Dual-source CT

Dual-source CT (DSCT) was designed to overcome the limitation in gantry rotation time by doubling the number of X-ray sources and detectors. By mounting two X ray sources and detector arrays at angle of 90^oon the rotating time. This has the effect of doubling temporal resolution compared to standard MSCT. For example, a dual source scanner with gantry rotation time of 330ms will have a temporal resolution of 83 ms. Several studies have shown the value of DSCT in producing diagnostic image quality in patients with heart rates up to 100bpm,potentially still improved if the heart rate controlled

Dual source data are acquired simultaneously in the same relative phase of a patients cardiac cycle and at the same anatomical level. The two rows detectors in DSCT tend to vary in the entire filed view (FOV; about 50cm), whilst the other is restricted to a smaller central FOV. The radiation burden from DSCT is roughly equivalent to a similar single source MSCT acquisition.

 

Comparison of (a) single –source and (b) dual-source CT. Doubling the number of X ray tube/detector array pairs reduces the arc required to obtain an image to 90⁰. thus improving temporal resolution two-fold

Another important feature of DSCT is that the voltage and current of each x ray can varied individually, allowing dual energy CT (DECT) scans to be performed. This technique uses the phenomenon that tissues attenuate x ray beams of different energies to differing energies to differing degrees, depending on their density. Although not extensively validated, cardiac applications of DECT potentially include digital subtraction of calcium from coronary arteries (although spatial resolution is currently a limiting factor) and characterization of myocardial enhancement patterns.

Why Do Slice Images Offer Higher Contrast?

Conventional film radiography offered a valuable non-invasive means of diagnosis for many decades. Its limitations were and are severe, however we are increasingly less aware of these due to the availability of alternative procedures. Imaging the brain by radiographs, for example, yields insufficient result in most cases. Roentgenologist tried to solve this dilemma by various measures, for example by using contrast media such as air in pneumoencephalography. In spite of the high effort and the great discomfort to patient these procedures only yielded very limited additional information. Computed tomography for first time offered the possibility to image brain structures in diagnostic quality with high contrast. How is this achieved? Several radiogical books offered the explanation that the high contrast of CT images is due to higher dose. I think this explanation is wrong. Although both radiography and CT use X rays, CT images display a quantity different from that in radiographs.

The principle of radiography is to record the radiation which is emitted from the focus of an x ray tube and attenuated by object to be examined with a detector, traditionally by film. This a conventional radiograph present the modulated distribution of radiation intensity and always offers a superposition image: all structure along the rays forms x ray focus to detector, i.e all volume element passed by any ray, contribute display the sum of all contribution to attenuation or,, mathematically speaking, the integral of attenuation component along time. We will return to these “line integrals” again when discussing the principle of computed tomography in next post.