Cardiophile MD Archive » Cardiac CT scan

Multi slice CT for coronary artery imaging

Johnson Francis — Sun, 16 May 2010 10:04:52 +0000

Multi slice CT (MSCT) scanners are becoming popular as a non-invasive imaging modality for screening coronary artery disease. The era of MSCT started with 4 slice scanners and progressed to the 64 slice MSCT which is commonly used now. 256 slice and 320 slice CT scanners are also being evaluated for clinical use. One of the difficulties with cardiac CT for imaging coronaries is the constant motion of the heart. This can be partly overcome by using ECG gating of the signals. Conventionally this has been done by retrospective ECG gating. This would mean that CT data is acquired through out the cardiac cycle and only those signals during a particular part of the cardiac cycle, usually a diastolic frame is used for final reconstruction of the coronaries. This would mean a much higher dose of radiation. Now prospective ECG gating is being evaluated in MSCT to reduce the radiation dose so that data acquisition is done only during the selected phase of the cardiac cycle. Prospective ECG gating can reduce the radiation dose up to 90% as compared to retrospective ECG gating. But the quality of images will come down if the heart rate or heart rate variability is high in prospective ECG gating. So also, higher body mass index (BMI) also causes lower image quality. Prospective gating can cause stair step artefacts due to patient motion on the CT table as multiple acquisitions may be needed to cover the z-axis with 64 slice CT. This is avoided in 320 slice CT as the gantry can cover the whole heart in one cycle due to better z-axis coverage. Dual source CT also gives better results.

Imaging of the coronaries is ideally done at lower heart rates and beta blockers are often administered to lower the heart rate during MSCT if the basal heart rate is high. As the number of slices increased, the resolution of the system to pick up more and more details of the coronary anatomy has improved. Main coronary arteries as well as side branches can be visualised well with ECG gated 64 slice CT. 320 slice CT will improve the quality of images further. The whole imaging can be completed in one cardiac cycle, even with prospective ECG gating. This would mean that patients with cardiac arrhythmias are no longer excluded from cardiac CT evaluation of the coronaries.

Cardiac CT image of pulmonary artery bifurcation

Johnson Francis — Fri, 12 Mar 2010 01:57:27 +0000

Cardiac CT scan image at PA bifurcation

Cardiac CT scan image at the level of the bifurcation of the main pulmonary artery (MPA) into right pulmonary artery (RPA) and left pulmonary artery (LPA). Ascending aorta (AAo) and descending aorta (DAo) are seen as two contrast filled circles (cross section) anteriorly and posteriorly. Descending aorta is just to the left and anterior of the vertebra which is seen as a semicircular radio-opaque structure. Two elliptical radioluscent structures seen between the LPA and RPA are the left and right bronchi filled with air. This shows that the level of the section is below the tracheal bifurcation. As you go down, the bronchial images can be seen diverging (not seen in this single image). This is a good view to visualise pulmonary emboli in the main pulmonary artery or its proximal branches in cases of massive pulmonary embolism. Thrombi will be seen as filling defects in this contrast filled image. Aortic aneurysms can also be picked up in this view.

Cardiac CT images of coronary arteries

Johnson Francis — Fri, 12 Mar 2010 01:24:59 +0000

Cardiac CT scan image of LAD and RCA

Cardiac CT scan image of left anterior descending coronary artery (LAD) and right coronary artery (RCA). Ao: aorta. Though the distal LAD appears tapered, it could be due to change in the plane as this is only a tomographic section and not a total angiographic image of the LAD. Proximal RCA is seen well. Only a short portion of the left circumflex coronary artery (LCX) is seen arising from the left main coronary artery (LMCA) [not marked] just beyond the aorta (Ao).

Cardiac CT scan image of LCX

Cardiac CT scan image of left circumflex (LCX) coronary artery in the atrioventricular (AV) groove, seen arising from the aorta (A0). Though only a short segment of the LCX is seen in the previous view, a good extent is seen in this section. Hence caution has to be exerted before committing that a short segment visualised is the stump of an occluded artery as the segments may become invisible when the plane of imaging changes in tomographic images.

Cardiac CT scan image of LM, LCX and RCA

Cardiac CT scan image of left main coronary artery (LM), left circumflex (CX) coronary artery and right coronary artery (RCA). The left main and RCA are seen arising from the left and right coronary sinuses of the aorta (Ao). The non coronary sinus of the aorta from which no coronary artery arises is also seen. These are the sinuses of Valsalva.

Cardiac CT image of right coronary artery

Cardiac CT image of right coronary artery (RCA), seen almost to the full extent, including the bifurcation. Right ventricular branch is also seen. The cross sections of the left ventricle and aorta are seen filled with contrast. A portion of the left coronary system is also seen incompletely on the right side of the image (not marked).

Cardiac CT scan image of LAD and RCA

Normal cardiac CT images: Coronaries on reconstructed views

Johnson Francis — Thu, 11 Mar 2010 17:25:59 +0000

Cardiac CT angiograms are increasing in popularity as a non-invasive screening tool for detecting significant coronary artery disease. The angiograms are reconstructions from 64 or more slice CT scans following intravenous injection of radiocontrast dye. As of now it cannot replace conventional coronary angiograms for assessing the detailed coronary anatomy.

LAD, LCX and Left main on reconstructed cardiac CT

Reconstructed cardiac CT scan image as viewed from the left anterior aspect, showing left main (LM) left main coronary artery, left circumflex (LCX) coronary artery and left anterior descending (LAD) coronary artery. Ao: aorta. Two diagonal branches are also seen arising from the LAD. LCX is occupying the atrioventricular (AV) groove.

LMCA, LAD, LCX and coronary sinus on reconstructed cardiac CT

Reconstructed cardiac CT image as viewed from the posterior aspect showing the left anterior descending (LAD) coronary artery, left main coronary artery (LMCA), left circumflex (LCX) coronary artery and the coronary sinus (Coron sinus). The main tributaries of the coronary sinus are also seen joining it. A diagonal branch is seen arising from the LAD and an obtuse marginal branch from the LCX (not marked).

RCA in the AV groove on reconstructed cardiac CT

Reconstructed cardiac CT image showing the right coronary artery (RCA) in the atrioventricular (AV) groove. The structure to the left of the RCA is the right atrium and that to the left is left ventricle. The left anterior descending (LAD) coronary artery is seen to the left extreme of the image, though it is not visualised well. A diagonal branch of the LAD is also visible.

Distal RCA and coronary sinus on on reconstructed cardiac CT

Distal right coronary artery (RCA) and coronary sinus seen in the atrioventricular (AV) groove seen on the posterior aspect view of reconstructed cardiac CT. Left circumflex coronary is seen adjacent the coronary sinus in the AV groove.

Aneurysm of proximal descending thoracic aorta on X-ray chest PA view and CT scan

Johnson Francis — Tue, 01 Sep 2009 03:11:32 +0000

Aneurysm of proximal descending thoracic aorta on X-ray chest PA view

Click on the image for a larger view

The saccular aortic aneurysm of the proximal descending thoracic aorta is visible within the cardiac silhouette just below the aortic knuckle on the left border. It is partly overlapped by the main pulmonary artery shadow. The cardiac size is normal and so is the asecending aorta.

Aneurysm of proximal descending thoracic aorta on CTscan

CT scan demonstrates the aneurysm very well. The wall of the aneurysm is partly calcified. The aneurysm measured about 5.5 cm and was saccular. There was a past history of major trauma several years back producing multiple fractures.

Dilated aorta on contrast enhanced CT scan

Johnson Francis — Sun, 28 Jun 2009 05:59:54 +0000

Dilated aorta on contrast enhanced CT scan

Dilated ascending aorta seen on contrast enhanced CT scan of thorax in coronal section. Aortic sinuses of Valsalva are also seen. Right brachiocephalic artery arising from the aortic arch is also visible. LV: left ventricle.

Cardiology question answer session 26

Johnson Francis — Mon, 19 Jan 2009 13:10:14 +0000

Electron beam CT vs Multi slice CT

Electron beam computerised tomography (EBCT) and multi detector computerised tomography (MDCT) are the two modalities of CT scans used for evaluating the coronary arteries. EBCT has higher temporal resolution, since there is no gantry movement; electron beam is rotated using magnetic fields. MDCT has higher spatial resolution, but the temporal resolution is lower as more time is required for gantry movement. Multiple detectors, even upto 320 slice MDCT have been developed for clinical use. But the major studies have been conducted only with upto 64 slice MDCT.

Glagov phenomenon

Glagov phenomenon is the positive remodeling of arteries described by Seymour Glagov in 1987. By meticulous serial sectioning of left main coronary arteries in necropsy specimens, Glagov observed that the arterial size is proportional to the plaque burden. The process of enlargement of the artery to accomodate the plaque and maintain the lumen has been called as the Glagov phenomenon. It also known as compensatory enlargement or positive remodeling of the artery.

Calcium scoring of coronary arteries

The scoring can be done with EBCT, while the vessel assessment can be done only with MDCT. The calcium scoring scale is known as Agatston score. A score of 0 means that there is no identifiable calcium.

If there is no calcium there is no obstuctive coronary artery disease (CAD); if calcium score is more than 100 , it predicts obstructive CAD.

Calcium scoring is useful to differentiate ischemic dilated cardiomyopathy from the idiopathic variety. If the calcium score is negative, it is unlikely to be ischemic in origin.

Radiation exposure in cardiac CT angiography

CT angiography has higher radiation dose than conventional coronary angiography, almost 5 times. Radiation exposure is low for EBCT.

Contrast load is also higher with CT angiography.

Scout scan of an implanted pacemaker and lead

Johnson Francis — Wed, 12 Nov 2008 15:07:32 +0000

The initial image obtained during CT scanning is usually called a scout image, pilot or topogram. Sometimes it is also called a scanograph. The scout image is used to determine the exent of slices to be taken during axial scan. The gantry is kept in a fixed position and the table is moved as the x-ray beam is delivered to record the scout scan. The scout image is similar to radiograph in appearance. The image below shows an implanted pacemaker and the lead very well unlike in a conventional radiograph.

Scout scan of an implanted pacemaker with lead

Pacemaker is seen in the right pectoral position, a single chamber device used for ventricular pacing. The lead entering the subclavian vein and tracking down through the superior vena cava into the right atrium and across the tricuspid valve (more or less in the midline) into the right ventricle is seen well. Both the proximal ring electrode and the distal electrode of the lead are seen fairly well. A penetrated view of x ray chest PA view can also show the lead fairly well, but it will darken out the lung fields very much.

64-Slice computed tomography angiography has a high sensitivity and negative predictive value

Johnson Francis — Tue, 21 Oct 2008 02:15:16 +0000

64-Slice computed tomography angiography as diagnostic tool in coronary artery disease has been evaluated in a meta analysis by Mowatt et al (Heart 2008;94:1386-1393). Of the 40 studies identified, 28 had sufficient data for incusion in the meta analysis. The studies included were those comparing 64-Slice computed tomography angiography with standard coronary angiography. The pooled data offered a sensitivity of 99% and specificity of 89% taking 50% or more coronary stenosis as a cut off point. The median positive predictive value across studies was 93% (range 64–100%) and negative predictive value 100% (range 86–100%).

The authors suggest that 64-Slice computed tomography angiography may have a role in the assessment of undiagnosed chest pain when simple non-invasive investigations are non-diagnostic and can rule out significant coronary artery disease in this context.