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. 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 xray imaging. 2008.
http://www.acr.org/secondarymainmenucategories/quality_safety/gu
idelines/med_phys/reference_levels.aspx, accessed 11/03/2011.
McCollough CH, “Diagnostic reference levels”, image wisely,
http://www.imagewisely.org/Imaging-Professionals/MedicalPhysicists/Articles/Diagnostic-ReferenceLevels.aspx?CSRT=3768001513176777918 accessed 11/03/2011.
McCollough CH, “Strategies for Reducing Radiation Dose in CT”,
Radiol Clin N Am, 49:27-40, 2009 .