Chest: Angiography: Coronary Artery Calcification Quantification
Elliot K. Fishman, M.D.
One of the hottest areas of interest in CT today is the use of the spiral or helical scanner for the quantitative evaluation of coronary artery calcification. Although the study has been around for a number of years it has had limited use due to the fact it required an election beam scanner (Imatron). The new multidetector scanners can also provide this functionality at a cost significantly lower than that of the Imatron scanner. Several recent papers found the results of both scanner systems to be nearly equivalent.
The study requires attention to a strict exam protocol if a successful study is to be obtained. These include:
1.Correct placement of the three EKG leads on the patient.
2.EKG monitoring of the patient’s cardiac rhythm including pulse rate to define the best time for prospective gating.
3.Selection of the correct parameters for scan acquisition so that data is obtained during diastole.
4.Acquisition of a sequence of images through the diastolic phase of the cardiac cycle using subsecond CT scanning.
5.Use of a scoring program to get a specific calcium score with the Angston scale.
The analysis portion of the study is relatively straightforward but must be done carefully in order to ensure that the coronary artery calcium score is both correct and reproducible. Specific parameters on the Siemens Plus-4 Volume Zoom scanner are as follows:
Sequential Scan Acquisition
Single scan time;.36 sec (of a .5 sec rotation)
Cycle time1.5 sec
Collimation2.5 mm
Reconstruction algorithmB30 (medium smooth)
mAs149
kVp120
Check pulse rate to set acquisition timing of gating. If
Pulse= 60 then 50% delay
Pulse = 70 then 40% delay
Pulse = 80 then 35% delay
for faster heart rates (> 90) use T-reverse and a 450 m/sec time
Recent articles have also noted limitations with the reproducibility of the EBCT for coronary scoring. In one recent article scanning the patient twice only seconds apart resulted in scores that were different enough to effect patient management decisions in nearly half the cases. For this reason some sites are obtaining two sets of data and then treating patient based on the higher score. Whether this becomes a standard practice is to be determined.
Although coronary artery calcification is in many ways a relatively straightforward exam that provides a quantitative score, there is increased interest in using MDCT for creating true coronary artery angiograms much like MDCT has done for other areas like the aorta, renal arteries, carotid arteries, etc. Initial work on coronary artery stenosis evaluation has shown promise but obviously additional research will need to be done. Becker et al. has shown that CT is capable of showing soft plaque which is in an exciting development. The development of even faster acquisition times with MDCT as well as the development of better 3D rendering techniques will be needed if this application is to become part of the radiology armamentarium.
Coronary Artery Stenosis Evaluation
Cardiac gating for coronary artery stenosis evaluation protocol:
Heart rate > 80 use pitch 2
< 80 use pitch 1.5
30-40 sec scan time
3 ml/sec 25 sec scan delay
of 120-150 cc (Omnipaque-350)
400 mAs
1.0 thickness
1.0 collimation