During the 2017 Landauer Clinical Dose Optimization Symposium one of the session speakers, Douglas Pfeiffer, a medical physicist with Blackthorn Medical Physics, reported he had found the table-head rest extension connection mechanism increased CTDIvol and DLP when the body part being imaged was positioned over the connection mechanism. Following the symposium, we worked with a facility for whom we provide Dose Index Registry Support and DoseID services to see if we could replicate Mr. Pfeiffer’s findings. The results: We found that scanning through the table-head rest extension connection mechanism increased doses by 28.5% to 31.7%. In the following article, I describe how we performed the test, our findings, and changes we are making to reposition away from the connection mechanism when possible.
Background: This health system operates several scanners, two of which are Siemens Sensation 16 scanners. The image below shows the head extension and the connection mechanism in the table end of one of the Sensation 16 scanners. The black bar extending from the head rest inserts into the metal insert in the table end. The facility uses this table configuration to perform CT Neck and CT Neck Angiograms, as well as to perform some extremity studies.
Test Procedure: We set the scanner to use the following scan parameters: 120kV; 217 Ref mAs; Auto-exposure set to On; 0.75 scan rotation time; with a 0.8 slice thickness. We then imaged the 32cm phantom twice. For the first scan we placed the 32cm phantom on the scanner table over table-head rest extension connection mechanism. We positioned the phantom at isocenter and scanned through the visibly denser area in the table which represented the connection mechanism. We noted the scan length. We then repositioned the phantom so it was positioned on the table away from the table-head rest extension connection mechanism, again positioning the phantom at isocenter. We rescanned the phantom using the exact same parameters and scan length.
Results: As the table below indicates, scanning the phantom when it was placed over the table-head rest extension connection mechanism increased CTDIvol by 28.5% and DLP by 31.7% when compared to scanning the phantom positioned on the table, but not over the connection mechanism. Effective mAs (governed by auto-exposure) was 27.7% higher when the scan was performed through the table-head rest extension connection mechanism compared to scanning through the table top alone.
Discussion: This confirms that some scanners’ head rest-table connection mechanism results in higher patient doses when the scanned anatomy is positioned over the connection mechanism than when the scan is performed through the table top and away from the connection mechanism. We have not tested other system scanners so we can only report on what we found on the Siemens Sensation 16 scanner used to perform this test. Also, depending upon the scan length, the overall dose increase during an actual study could be less than indicated here since our test scan was limited to the mechanism length while an actual scan may use a longer scanning range during which the scanner’s auto-exposure would compensate for the less dense areas above and below the mechanism.
Summary: We found that scanning through the table-headrest connection mechanism resulted in an increased CTDIvol and DLP 28.5% and 31.7%, respectively, compared to scanning through the table top itself (avoiding the connection mechanism altogether). The facility is taking steps to change how they position patients relative to the head rest – table connection mechanism when possible to reduce patient dose.