Current Clinical Concerns in CT: Results : Protocols
Renal Stone
1. With the 16 slice MDCT, what is the optimal collimation, slice thickness and image reconstruction for renal stone protocol? Do we actually see more stones with thinner than 5 mm collimation, or are we just getting more degraded images?
| Answer: Our routine protocol is 3 mm thick sections at 3 mm intervals. Obviously, the thinner the sections and the closer the interscan spacing, the more likely you are to pick up the 1 to 2 mm stones. |
2. Some of our radiologists like renal stone protocols from the top of the kidneys down to the pelvis, to cut down on exposure, especially breast tissue in young females. Others want diaphragm down. Any thoughts?
| Answer: We start midliver which gets us the gallbladder, which may mimic renal colic when diseased. This is a good compromise. |
3. We have a Somatom Plus 4. We currently scan a stone study 5 mm/feed 8mm, .75 sec rotation, top of kidney through bladder. Do you prefer 3 mm/ feed 6mm/0.75 second rotation to the 5 mm scans. I know the dose and breath hold is more, but the MPR data is very good and small stones can be seen in the ureter and not overlooked as incidental calcification. Is the extra dose worth the effort for small stones? We reconstruct the 3 mm slice to 5 mm to limit the images the rad has to look at.
| Answer: Our routine protocol is 3 mm thick sections at 3 mm intervals. |
4. Do you have age restriction with regard to renal stone studies? In particular, women under the age of 35. Are there concerns about limiting dose to the ovaries?
| Answer: We of course are concerned with dose, so you can use a lower dose in these patients. The key is to scan only through the needed volume. People have been scanning with lower dose studies. On the newest scanners, automated dose reduction software are valuable. |
5. We routinely scan our patients prone for stone protocol unless they are unable. You stated at a meeting that you didn't feel it necessary to scan the patients in the prone position. Our radiologists have questioned the rationale for that, since it may be better to visualize the position of a stone at the UVJ vs. sitting on the posterior wall of the bladder. You stated that if the stone was at the UVJ, it would probably be pushed into the bladder and excreted eventually and that this did not warrant the uncomfortable positioning of a patient in agony.
| Answer: Although some people do studies prone, we do ours supine. I would not recommend doing both supine and prone routinely as this will make the radiation dose too high. |
References
Spielmann AL,Heneghan JP, Lee LJ, Yoshizumi T and Nelson RC. Decreasing the radiation dose for renal stone CT: A feasibility study of single- and multidetector CT. AJR 2002; 178: 1058-1062.
- Summary: Using retrospective clinical data, the authors report that 33% of the CT examinations requested for renal calculi in their institution between 1999 and 2000 were performed on women. They estimated the uterine dose to be 23 mGy (2.3 rad) from the MDCT QX/I scanner. The dose concerns prompted this study using human calcium oxalate stones in porcine kidneys, scanned with single detector helical and MDCT. The kidneys were scanned with progressively decreasing mAs levels and evaluated for stone detection and measurement. Results showed that stones ranging from 2-8 mm were all visible using single-detector CT with an mA of 80, and using MDCT with an mA of 60. There was no significant difference in the size measurements with the lower dose scan. The lower mA resulted in a 3 fold decrease in estimated radiation dose.
Heneghan JP, McGuire KA, Leder RA, DeLong DM, Yoshizumi T and Nelson RC. Helical CT for nephrolithiasis and ureterolithiasis: Comparison of conventional and reduced radiation dose techniques. Radiology 2003; 229: 575-580.
- Summary: In this study, 50 patients weighing less than 200 lbs. were scanned prone using either single detector helical (5 mm collimation, pitch of 1.5) or 4 slice MDCT (5 mm collimation, 0.75 pitch.) Standard protocol included 240 mA, .8 second gantry rotation and 140 kVp for single detector and 170 mA, .8 or .5 second gantry rotation and 140 kVp for the MDCT. Following the initial acquisition, a low dose scan was obtained using 100 mA (76 mean mAs) and 140 kVp. The lower dose scans resulted in accuracy rates of 91% for nephrolithiasis, 94% for ureterolithiasis, 91% for obstruction and 92% for normal findings. With MDCT, the dose was reduced 25% using the lower dose technique.