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08 July 2008

Answer to Question #2101 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

The following question was answered by an expert in the appropriate field:

Do you have any information on the radiation dose for CT coronary, CT colonoscopy or CT abdominal angiography using 4, 8, OR 16 channel multihelical CT? These tests are becoming popular and I have not seen any data on the radiation dose.

With regard to virtual colonoscopy on a multislice CT scanner, I have estimated, based on the techniques we are using at the Memorial Sloan-Kettering Cancer Center (MSKCC) on our GE LightSpeed Ultra (8 slices per rotation) CT scanner with the following techniques: 120 kV, 220 mA, 0.5 sec/rotation, 13.5 mm/sec table speed, 5 mm slice reconstruction, and 1:35 pitch. The average dose to the tissues irradiated (CTDI or MSAD) is about 10 mGy (1 rad).

Some approximate doses for CT procedures which involve angiography are given below. The number of passes and study phases are indicated in parentheses for the tissues which receive the listed doses. I have assumed abdomen doses for our GE 8-slice LightSpeed Ultra CT Scanner running at 120 kV and 300 mA, with varying pitches and slice acquisition widths that are used clinically.

Organ	Procedure	Dose	Passes/Study Phases
Abdomen	Triphasic Liver - Diagnostic scan with pre-operative CT angiography	70 mGy (7 rad)	Four passes - precontrast, hepatic arterial phase, hepatic arterial phase 2, portal venous phase
Abdomen	CT angiography for hepatic surgery planning	56 mGy (5.6 rad)	Two passes - hepatic arterial phase, portal venous phase
Pancreas	Preoperative planning with CT Angiography	100 mGy (10 rad)	Four passes - precontrast, early arterial, parenchymal, portal venous
Kidney	3D work-up for renal mass or renal collecting system lesion	100 mGy (10 rad)	Four passes - kidneys precontrast, kidneys arterial, kidneys parenchymal, kidneys excretion

In summary, depending on the number of passes and including the precontrast injection scans, the typical CT angiography doses vary from about 50 to 100 mGy (5 - 10 rad).

Note: the doses presented are the CTDIvol (volume Computer Tomography Dose Index) doses, which was formerly called CTDIw or weighted CTDI. This involves a weighted average of 2/3 of the surface CTDI and 1/3 of the central axis CTDI. The CTDI is a reasonable estimate of the multi-slice average dose. These numbers are available as pre-scan estimates on the operator console screen on most new multi-slice CT scanners from the major companies, e.g., GE, Siemens, Toshiba, Philips, etc.

Lawrence N. Rothenberg, Ph.D.
Attending Physicist
Department of Medical Physics
Memorial Sloan-Kettering Cancer Center

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