnostic image quality.

Discussions Scanning technique should be adjusted based on the patient size in order to achieve acceptable diagnostic image quality. Patient attenuation may vary substantially from infants to obese patients. It is appropriate to increase the technique for big size patients and decrease the technique for small size patients. For CT imaging of the body, typically a reduction in mAs of a factor of 4 to 5 from adult techniques is acceptable in infants; whereas for obese patients, an increase of a factor of 2 is appropriate. To achieve sufficient exposure levels for obese patients, either the rotation time or the tube potential may also need to be increased. Diagnostic reference levels (DRLs) are supplements to professional judgment and do not provide a dividing line between good and bad exams. DRLs recommended by ACR practice guideline are based on radiation doses for specific procedures measured at a number of representative clinical facilities. DRLs have no link to dose limits or constraints. DRLs are not the suggested or ideal dose for a particular procedure or an absolute upper limit for dose. Rather, they represent the dose level at which an investigation of the appropriateness of the dose should be initiated.

References ACR practice guideline for diagnostic reference levels in medical x-ray imaging. 2008. http://www.acr.org/secondarymainmenucategories/quality_safety/guidelines/med_phys/reference_levels.aspx, accessed 11/03/2011. McCollough CH, “Diagnostic reference levels”, image wisely, http://www.imagewisely.org/Imaging-Professionals/Medical-Physicists/Articles/Diagnostic-Reference-Levels.aspx?CSRT=3768001513176777918 accessed 11/03/2011. McCollough CH, “Strategies for Reducing Radiation Dose in CT”, Radiol Clin N Am, 49:27-40, 2009 .