Study (Audit) Design:
Retrospective audit of radiation dose in 50 subjects referred for CT KUB to the department of radiology, SAK, Akureyri was performed over a period of 3 months. All the studies were performed on 128 Slice CT scanner. The study group consisted only of adult patients (pregnant females were excluded).
For each patient, following parameters were recorded:
-Dose Length Product(mGy.cm)
-Scan length (in cms)
-Patient width (in cms).
Effective dose was calculated in mSv for each examination according to European commission formula- Effective dose(E) in mSv = K (Conversion coefficient) x DLP. Conversion coefficient for abdomen being 0.015.
CT KUB study was considered low dose when effective dose in mSv is <= 3 mSv. We aimed at target of 80 % or more CT KUB examinations being low dose.
Initial data analysis revealed effective dose far more than set target of <= 3mSv in more than 80 percent examinations (Fig. 1,2, 3 and Table 1).
Fig 1: Pie chart summarizes the effective radiation dose in millisieverts(mSv) in 50 examinations studied during initial audit- Actual low dose examination was achieved in only 9 out of 50 studies i.e. 18%.
Fig. 2: Bar chart detailing effective dose in mSv in remaining 41 patients. Clearly in most of the studies i.e. 29, dose was in range of 3-6 mSv which was felt to be the most suitable target group regarding implementation of necessary changes.
Fig.3: Chart illustrating split of effective dose received by these 29 patients, with maximum number of patients in range of 4-5 mSv.
Table 1: Comparison of acquisition parameters during CT KUB and CT abdomen- Obvious factors affecting DLP are the SCAN LENGTH and TUBE CURRENT.
1. Optimising the scan length to one cm above the kidneys to the pubic symphysis;
2. Adjusting the noise tube current modulation parameter to increase the noise level, setting a maximum and minimum tube current range; and use [or increase the level] of iterative reconstruction.
3. Image quality to be closely monitored to ensure that the resulting images are of sufficient diagnostic quality.
4.To REAUDIT the studies after implementation of above measures.
After optimising for Low Dose CT KUB protocol and implementing changes, reaudit was performed. Data analysis was done in 40 CT KUB studies over a period of 3 months. Reaudit results were impressive with 87.5% of the exams (35/40 studies) recieved <= 3 mSv effective dose( Fig. 4). In remaining 5 patients, the dose was marginally outside the limit(3-4 mSv). Can we do better in these patients? Answer is YES.
Table 2: Summarizes the various dose parameters and scan length in these 5 patients. Modifiable variables are scan length and KVp.
To decrease the effective dose further local standards were set with these check points for radiographers:
1. Scan from upper border of Thoracic 12 vertebra till mid of pubic symphysis which was convenient for radiographers for planning rather than searching for renal outline(Fig.5).
2. Check the DLP display on CT console ( should not be more than 200 mGy cm which leads to effective dose of > 3 mSv).
3. Keep the Kvp to the lowest possible(preferably 100-120).