Dual-energy cardiac CT postprocessing as easy as a mouse click
Researchers from the Medical University of South Carolina in Charleston are routinely using dual-energy cardiac CT imaging on low- to intermediate-risk patients with chest pain. The approach delivers stenotic data, as well as its hemodynamic relevance.  

They published their experience with 35 patients in the June issue of European Radiology.

  Contrast-enhanced retrospectively ECG-gated dual-energy CT study in a 72-year-old man without prior history of CAD but high cardiac risk. Coronary CTA reconstruction displayed as conventional 3D volume rendering with right coronary artery (arrowheads show the coronary stenosis)
  Same patient, curved multiplanar reformat shows stenosis (arrowhead) of the first diagonal branch due to mainly calcified atherosclerotic plaque.
  Dual-energy CT reconstruction in three-chamber view. Myocardial blood pool deficit can be observed in the antero-apical and infero-basal wall in good correlation with findings at cardiac catheterization.
  Dual-energy CT reconstruction through the left ventricle shown as 3D volume rendering shows infero-basal myocardial ischemia (arrowheads).
Balazs Ruzsics, MD, and colleagues from the divisions of cardiology and radiology retrospectively reviewed data of 35 consecutive patients who had undergone coronary CT angiography using a dual-source CT in dual-energy mode (Definition, Siemens Healthcare).

Dual-energy mode allows two x-ray spectra to be simultaneous applied to the patient.
One x-ray tube was operated with 150 mAs at 140 kV, the second tube with 165 mAs at 80 kV for slim (≤140 lbs) individuals and 165 mAs at 100 kV for average-sized (≤200 lbs) and larger patients.

“The advantage of this method lies mostly in the ability to assess stenoses and their hemodynamic significance with the same scan,” senior author U. Joseph Schoepf, MD, told Cardiovascular Business News.

“We know that the same degree of coronary artery stenosis does not mean the same thing in different patients. Depending on their level of vascular collateralization, the same degree lesion may or may not be hemodynamically significant,” he said. “In addition, accurate stenosis grading is still challenging for coronary CTA but the dual-energy approach enables the assessment of the effect of lesions on the myocardial blood supply on the spot, from a single scan and without additional tests.”

Researchers correlated coronary CTA with SPECT and catheter angiography on a segmental basis. For detection of >50 percent stenosis, they found coronary CTA had:
  • 98 percent sensitivity
  • 88 percent specificity, and
  • 92 percent accuracy.
For detection of myocardial ischemia, dual-energy CT had:
  • 84 percent sensitivity
  • 94 percent specificity, and
  • 92 percent accuracy.
From the single dual-energy CT data acquisition, three different image reconstructions were performed with 0.75-mm reconstructed section width and 0.4-mm reconstruction increment using the routine dual-energy reconstruction algorithm implemented on the scanner platform.

The first set of transverse gray-scale images was aimed at optimizing spatial and contrast resolution by merging 70% of the 140 kV spectrum and 30% of the 80 kV/100 kV spectrum. These data sets were used for clinical coronary CTA interpretation of coronary artery morphology for stenosis detection and grading.

Another set was based only on the low kV (80 kV/100 kV) x-ray spectrum and yet another only on the 140-kV X-ray spectrum.

The myocardial blood pool was analyzed by determining the iodine content within the myocardium based on the unique x-ray absorption characteristics of this element at different kV levels. The resulting color-coded “iodine maps” represent the myocardial
blood pool.

The iodine maps were then superimposed onto grayscale multiplanar reformats of the myocardium in short- and long-axis views, from which the iodine content in the voxels had been digitally subtracted using the “virtual non-contrast” application of the dual-energy reconstruction algorithm. The resulting final images, which provide combined information on cardiac morphology and the myocardial blood pool, were used for the detection of myocardial ischemia based on dual-energy CT.

The average radiation dose in the study population was 15.23 mSv. Schoepf said it was not unreasonably high.

“If you compare this radiation dose to the combination of tests that is required to arrive at the same information, e.g., catheter angiography plus a nuclear study, this is actually less,” he said. “It is also well within the average for a coronary CTA.”

Schoepf was challenged with the fact that recent research has shown that a prospectively ECG-triggered CTA can be performed with as little as 3 mSv, a big difference from the 15 mSv for the dual-energy approach.

“It is correct that one can perform prospectively ECG-triggered CTA with much lower radiation. However, patients often have another test after a positive or equivocal CTA, such as SPECT imaging, which carries a similar radiation burden,” he said.

A dual-energy scan could possibly obviate the radiation from additional tests by triaging low- to intermediate-risk patients to go home, he said.

Schoepf admitted that some patients—those with a negative CTA—will get more radiation with a dual-energy scan than if they had received a prospective ECG-gated exam.

“Overall, I think it would be a wash in terms of radiation, but could incur significant cost savings,” Schoepf said.