General: Multidetector CT Scanning (MDCT): A Primer of Practical Decision Making

Elliot K. Fishman, M.D.

Also see our lectures CT Technology, Principles, & Techniques with an Emphasis on Multidetector CT in our CT Lectures Series section.

Multidetector CT provides the radiologist with unparalleled capabilities for detailed analysis of normal anatomy and pathology. Unlike classic single detector spiral CT where slice selection was determined prior to the study and was fixed, with multidetector CT there is more flexibility as with each individual detector bank a range of possible variable slice thickness can be obtained which can be selected even after the study is completed. Multidetector CT provides potential true isotropic datasets, which may be important in the wide range of clinical applications ranging from 3D imaging to early detection of occult lung masses to vascular imaging. In order to optimally use a multidetector scanner one must be aware of the possible options and potential pitfalls in determining scan protocols. This article provides our experience with multidetector CT through a series of over 100 specific protocols. It should be noted that for many of these protocols we have had extensive experience while for others we have had a bit less experience. One also should be aware that these protocols are subject to change based on our own continuing clinical experience and/or based on the experience in the published literature. We are very excited about the expanding possibilities of multidetector CT and trust that this excitement is also true in your institution.

As with any new technology or advance in imaging there are potential pitfalls and limitations of the technique. One obvious one is that although in fact multidetector CT can scan each patient at 1 mm thick sections and 1 mm intervals, simply doing this across the board is not ideal as it results in very expensive medicine as well as poor patient care by giving an unnecessarily high radiation dose to our patients. Instead, protocols are designed as problem solving tools. We attempt to generate protocols which are the most efficient for study performance, and provide high patient compliance without sacrificing the optimal diagnostic capability of the scanner. Several helpful hints in the use of this protocol guide and to multidetector CT in general are as follows:

1. These protocols match the Siemens Somatom Plus-4 Volume Zoom scanner. Protocols can be extended to other scanners but it is important to recognize the variability of multidetector scanners and adjust protocols accordingly.

2. Many of the protocols are pre-programmed into the Siemens Somatom Volume Zoom scanner. Other protocols, need to be placed in the scanner by either the product specialist or someone on site who is experienced with the scanner. Classically, all abdominal protocols are under abdominal modes, lung protocols are in chest modes and bone protocols are in musculoskeletal protocols.

3. One of the realizations is that with multidetector CT using a pitch of 6 or 8, will result that the time of acquiring a study may be in as little as 10 seconds. Therefore, one must adjust IV contrast injections to take advantage of this rapid scan acquisition. For example, if injection of 2 cc of contrast per second were done with a 120 cc of contrast and of 50 second delay, and 5 mm thick sections were obtained with a pitch of 6, then contrast will still be injected into the patient when the data acquisition phase study was long gone. One of the advantages of this of course is that the logical conclusion is that contrast volumes can be decreased (which is definitely true). At this time we are not certain of the extent of contrast reduction but 20-30% is probably not unreasonable for an estimate.

4. Many of the protocols listed require 3D imaging while others suggest that it can be helpful in select cases. All 3D imaging that we currently do is on the 3D Virtuoso (Siemens Medical Systems, Iselin, NJ) or on 3D virtuoso prototype software. It is important to recognize that the quality of 3D imaging will be dependent on the quality of 3D imaging system if all other factors are held constant.

5. The Siemens Somatom Plus-4 Volume Zoom scanner reconstructs data at a rate of better than 2 slices per second. This very rapid reconstruction time even in the face of large datasets allows the user to take advantage of varying reconstruction algorithms for the same patient. For example, on a CT scan of the chest, one can reconstruct the lung parenchyma with high resolution technique, then reconstruct the soft tissues with a classic soft tissue algorithm and finally reconstruct the bony thorax with a bone algorithm. In this way one does not compromise one part of the study to obtain a better image in a different part of the study.

6. We currently have the choice of three different collimators which are in banks of 4 detectors. They are 1 mm, 2.5 mm, and 5 mm. Although the 5 mm collimation will allow for longest travel time we do not find it necessary in practice to use it. You will find no protocol in this program that uses the 5 mm collimation. Rather our protocols tend to focus on the 2.5 mm and the 1 mm collimation.

7. Although it is possible to reconstruct any dataset at 1 mm intervals certain questions need to be answered before this is done on a routine basis. These questions include the following:

• (a) Will more lesions be detected by reconstructing at 1 mm intervals than at 3 or 5 mm intervals?
• (b) Do we run the risk of missing lesions because there are too many images for the radiologists to review?
• (c) If one does not have a PAC system how do we handle 300-500 slice datasets?
• (f) Can your workstation handle 300-500 slices on a routine basis or does it become prohibitively slow?
• (d) Is your network capable of handling these large datasets on a routine basis without crashing?
• (e) What information will you achieve? We now archive all the initial slices with select images. This is costly both the time as well as in retrieval. It will not be a long term solution in my opinion.

I believe we are reaching the era in CT where the best study we can obtain may not be the study that is always necessary. Therefore, our protocols try to form a reasonable bridge between what is possible and what is necessary. As always our goal is in the best patient care. In CT means the best possible study for detection of disease, accurate determination of the disease process and correct staging of disease. Multidetector CT provides the promise of achieving these needs better. Hopefully this program will help you begin this journey.