Generally speaking, today’s scanners are much more dose efficient than the ones they are replacing. When a site installs a new scanner I like to provide them with an overview of just how their doses changed so they can share their success within the institution.
One of my clients installed a new scanner a few months ago so I prepared my report for them. Their new scanner was imparting doses that were 30% to 80% lower than their previous scanner, depending upon the study group (RPID). That is, with the exception of one study – CT Brain. The doses for CT Brains were about 30% higher on the new scanner than they had been on the scanner they replaced. This seemed counter-intuitive to me so I decided to look into it a little further
My initial review found that CTDIvol was essentially identical between the two scanners. However, Total DLP was approximately 30% higher. The chart below plots both CTDIvol and Total DLP for CT Brain w/o Contrast studies for both the old scanner and the new scanner. Each dot represents a single study, and plots both the study’s CTDIvol and Total DLP. As you can see, the CTDIvol was consistent between scanners. However, Total DLP was markedly higher on the new scanner.
CTDIvol essentially represents base scanner settings while DLP is based on CTDIvol, but then adds a scan length component (hence the name Dose Length Product). Was it possible the protocol or some staff could be setting up the scans so they are scanning further (above and/or below) the desired area on the new scanner than they used to scan on the old one? Were there other factors causing the increase?
To answer these question I ran the DIR’s Exam Detail Report which details the scan parameters of each irradiation event within a study, and includes data such as actual scan length, pitch factors used, kVp and scan rotation time.
The table below represents 3 studies from the old scanner in the top heavy-bordered section, and 3 studies from the new scanner in the lower heavy-bordered section. Each study is grouped using the same color-coding within each section.
The Exam Detail Report confirmed that the old and new scanner used approximately the same CTDIvol per study, but the new scanner’s Total DLP is approximately 30% higher than the scanner it replaced. It also showed that
- The new scanner uses 3 irradiation events where the old scanner used 2 events
- probably two scouts and one study acquisition event;
- Scan length of the acquisition stage is longer on the new scanner;
- Pitch is set to .625 on the new scanner (Spiral) and 1.0 on the old scanner (constant angle);
- kVp is the same on both scanners; and,
- Scan rotation is .75 seconds on the new scanner and 1 sec on the old scanner.
My role is not to recommend specific protocol changes. My role is to help facilities identify potential opportunities for dose reductions and point them in the right direction. To that end, I shared this information with the site so they could work with their radiologists, physicists, and application specialists to determine if there is an opportunity to reduce image dose while maintaining image quality. Perhaps the CT Brain protocol in use is the protocol this site wants and needs. But again, what struck me was that virtually ever other study group had significantly lower doses on the new scanner, except CT Brain. On that fact alone, I felt it was worth reviewing the exam details.
Given the information above, do you see a potential to reduce these doses? If so, where?
Please keep Dose Registry Support Services in mind if you or your facility are looking for a low-cost, staff-friendly, and Joint Commission compliant dose monitoring solution. or more information visit www.doseregistry.com or email me by clicking here.