Multislice CT scanning (MSCT) has and will continue to open new opportunities to see the heart and its surrounding tissue in greater depth and detail. Recent advances in 16 and 64 slice CT technology mean you can acquire several thin slices with a single rotation of the gantry. The introduction of ECG synchronised CT scanning of the heart means that the whole procedure can be done in a single breath hold (total scanning times of 10-12secs). Most importantly this means that rapid scanning ‘freezes’ the heart.
Prior to fast scanning techniques in CT, the heart was very difficult to visualise other than with conventional angiography. Electron beam computed scanners (EBCT) were first used to image the heart in 1979.Until recently; EBCT was commonly used to detect plaque in the coronary arteries and therefore was the gold standard for Coronary artery calcification (CAC)(1-6)
However cardiac optimised CT scanners now have higher temporal and spatial resolution, which allows us to see small structures without motion artefact. Ultimately it gives us better actualisation and visualisation of the vessels of the heart, surrounding vessels and coronary anatomy. With further advancement of new technologies, post-processing packages have become available. This has meant the visualisaton of anatomy can be done in several planes. The manipulation of the acquired data allows us to show detail in several formats. E.g. curved reformats, multiplanar reconstruction, volume rendered images etc.
The use of specially adapted cardiac CT techniques allows you to visualising the heart and surrounding vessels from adjacent structures, which can be difficult to differentiate as they all lie very close to each other.
CT cardiac imaging can be performed with or without the use of non-ionic contrast (x-ray dye) depending on the clinical indications and diagnosis required. Simply put, non-ionic contrast is used within the body to highlight/opacify relevant structures.
The use of contrast to delineate structures in and around the heart is already used in the catheter labs whilst performing coronary angiography.
When utilised properly, MSCT can aid in the reliable diagnosis and exclusion of coronary artery disease (CAD). With one cardiac CT scan it is possible to determine causes of unexplained chest pain and shortness of breath. (E.g. Heart attack, pulmonary emboli, and aortic dissection –‘triple rule out’).
The most common non-invasive cardiac imaging procedure carried out in CT scanning departments is a calcium scoring CT scan. The images produced are used to assess the calcium in coronary arteries. This technique can identify non-calcified (soft) and calcified plaques. The calcium that is detected in all the coronary arteries is reported as a “calcium score”. The higher the calcium score, the higher your chance of having a heart attack or needing coronary bypass surgery, unless you are treated appropriately with medications.
To date CT cardiac imaging and CT angiography (CTA) of the heart is clinically being used for the following examinations:
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Coronary artery imaging, using CTA, looking at the vessels surrounding the heart
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Functional assessment of the heart, assessing the state of the chambers
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Coronary artery stenosis as explained above
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Post stent visualization
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Graft patency post coronary artery by-bass surgery (CABG)
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Detection of coronary artery anomalies: - cardiac mass and congenital heart disease (CHD)
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Aneurysms
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Pulmonary vein mapping
Using CT as a diagnostic tool to aid diagnosis in cardiac cases has the advantage that:
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It is a cheaper diagnostic tool
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It is quick and non-invasive compared to standard coronary angiography
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Has a lower risk than conventional angiography
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Can help in earlier detection of disease
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Can be used to visualise anatomy in 3D and 4D
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Requires less or no time in hospital therefore frees up vital bed spaces
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Frees up and makes available other diagnostic services
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Can be cost effective compared to 2/3 investigations required to make a diagnosis.
However as with all great advances in technology there are disadvantages, and the use of cardiac CT as a diagnostic tool has quite a few!
We still need to be aware of the importance that there is a radiation dose involved. Cardiac MRI does not use radiation to acquire images.
Cardiac MRI of the heart still produces superior images. But cardiac MRI imaging in itself is very long procedure and is still a very expensive diagnostic scanning tool. The confined space and constant ‘noise’ in an MRI scanner can make the whole procedure uncomfortable for the patient. As mentioned before, multislice CT on the other hand can be a very fast cost effective imaging tool.
The time taken to perform a cardiac CT can take up to 20 minutes compared to an MRI scan that can take up to 1.5 hrs.
Disadvantages of Cardiac CT:
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A non compliant patient, leads to movement artefact
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Lack of IV access makes the CT procedure difficult to interpret
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CTA is unsuitable when there is a large amount of existing coronary artery calcification
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Arrhythmias and unusually high heart rates (>85 beta blocker may be administered)
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Artefact due to pulsation from blood flow in vessels and the chambers of the heart.
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Respiratory artefact
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Contrast induced artefacts
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Implant/metal artefacts
Currently at Harefield Hospital cardiac CT is not routinely performed. However, we have a one-stop GP referral clinic in operation for patients with non-urgent chest pain. Patients are routinely referred for CT calcium scoring if clinically indicated e.g. strong family, history diabetics, all men over 40 and all women over 50 years of age. Occasionally we are asked by the Cardiologist to perform CTA on difficult Cath lab patients when the Cardiologist is unable to locate the origin of an artery and more often for graft studies and to clarify anomalies.
However as we are due to have a 64-slice Toshiba scanner (Aquilion) installed in early January 2007, we hope to offer specialised Cardiac CT scans as a part of the service for patients referred to Harefield.
Future of Multislice CT Scanning in Cardiac imaging.
As the use of Multislice CT in cardiac becomes more frequent, it is apparent that we need to be more aware of coincidental findings that may be malignant and non-malignant In some cases we may also find pulmonary nodules, aortic aneurysms, acute dissections, pneumonia. PE, non-urgent findings can include benign liver/adrenals lesions emphysema. Therefore we all need to look beyond the area of our interest.
With faster scanning times and the imminent release of 256-technology and dual source technology Cardiac CT is evolving into it own. The advantages of using Coronary artery CT are growing with advances in multislice scanning.
References:
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Mautner, G.C., et al., Coronary artery calcification: assessment with electron beam CT and 8histomorphometric correlation. Radiology, 1994. 192(3): p. 619-23.
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Fallavollita, J.A., et al., Fast computed tomography detection of coronary calcification in the diagnosis of coronary artery disease. Comparison with angiography in patients < 50 years old. Circulation, 1994. 89(1): p. 285-90.
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Wexler, L., et al., Coronary artery calcification: pathophysiology, epidemiology, imaging methods, and clinical implications. A statement for health professionals from the American Heart Association. Writing Group. Circulation, 1996. 94(5): p. 1175-92.
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Kaufmann, R.B., et al., Detection of heart calcification with electron beam CT: interobserver and intraobserver reliability for scoring quantification. Radiology, 1994. 190(2): p. 347-52.
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Becker, C.R., et al., Helical and single-slice conventional CT versus electron beam CT for the quantification of coronary artery calcification. AJR Am J Roentgenol, 2000. 174(2): p. 543-7.
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Broderick, L.S., et al., Measurement of coronary artery calcium with dual-slice helical CT compared with coronary angiography: evaluation of CT scoring methods, interobserver variations, and reproducibility. AJR Am J Roentgenol, 1996. 167(2): p. 439-44.
