Clinical Techniques of Cardiac Computed Tomography

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CHAPTER 9 Clinical Techniques of Cardiac Computed Tomography

Over the last decade, cardiac computed tomography (CT), and in particular coronary CT angiography (CTA), has undergone significant technical improvements, resulting in a reliable ability to evaluate coronary arteries with a noninvasive technique. There are still many questions to be answered with respect to how this test will be used in specific patient populations and the impact it will have, not only on patient management but also on patient outcomes. This chapter is meant to be an overview of the technical considerations for performing a clinical cardiac CT scan in a routine daily clinical practice.

PATIENT PREPARATION

The patient should be kept NPO for at least 4 hours prior to the scan as a precaution, as is standard for all contrast CT examinations, because of the potential for nausea and vomiting associated with contrast administration. It is extremely important that the patient not consume caffeine for at least 12 hours prior to the examination; this helps with heart rate control, which is critical for optimal image quality. With the newer generation of scanners having improved temporal resolution, high-quality images can be obtained with higher heart rates.

Electrocardiogram (ECG) leads need to be attached, which are used by the scanner to determine data reconstruction and electrocardiographic tube modulation, if used. In addition, this allows monitoring of the patient for absolute heart rate, heart rate variability, and arrhythmias, which are critical in determining the optimum protocol. A good electrocardiographic tracing (Fig. 9-1) is required, especially if using electrocardiographic tube modulation for overall dose reduction, because the x-ray tube modulation is synchronized with the cardiac cycle, using the R wave as a reference. It is important to ensure that the wire leads and ECG patches are outside the scanning field as much as possible, and in particular to ensure that the wires are not coiled on the patient’s chest, which can result in significant streak artifact. For someone who is very hairy, good adherence and electrical conductivity of the pads may require shaving a small area. In particular, noisy electrocardiographic tracings (Fig. 9-2) can lead to inappropriate modulation of the tube current, resulting in a poor signal-to-noise ratio at the specific phase of the cardiac cycle required for coronary evaluation. In addition, be sure that the R wave peak is of greater amplitude compared with the T wave to prevent tube current modulation synchronized to the wrong phase of the cardiac cycle. Some scanners allow switching of the lead at the scanner console; if not, manual switching of the lead attachments to the ECG patches on the patient can often correct a situation in which the T wave has a higher peak.

If the patient’s baseline heart rate is above an acceptable limit for optimal image quality, the use of β blockers is recommended, provided no contraindications (e.g., asthma, aortic stenosis) are present.1 This acceptable limit is increasing with the latest generation of scanners. However, with the current generation of 64-slice scanners, a heart rate below 65 beats/min is highly recommended to increase the likelihood of an optimal high-quality scan with the least amount of motion artifact. There are multiple protocols for the administration of β blockers. The two basic protocols include the use of oral administration, both the night before and the morning of the scan, or just the morning of the scan, versus intravenous (IV) administration just prior to the scan.1 In general, oral administration seems to result in a more predictable result, and it also requires less time and personnel commitment from the support staff. However, IV administration can be effective if the patient did not have prior administration of an oral β blocker or the target heart rate was not achieved after the administration of an oral dose. IV administration tends to have an all or none effect, meaning that usually if there is a noticeable decrease in heart rate with the first dose administered, a desired heart rate can usually be achieved. However, in many patients, the maximum dose (25 mg metoprolol at our institution) is administered without a noticeable change in heart rate. If the patient’s heart rate does not reach the target, then the scan can be obtained with the understanding that it will likely be suboptimal, with the potential for nonevaluable segments of the coronary arteries, or the patient can be rescheduled with more aggressive oral β blocker administration prior to the scan.

If the patient has severe arrhythmias, then the patient is generally not a good candidate for cardiac CT and requires management of the arrhythmia prior to a coronary CTA. Usually, this is the obligation of the referring physician unless the performing physician is comfortable with antiarrhythmic medications.

A perfect breath-hold is critical for coronary imaging in particular. This requires the patient to have a steady breath-hold throughout the entire scan and needs to be relaxed—that is, no Valsalva maneuver. Therefore, the patient is coached on the breath-hold technique, which is practiced at least twice with the patient prior to the scan. The patient is instructed to take a breath in and hold the breath, with additional instructions during the breath-hold to be relaxed, not to bear down, and not to let any air out or take any air in once the breath is held. The breath-hold is practiced for approximately 5 seconds longer than the time required for the actual scan being performed.

A baseline heart rate and blood pressure are documented. Then, a brief history is obtained from the patient, including contraindications to the medications that will be administered for the examination, which may include a beta blocker and nitroglycerin. The patient is questioned about history of asthma, heart failure, and heart murmur with respect to β blockers and erectile dysfunction medications, and sildenafil with respect to nitroglycerin. Nitroglycerin is administered to dilate the coronary arteries for optimal visualization.2 If there are no contraindications, the medications to be administered for the examination include up to 25 mg of metoprolol IV, in 5-mg push increments, and two 0.4-µg sublingual tablets. The onset of action for a sublingual nitroglycerin tablet is 1 to 3 minutes; it reaches maximum effect at 5 minutes, lasting at least 25 minutes. Therefore, these will be administered just prior to the scout and coronary CTA will be performed at approximately 5 to 6 minute after administration.

Additional history, which is collected while administering the β blocker, includes any symptoms, as well as their character and length of duration. Also, risk factors, including smoking, diabetes, lipid profile, hypertension, and family history, are documented. If additional testing has already been performed, these data are collected. especially if the previous testing led to the performance of cardiac CT.

SCANNING TECHNIQUE

The scan can be performed using retrospective electrocardiographic gating or a prospective electrocardiographic triggering (step and shoot) technique. With the retrospective electrocardiographic gating technique, the scan is performed using a standard helical scan acquisition, as for routine chest or abdominal CT scans, except a much smaller pitch, usually around 0.2, is used to ensure adequate data acquisition at all anatomic locations throughout the entire cardiac cycle. At the same time that the helical CT scan is obtained, the electrocardiographic tracing from the patient is recorded. Unless electrocardiographic tube modulation is used to reduce radiation exposure, the electrocardiographic tracing has no effect on the helical CT scan at the time of data acquisition. Rather, the tracing will be used to reconstruct axial source images from the raw data during the predefined portion of the cardiac cycle chosen after the scan. The tracing is used as a reference to determine which data at a specific anatomic location were collected during the defined phase of the cardiac cycle.