In which one of the following is ECG-gated coronary CT angiography not indicated?
Coronary CT angiography has a very high (>99%) negative predictive value and therefore is an excellent tool for the exclusion of significant (>50% stenosis) coronary artery disease in patients with a low to intermediate risk of coronary artery disease. Patients with a high pre-test probability of coronary artery disease often have a high coronary artery calcium burden which reduces the accuracy of CT angiography. This patient group is more likely to require coronary angiography. Coronary CT angiography gives excellent anatomical delineation of the origins and course of the coronary arteries and therefore is extremely useful in the three-dimensional assessment of anomalous coronary arteries, including in the setting of failed intubation at catheter angiography. Grafts may be occluded or difficult to intubate at catheter angiography, particularly when there is uncertainty over the details of the initial operation. Pre-procedural planning using CT may be considered. The accuracy of graft stenosis detection compared with catheter angiography is high. Ollendorf D, Kuba M, Pearson S. The diagnostic Performance of Multi-slice Coronary Computed Tomographic Angiography: a Systematic Review. Journal of General Internal Medicine 2010; 1–10. Stein P, Yaekoub A, Matta F, Sostman H. 64-Slice CT for Diagnosis of Coronary Artery Disease: A Systematic Review. The American Journal of Medicine 2008; 121: 715–25. Abdulla J, Asferg C, Kofoed KF. Prognostic value of absence or presence of coronary artery disease determined by 64-slice computed tomography coronary angiography. A systematic review and metaanalysis. International Journal of Cardiovascular Imaging 2010; 1–8.
Which one of the following patient characteristics is ideal for performance of a good quality coronary CT angiogram?
Many patient characteristics have an effect on the quality of a CT coronary angiogram. A slow (<65 bpm) regular heart rate is optimal for cardiac CT as a slow rate increases the length of diastole and thus increases the time when the heart is relatively still in which image acquisition can be performed. A regular heart rate with low respiratory variation is also important to minimize reconstruction or ‘step’ artefacts caused by ectopic or irregular beats. Thus, even with a slow ventricular rate, atrial fibrillation may significantly degrade image quality. In view of the need for a slow heart rate, a contraindication to beta-blockers is not ideal, as this is the main pharmacological technique employed in cardiac CT for heart rate control; calcium-channel antagonists with a negatively chronotropic effect may be used as an alternative. Recent work on the use of sinus node blockers is under investigation. Depending on the exact specification of the CT scanner and the field of view of image acquisition, the breath-hold time is 5–10 seconds. Thus if a patient can only breath-hold for 2 seconds, there is likely to be motion (respiratory artefact) which will degrade the image quality.
A cardiac CT is performed with the patient’s arms extended above the head to ensure that there is no ‘beam-hardening’ artefact from the arms which reduces image quality.
Concerning heart rate in cardiac CT, which of the following statements is false?
A heart rate of ≤65 bpm is ideal for cardiac CT (see Answer 2).Beta-blockers are the main pharmacological agents used for heart rate control in the context of cardiac CT, and metoprolol is commonly used. A well-recognized protocol involves 50–100 mg oral metoprolol 2 hours prior to the scan. If this is insufficient, further metoprolol can be given intravenously on the table immediately prior to the scan, given its short onset and duration of action. Typically, 5 mg IV is given slowly, with pulse and blood pressure monitoring. This can be repeated with careful titration to an optimal heart rate. Oral metoprolol on the table is not useful given its long onset of action. Therefore it is ideal if patients referred for cardiac CT are optimally prepared with cardiology advice. Contrast-induced nephropathy is a major cause of hospital-acquired acute renal failure, and its risk is significantly increased in patients with diabetes mellitus. The standard non-ionic low-osmolar contrast agents used routinely in contrast-enhanced CT are much safer and have fewer side effects than ionic low-osmolar agents. In addition, it has been reported that newer non-ionic iso-osmolar agents have less tachycardic and arrhythmic effects.
Concerning ionizing radiation in cardiac CT, which one of the following statements is true?
Historically, cardiac CT has been recognized as a high-radiation investigation and this has been cited as an adverse reason for considering its routine use. Whilst the dose can be high, careful optimization of scanning parameters with aggressive ECG-gated dose-modulation techniques can result in cardiac CT being performed with a dose of <1 mSv.
Calcium scoring is a relatively low-dose study (currently <0.5 mSv) and is lower than a CT coronary angiogram.
Techniques to limit the radiation dose include reducing the kilovoltage (dose is proportional to kV2 ) and limiting the scanned field of view. Prospective gating, in which the scan is limited to a fixed segment of the cardiac cycle (usually mid to late diastole), will also reduce the radiation dose compared with retrospective scanning, which acquires data (and therefore administers radiation) throughout the cardiac cycle. If a retrospective method of scanning is used, the dose can be minimized by using of ECG-gated dose modulation, with only a fraction of the maximal tube current (e.g. 4% or 20%) administered outside the useful diastolic reconstruction window.
The dose administered will increase with increasing BMI, as a higher kilovoltage (kV) and tube current (mA s) are required to penetrate an increased depth of tissue and maintain an adequate signal-to-noise ratio (good image quality).
Concerning coronary artery calcification, which one of the following statements is false?
Coronary artery calcium scoring (CACS) was initially investigated using electron beam CT. Several scoring systems were devised, the most widely used being the Agatston score. This combines assessment of the volume and the density or mass of coronary calcium. Studies have shown that total coronary calcium is a good independent predictor of future cardiac events. The presence of coronary calcium confirms the presence of atheroma and correlates with the total plaque burden but does not always correlate with the location of a stenosis.
A coronary calcium score of zero is associated with a <1% risk of future coronary events. However, flow-limiting non-calcified atheroma is clearly not excluded and therefore clinical correlation is necessary.
A high coronary calcium score will decrease the negative predictive value and overall accuracy of a coronary CT angiogram, as calcium results in partial volume or blooming artefact in which dense structures can appear larger than their true size. This impairs visualization of the adjacent coronary lumen and can lead to an over- or underestimation of coronary stenosis when compared with catheter angiography.
Therefore CACS can be used as an important adjunct to the Framingham risk assessment and thereby significantly increase or decrease this risk.
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