CT Technology: An Overview of the Latest Menu

Twenty years ago, there was a whiff of obsolescence about CT and the feeling that MRI was inherently superior and would own most CT applications. However, relentless engineering, forged by tenacious competition between the vendors, advanced CT technology.

Coronary imaging has become routine. With wide-detector CT (Toshiba Medical Systems and GE) we can analyze organ perfusion quantitatively and imaging of the heart in a single rotation is a one-stop-shop. We can quantify coronary artery stenosis and match the stenosis with functional imaging. This avoids a nuclear medicine stress test in cases of medium-grade coronary artery stenosis. Coronary calcium score, coronary CTA, and cardiac CT perfusion can be performed with a dose less than 5 mSv, with adenosine or stress/rest protocols. Whole heart imaging is the key to perfusion because of temporal uniformity – ie, the whole organ is in the same contrast time zone.

The dual X-ray tubes improved the temporal resolution, and brought dual energy, or spectral, imaging, which characterizes tissue better than traditional CT. Spectral imaging is achieved in multiple ways - two X-ray tubes (Siemens Healthcare), a single tube with rapid peak kilovoltage (kVp) switching (GE), and a spectral detector (Philips Medical Systems).

Meanwhile, radiation dose has declined because of modulation of kVp and tube current, improved detector efficiency, and iterative reconstruction – whereby an acceptable image is made from fewer photons.

The four major vendors for CT - Philips, Toshiba, GE, and Siemens - do most things well but each vendor has a unique signature.

IQON (Philips Medical Systems)[[{"type":"media","view_mode":"media_crop","fid":"43512","attributes":{"alt":"","class":"media-image media-image-right","id":"media_crop_7820380192627","media_crop_h":"0","media_crop_image_style":"-1","media_crop_instance":"4755","media_crop_rotate":"0","media_crop_scale_h":"0","media_crop_scale_w":"0","media_crop_w":"0","media_crop_x":"0","media_crop_y":"0","style":"float: right; height: 160px; width: 250px;","title":"Spectral Liver Imaging. Confirmed liver metastasis using a spectral iodine overlay versus conventional CT. Images courtesy of Philips.","typeof":"foaf:Image"}}]]
Spectral imaging has two impediments.  Many referrals are not reviewed by radiologists before scanning. Even when spectral imaging is correctly chosen, the radiologist must instruct which mono-energetic or material-specific reconstruction to use. The IQON CT achieves "spectral imaging always on" by utilizing a unique spectral detector which discriminates between photons of different energies in the X-ray beam, thus generating spectral data.

There is no need to correctly protocol spectral imaging before the scan. Technologists need generate only the regular series for interpretation. Should a lesion be present which can benefit from spectral data, this is available via a PACS link to a thin client workstation. Thus, spectral imaging is available prn, at little cost to the workflow and no penalty in dose.

With spectral detectors, there is temporal homogeneity of the spectral data – ie, no time lag between the acquisitions of different energies. The lag is short - 1ms for GE, tens of ms for Siemens and one rotation for Toshiba. The significance of this lag is debatable, but its absence in the IQON is particularly advantageous in decreasing blooming from calcified coronary plaque.

Aquilion One (Toshiba Medical Systems)[[{"type":"media","view_mode":"media_crop","fid":"43513","attributes":{"alt":"","class":"media-image media-image-right","id":"media_crop_7227294354342","media_crop_h":"0","media_crop_image_style":"-1","media_crop_instance":"4756","media_crop_rotate":"0","media_crop_scale_h":"0","media_crop_scale_w":"0","media_crop_w":"0","media_crop_x":"0","media_crop_y":"0","style":"float: right;","title":"Toshiba Aquilion ONE ViSION Edition, Dynamic 4D CT Digital Subtraction Angiography and Brain Perfusion. The inverted maximum intensity projection [MIP] and perfusion maps reveal right middle cerebral artery [MCA] stroke with perfusion abnormality.","typeof":"foaf:Image"}}]]
Toshiba designed the Aquilion One to move from morphologic to physiologic imaging. With 16 cm of coverage from its 320 detectors, it sees the brain, heart, and liver in a single gantry rotation.

Wide coverage helps imaging in suspected stroke. The unenhanced head CT is combined with an angiogram, followed by an assessment of the perfusion, to yield arterial, venous, washout, and delayed images as well as a virtual DSA, using as little as 50 cc of contrast with an effective dose as low as 5 mSv. The whole procedure, including acquisition, reconstructions, and perfusion maps, can be performed in five minutes.

In keeping with the pledge to reduce radiation dose, Toshiba has thoughtful designs. The ratio of iterative to filtered back projection reconstruction can be adjusted to body habitus. In Organ effective modulation the direction of the beam minimizes most dose to the most sensitive organ. For example, in acquiring images through the chest, the tube is posterior and the detectors are anterior. The dose to the breast is significantly reduced by this simple adjustment.

Revolution (GE)
Imaging to plan transcatheter aortic valve replacement (TAVR) is the event horizon for CT technology. There are two requirements. First we need a multiphase acquisition – ie, an acquisition at multiple points in the cardiac cycle - through the left ventricular outflow tract to size the annulus. Second, we need a CT angiogram from the axillary arteries to the femoral arteries to assess the access for the device. There is a penalty of time and in radiation.[[{"type":"media","view_mode":"media_crop","fid":"43514","attributes":{"alt":"","class":"media-image media-image-right","id":"media_crop_5976770457322","media_crop_h":"0","media_crop_image_style":"-1","media_crop_instance":"4757","media_crop_rotate":"0","media_crop_scale_h":"0","media_crop_scale_w":"0","media_crop_w":"0","media_crop_x":"0","media_crop_y":"0","style":"float: right;","title":"With the Revolution, high quality images can be obtained from two phases of the cardiac cycle which are not adjacent to each other. The dose can be minimized in the intervening phases of the cardiac cycle.","typeof":"foaf:Image"}}]]

The Revolution covers 16 cm in one rotation, and its gantry rotates fast, completing a rotation in 280 milliseconds. The scanner interrogates the volume for TAVR so quickly that the gantry may reach the femoral arteries before the contrast. This is a backhand compliment for the scanner.

The Revolution deals well with fast and variable heart rate during coronary imaging. It uses smart auto-gating and smarter motion correction. The scanner’s dose modulation algorithms permit high dose in non-contiguous phases of the cardiac cycle, with reduced dose in the intervening phases. For example, the tube current can be maximal at 25% and 75% of the cardiac cycle – ie, two disparate time points – so that the aortic valve is seen optimally in systole and the coronary arteries are seen well in diastole.

Because of temporal uniformity, high temporal resolution, and efficient gemstone detectors, Revolution particularly excels in imaging of coronary stents for in-stent stenosis.

Force (Siemens Healthcare)
Siemens has taken a different approach to the multi-detector arms race. Instead of increasing the number of detectors, Siemens added another tube orthogonal to the original. When two tubes, at right angles to each other, each rotate quarter a circle, they see what a single tube sees in half a circle of rotation. Dual-source CT immediately improves the temporal resolution by a factor of two. The temporal resolution of the Force is 66 ms.[[{"type":"media","view_mode":"media_crop","fid":"43515","attributes":{"alt":"","class":"media-image media-image-right","id":"media_crop_8188964625840","media_crop_h":"0","media_crop_image_style":"-1","media_crop_instance":"4758","media_crop_rotate":"0","media_crop_scale_h":"0","media_crop_scale_w":"0","media_crop_w":"0","media_crop_x":"0","media_crop_y":"0","style":"float: right; height: 172px; width: 300px;","title":"High pitch imaging enables assessment of the intracranial arteries and coronary arteries in one scan.","typeof":"foaf:Image"}}]]

The Force scans at a high pitch, as high as 3.2. To put it simply, the CT table moves fast, very fast – 737 mm per second. But the engineering feat is not the table speed. The feat is the acquisition of high quality images at such fast speeds. The secret is the Vectron tube, which permits delivery of a lot of power and a lot of milliamperes. The high milliamperes compensate for higher noise at lower kVp, so that decent images can be obtained at as low as 70 kVp. Imaging at lower kVp reduces the amount of contrast needed for angiographic studies.

You’ve heard of ‘triple rule out”. High pitch imaging permits ”quadruple rule out”. This is where you can assess the intracranial arteries, pulmonary arteries, aorta, and coronary arteries – ie, assess for stroke, pulmonary embolism, dissection, and acute coronary syndrome. This may not be clinically indicated but is still a technological marvel. More practically, high pitch imaging scans the chest in less than a second so patients need not hold their breath during the search for pulmonary embolism.

Based on their features, one clinical indication best achieved by each scanner is:

IQON: Composition of a renal stone.

Aquilion One: Acute stroke when thrombolysis is being considered.

Revolution: Pre-planning for TAVR.

Force: Pulmonary embolism in an intubated patient with renal impairment.

Importantly, all four scanners deliver good image quality at substantially lower doses of radiation. Any assessment of net radiation dose to the population from CT needs to account for new technology.