Dual-source CT advances coronary angiography

Dual-source CT represents a significant advance for coronary CT angiography. In our preliminary experience, it has made CTA more robust and provided reliable assessment of vessel wall irregularities and stenoses without motion artifacts. Radiation exposure can be reduced significantly compared with conventional CTA.

DSCT was primarily developed to increase temporal resolution, which is a crucial factor in cardiac imaging. As the coronary arteries move rapidly during most parts of the cardiac cycle, the benchmark for imaging is the ability to freeze these arteries during the small time window of slow motion. In low heart rates, this has been possible with gantry rotation times of 0.3 to 0.5 seconds, which define the temporal resolution of the system. In higher heart rates, however, the diastolic window of time for slow coronary motion, about 150 to 200 msec,¹ diminishes. The diagnostic evaluation of stenoses or vessel wall irregularities becomes unreliable or even impossible due to the increased blurring of the coronary arteries.²

CT system components are exposed to high g-forces, and it has been a major technical challenge to construct a gantry that can mechanically withstand them. To date, no manufacturer has been able to decrease the rotation time beyond 0.33 seconds in commercially available systems. Improving temporal resolution required a new approach: DSCT.

In recent years, developments in tube technology with externally cooled anodes³ have made the combination of two x-ray sources and two detectors in one gantry feasible. This concept is realized in the Siemens DSCT scanner,⁴ making it possible to acquire projection data for one image in a quarter rotation of the gantry. This doubles the temporal resolution at a given gantry rotation speed. As a result, a temporal resolution of 83 msec can be achieved at a gantry rotation time of 0.33 seconds.

We are now applying the theoretical advantages of the DSCT system in a clinical setting. Generally, the higher temporal resolution makes cardiac imaging more robust and improves image quality, especially in patients with irregular heartbeats or high heart rates. It is not necessary to lower the heart rate to below 60 beats per minute with beta blockers as we have done in the past. Instead, we now apply nitroglycerine spray, which has been shown to significantly dilate the coronary arteries.⁵ The reflex tachycardia resulting from vasodilatation is considered a drawback of this approach to coronary CTA, but the increased heart-rate insensitivity of DSCT outweighs this disadvantage, by offering a larger vessel diameter for assessing wall irregularities.

Regarding arrhythmia, in the past it was possible to eliminate ectopic beats by editing the ECG trace. Now the high image quality that can be achieved in the short systolic window of the cardiac cycle makes it possible to obtain diagnostic images even in patients with atrial fibrillation.

In our first clinical study,⁶ we were able to acquire diagnostic images in all 24 patients with heart rates between 44 and 92 bpm. Since then, we have successfully imaged various patients at heart rates over 100 bpm (Figure 1). Our study shows that a reliably high diagnostic image quality can be obtained at 70% of the cardiac cycle in heart rates of up to 70 bpm, while in higher heart rates the best image quality is frequently found in systole (i.e., at 30% to 40% of the cardiac cycle).

We have been able to decrease radiation exposure by reducing the pulsing window for ECG-triggered tube current modulation,⁷ applying the full current only for a duration of about 10% of the cardiac cycle in diastole. In higher heart rates, we set the pulsing window to 30% to 80% of the cardiac cycle, which makes both systolic and diastolic reconstruction possible. Still, using a heart rate-dependent pitch adaptation, the radiation exposure is significantly reduced as compared with conventional CTA, as we have been able to verify in our own dose measurements.

The increased image quality and stable continuity of the depicted vessels over the slabs in the z-axis also improve the performance of postprocessing software. In most data sets, it is now possible to segment the whole coronary artery tree with a single click into the aortic root. The arteries can be evaluated at a glance, and curved multiplanar reconstructions help evaluate vessel wall changes. Quantification of stenoses appears more reliable and correlates better with quantitative analysis of invasive biplanar coronary angiography. Preliminary results from our prospective study demonstrate more reliable results from software-assisted quantification compared with "eyeballing."

UNEXPECTED ADVANTAGES

Static morphological depiction of the coronary arteries is not all that is improved with DSCT. The high temporal resolution makes it possible to visualize the vessel lumen during most of the cardiac cycle. This can be of diagnostic value in intramural segments, or myocardial bridging, of the coronary arteries, which can represent a cause of exercise angina that is frequently missed in conventional coronary angiography. The dynamic visualization of the lumen in this segment can help to assess the severity of compression during systole.

This dynamic information provides a new aspect to functional imaging with coronary CTA. In addition, wall motion is depicted without distortion as a concentric contraction in short-axis slices, which should make the detection of wall motion abnormalities more reliable. With 83 msec temporal resolution, it should also be possible to match the smallest ventricle volume in an end-systolic image exactly, offering a reliable quantification of ejection fraction. Limited temporal resolution and an underestimation of ejection fraction have been drawbacks of functional evaluation in CT, but this important information is now easily assessable and can be quantified routinely from CTA data sets.

DSCT also yields another rather unexpected advantage: very good depiction of the heart valves (Figure 2). Depending on the contrast injection regimen, typically only the blood around the mitral and aortic valve is sufficiently opacified. As a result, the individual leaflets of the valve and their motion during the cardiac cycle can be observed, making it possible to exclude valve stenoses or high-grade insufficiency. In an orthogonal reconstruction (Figure 2B), the exact opening area can be quantified as well, if the pulsing interval is adapted to depict the phase of the largest opening at about 10% with full tube current.

Another advantage of the new system is a protocol specifically designed for chest pain assessment. With this protocol, one tube and detector acquire a normal spiral scan of the chest. The second tube is switched on at the level of the carina to provide the high temporal resolution necessary for coronary imaging. Thus radiation exposure is limited to a minimum, and the whole vasculature of the chest can evaluated for the cause of the chest pain. With a suitable contrast injection regimen,⁸ pulmonary arteries, coronary arteries, and the aorta can be evaluated for pulmonary embolism, coronary artery stenoses, or aortic dissection, with full diagnostic angiographic image quality in a single breath-hold scan at a radiation dose significantly below 10 mSv.⁹

In our experience, DSCT has made coronary CTA more robust and eliminates the need for beta blockers. Reliable assessment of vessel wall irregularities and stenoses is improved by the absence of motion artifacts and facilitates postprocessing. With optimized ECG-triggered tube current modulation, radiation exposure can be reduced greatly compared with conventional CT. The added dynamic information, including wall motion, ventricular function, and valve function, increases the diagnostic value of cardiac CT beyond coronary angiography.

DRS. JOHNSON and NIKOLAOU are radiologists in the department of clinical radiology at Munich University. DR. BECKER is an associate professor of radiology and section chief of computed tomography at Grosshadern Hospital at the University of Munich.

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