|  
Return to: home page
| news page
Most MDCT Scanners Fall Short in
Cardiac Imaging 9/3/04
AuntMinnie.com
is pleased to present a guest editorial by two heart imaging experts,
Dr. John Rumberger and Dr. James Ehrlich, on the use of MDCT versus EBT
in coronary CT imaging. If you're interested in writing an alternative
viewpoint, please contact us at editorial@auntminnie.com.
Radiology's enthusiasm for the vast capabilities of multidetector CT technology
has grown exponentially with the increase in detector arrays. In many
cases the excitement is completely justified: The spatial resolution of
new spiral scanners is truly extraordinary, producing strikingly clear
images of stationary organs in the body.
At the same time, the search for new radiologic applications for these
high-throughput scanners has produced a flourishing cottage industry that
was previously the sole domain of cardiologists and cardiac radiologists
-- coronary calcium quantification. Now many general radiology groups,
that may or may not have formal training in cardiovascular CT, offer "calcium
heart score" exams, while others promote more advanced procedures
including CT coronary angiography.
At the risk of throwing cold water on contemporary radiology practice,
it appears to be an appropriate time to re-examine the value and limitations
of the typical MDCT coronary imaging service, as well as the inherent
benefit presumptions and risk that lie too quietly under the surface.
Certainly lay publications and some physicians are questioning a procedure
they believe (incorrectly) to be associated with numerous false-positives,
and that exposes the unwary patient to a radiation dose equivalent to
"hundreds of chest x-rays."
The question that needs to be asked is: Can calcium scoring conducted
on a 4-8-slice spiral CT (the most common scanner in the U.S. installed
base) be justified from a risk/benefit ratio, and can it withstand scrutiny
from outside entities like OSHA, medical task forces, and consumer groups?"
It's not difficult to question the actual benefit of providing MDCT calcium
scoring. One would be hard-pressed to find literature specifically demonstrating
that a spiral CT calcium score predicts coronary events, provides value
beyond office-based risk assessment, or has histopathologic, nuclear medicine,
or angiographic correlations with disease.
On the other hand, there is impressive evidence that electron beam tomography
(EBT) imaging provides an index of plaque burden (via the Agatston calcium
score) that is a powerful predictor of future hard coronary events, while
rampant progression of the calcium volume score on serial examination
has independent and worrisome prognostic significance.
Meanwhile, the so-called science behind MDCT calcium scoring is based
almost completely on the years of peer-reviewed literature validation
from EBT technology, whose superior temporal characteristics are ideally,
by design, suited for cardiac imaging applications. It has become commonplace
for radiology reports to make clinical recommendations (without appropriate
disclosure), borrowing guidelines and patient databases that were published
solely for EBT and Agatston calcium-based data.
Acquisition speed is critical for accuracy of coronary imaging, and MDCT
spirals are too slow to image the fast-moving coronary arteries without
showing motion artifact, especially at heart rates above 70 bpm.
A study by Goldin et al compared 70 asymptomatic patients who were scanned
with "sub-16-slice" spirals and EBT, concluding that "spiral
CT has not proved to be a feasible alternative to electron-beam CT for
coronary artery calcium scoring." There was also concern about high
interscan variability and high interreader variability using MDCT. The
mean interreader variability of 4.5% for EBT, compared to 41.5 % for spiral
CT, prompted the group to suggest double- reading all studies to better
assess coronary calcium scores (Radiology, October 2001, Vol. 22:1, pp.
213-221).
Another study demonstrated that 4- and 8-slice MDCT scanners frequently
mischaracterize true risk, especially when the CAC score is less than
100 (Becker et al, American Journal of Roentgenology, May 2001, Vol. 176:5,
pp. 1295-1298).
On a positive note, it appears that the future of cardiac imaging with
scanners that have 16 slices and higher may be exceptionally bright compared
to those with eight detectors or fewer.
Radiation dose
What about risk? A presentation at the 2003 RSNA meeting revealed the
effective radiation dose to sensitive structures (breast) from an MDCT
heart scan averaged an alarming 3.7 rads. The presenting radiologist,
questioned about the advisability of exposing patients to such doses,
said that a careful individual consideration of the risk/benefit ratio
should be made in each case before presenting the procedure to the patient
("CT screening exams deliver higher incidental radiation dose to
breasts,"AuntMinnie.com, December 4, 2003).
In comparison, the more highly validated EBT heart scan exposes anterior
radiosensitive structures to a fraction of that dose, certainly less than
0.5 rads. If one adheres to the ALARA ("as low as reasonably achievable")
principle in communities where patients have access to both modalities,
the choice of EBT coronary imaging for broad-based screening and clinical
indications would be abundantly clear. The switch to EBT would ensure
a dramatically improved risk/benefit ratio as compared with the spiral
CT alternative.
The journal Progress in Cardiovascular Diseases recently devoted an entire
issue (September/October 2003) to the choice of imaging modalities. For
a concise discussion of the differences between imaging modalities, we
especially recommend Dr. Matt Budoff's article ("Atherosclerosis
Imaging and Calcified Plaque: Coronary Artery Disease Risk Assessment,"
Progress in Cardiovascular Diseases, September/October 2003, Vol. 46:2,
pp. 135-148).
Although radiation concerns are lower in true diagnostic settings, they
are of potential relevance to the practice of CT coronary angiography.
Patients undergoing MDCT coronary angiography will be exposed to a radiation
dose that far exceeds that obtained in a cath lab (conventional angiography).
Most patients will also need beta blockade, which introduces additional
risks. Electron beam angiography is associated with a small fraction of
the radiation dose, and the heart rate does not need to be slowed. The
advent of 64-slice scanners should bring coronary calcium imaging at more
acceptable radiation doses, as the imaging time shortens appreciably.
A good summary of the radiation exposure during coronary heart scans and
CT angiography was published recently in Radiology (January 2003, Vol.
226:1, pp. 145-152).
Technologic considerations aside, radiology groups offering coronary imaging
services should be well versed in the clinical significance of the coronary
calcium score and how test results may alter conventional risk assessments
and therapeutic decisions. Furthermore, the requirements for performing
cardiovascular CT by individuals within a specific radiology group should
be determined, as there will soon be strict accreditation requirements
to practice as a "cardiovascular radiologist."
Since it is clear that a coronary calcium score has the potential to dramatically
modify clinical decisions and positively influence patient lifestyle choices,
we urge radiology groups to work closely with primary care physicians
and cardiologists. The early radiographic identification of coronary disease
provides a wonderfully powerful technique to help the referring physician
set precise LDL cholesterol goals and provide necessary patient risk factor
counseling or reassurance.
Efforts to establish more universal calcium scoring methods, such as "mass
score" (integrated area above background) to define a method that
would ultimately be machine-independent and accounting for issues such
as spatial resolution, field of view, and most importantly, temporal resolution,
should be encouraged. (For a further discussion, see Rumberger JA, Kaufman
L: "A Rosetta Stone for Coronary Calcium Risk Stratification: Agatston,
Volume, and Mass Scores in 11,490 Individuals," AJR 2003; 181: 743-748).
From the viewpoint of experienced EBT plaque imagers who function also
as clinicians, we express cautious optimism about the future of 16-slice
and higher MDCT calcium scanning procedures pending clinical validation
and the development of uniform scanning protocols. Essential to MDCT as
an accepted and useful modality for coronary evaluation will be an increased
focus upon accuracy, reproducibility, radiation dose, and the formation
of clinical standards.
By Dr. John Rumberger and Dr. James Ehrlich
AuntMinnie.com contributing writers
September 3, 2004
|