A new study shows that the radiation emitted by cardiac computed
tomography angiography (CCTA) varies widely, depending on a number
of variables, and that education of CT personnel is required to help
minimize radiation. Dr Jorg Hausleiter (German Heart Centre, Munich,
Germany) reported the findings of the Prospective Multicentre Study
on Radiation Dose Estimates of Cardiac CT Angiography in Daily
Practice (PROTECTION 1) during a late-breaking abstracts session at
the recent American College of Cardiology 57th Annual Scientific
Session.

"For the first time, we have demonstrated that radiation varies
significantly--both by site and by the machine employed. There is
great potential to reduce the dose," Hausleiter told
heartwire. "There are already strategies to reduce the dose, but
these are used very rarely in daily practice, and we need better
education of CT personnel, such as radiologists and cardiologists.
Also, they need to understand that there is no standard, one-size-
fits-all approach--they need to adjust the procedure to the
individual."

However Hausleiter stressed to heartwire that the benefits of CCTA
are still great in terms of the impact that the information obtained
may have on the diagnosis and treatment of cardiac disease. "And
most of the patients we are scanning are 60 years old, and it will
take 10 to 20 years before any cancer might develop from radiation,
so the risk is minimal to them. But obviously, the lower the dose,
the better," although he added this is of most importance in a
younger patient.

Radiation doses "all over the place"

As background, Hausleiter explained that use of CCTA is increasing,
and it is now used widely to rule out coronary artery disease, "but
the downside is that there is radiation associated with the
procedure. However, a lot of people--both physicians and patients--
are not aware of the dose of radiation emitted."

In the study, which was observational and industry-independent, the
researchers examined 50 sites worldwide and analyzed 1965 CT
angiographies performed during one month. The majority (94%) of
machines were 64-slice; the remainder 16-slice. Most were a single-
source configuration, with just one being dual source. They
determined the magnitude of CCTA radiation dose estimates and the
use and efficacy of radiation dose-saving algorithms.

Unexpectedly, the radiation dose estimates differed significantly
between the study sites, ranging from 5.7 mSv to 36.5 mSv, with a
median of 15.4 mSv. "We were very surprised by this. The doses were
all over the place at different sites. Some were 10 mSv and below,
while others were 25 or 30 mSv or higher," Hausleiter said.

Use of dose-saving strategies erratic

Next, they examined a number of dose-saving algorithms. First, they
looked at automatic exposure control, which Hausleiter explains
adjusts the tube current to the anatomy of the patient. They found
that this did not reduce the dose of radiation, something Hausleiter
said he was not surprised by, "because I've done previous work on
this that has shown it does not reduce radiation dose in cardiac
studies."

Second, they looked at so-called "ECG pulsing," whereby radiation is
used at 100% only during diastole and then reduced to 20% to 25% for
the remaining phases of cardiac contraction. Almost 80% of centers
did use this method, and this reduced radiation by 20%, he noted.

Third, the "100-kV" strategy, whereby the tube voltage can be
reduced from 120 kV to 100 kV in nonobese patients, "is actually
very effective, reducing the radiation dose by 50%," Hausleiter
said. But unfortunately, this was used in only 6% of the population
who were scanned. "There were only a few centers aware of it; the
majority were not using it," he noted. He added that only three of
the four manufacturers whose machines were studied actually have
this capability, with Philips being the one that doesn't. They also
studied image quality at 100 kV and found that it was as good as the
image seen at 120 kV.

Finally, they evaluated the sequential scan-dosing algorithm, also
known as "step-and-shoot." Hausleiter explained that this is "an old
methodology that is being reintroduced, and you can get dose
reductions of about 68% compared with conventional spiral imaging."
With spiral imaging, the table is moving all the time at the same
speed while the radiation tube rotates around the body, he noted,
but with the sequential method, the table stops at one position to
acquire images rather than being in continuous motion, so that
radiation is administered only in predefined snapshots during the
cardiac cycle, rather than during the whole cycle.

"We found it reduced the dose very effectively, and the image
quality was as good as in spiral data sets in this observational
study," he noted. But again, this strategy was employed in only 6%
of the population scanned. However Hausleiter points out that "most
of the scanners don't have 'step-and-shoot' capability. It is just
beginning to be installed or upgraded in machines, so it's not
surprising it's been used so rarely."

Other factors play a role, including, surprisingly, the make of
machine

Hausleiter explained that his team then performed a linear
regression analysis to look at individual predictors of radiation
dose and found some other surprises. The length of the scanned
arteries plays a big role, they discovered. "If you look carefully
and try to minimize the scan length rather than scanning the entire
thorax, you can reduce the radiation dose." Also, the length of time
that a center had been performing CCTA had some effect on radiation
dose, too--"it was a tiny effect, but it was significant," he noted.

"But most surprising of all was the tremendous impact of the
different CT systems: four vendors producing five 64-slice systems,"
he noted. After correcting for all other variables in a multivariate
adjustment, they found that some systems have a significantly higher
radiation dose than the others. The Siemens single-source 64 was the
best, emitting the lowest doses of radiation, followed by the
Phillips 64 and the Siemens dual-source 64. The worst two studied
were the Toshiba 64 and the GE 64.

But Hausleiter declined to say which system he would recommend
buying; rather, he commented: "You need to be aware of the
capabilities of the machine and keep looking at the updated systems,
as all the manufacturers have something in the pipeline. This is a
field that is rapidly evolving, and there are always additional
possibilities to reduce the radiation dose."

In conclusion, he noted: "This study will increase the awareness of
the radiation exposure of cardiac CTs. In addition, we think this
study will result in an increased use of already available dose-
saving algorithms, which will substantially lower the radiation
exposure of patients. Finally, it strengthens the need for further
education of cardiologists and radiologists about CT technology to
ensure low radiation exposure to the patient."