Vendor reported CTDI vs. actual CTDI in SPECT/CT: Initial experience

Objectives The addition of anatomic imaging in general nuclear medicine, i.e. SPECT/CT, has increased concern for patient (pt) radiation dose (RD). In the near future, pt reports will be expected to include the total RD for these types of exams. RD from the SPECT component is directly calculated based on the mCi dose; while, the RD from the CT component is quite variable. Vendors report estimated RD from the CT component of each study by using the computed tomography dose index (CTDI). The purpose of this study was to compare the vendor reported CTDI to the actual CTDI calculated from phantom studies (PS).

Methods PS were performed on a Philips Brightview XCT using a CT body phantom, a dosimeter, and an ionization chamber probe to measure the RD from penetrating x-rays from the CT scan. Data was collected by placing the probe into one of the five holes drilled into the phantom, running the CT scanner according to the given Philips protocol, and recording the measured RD reading from the dosimeter. This was done for each hole. The data was analyzed by a radiation physicist, who then used a standard equation to calculate and report the CTDI.

Results Preliminary results show that the actual CTDI from the PS is slightly lower, within 20%, of the reported CTDI (6.5mGy). The accuracy of such numbers is somewhat limited. Some error in the results could be due to dosimeter errors and phantom set up. This is particularly true in the case of the Brightview XCT where the CT is a flat-detector CT which is different than the more common multi slice CT in most SPECT/CT systems.

Conclusions While the preliminary results show that the actual CTDI is very similar to the reported CTDI, further testing should be done to validate the accuracy of using the CT body phantom in evaluating RD from our Brightview XCT scanner. More importantly, further studies are needed to elucidate the relationship between CTDI in phantom studies and the actual RD to pts undergoing clinical studies