Documented mean effective radiation dose for coronary CTA ranges from 6 to 25 mSv ³ and reported mean effective dose for conventional catheter angiography is 5.6 mSv.⁴ Further improvements in CT technology continue to reduce effective dose but there must be constant awareness with attempt to minimize dose wherever possible. Tailored coronary CTA is well-documented to have a high negative predictive value for coronary artery stenosis.^5,⁶

Indications

1. Patients with known or suspected chronic coronary artery disease.

2. As a screening test in asymptomatic high-risk patients or patients with atypical chest pain.

3. As an alternative to diagnostic coronary angiography when planning percutaneous angiographic intervention or as post-procedure follow-up.

4. Visualization of coronary artery grafts, including the entire course of the left internal mammary artery (LIMA) graft to its distal anastomosis on the coronary artery.

Technique

The exact technique will depend on individual CT scanner technology and requires a multidetector CT which is 16 slice or above. Each CT manufacturer will advise scan protocols tailored to their specific scanners; however, general parameters useful for coronary artery assessment cardiac CT are as follows:⁷

1. For optimal imaging, the patient should have a regular heart rate of 50–65 beats per minute during the scan. Either oral or intravenous (i.v.) beta-blockers are often given to slow the heart rate before scanning. ECG monitoring with clear QRS complex is required. Radiation dose can be reduced by cardiac gating with prospective ECG triggering and radiation dose modulation by decreasing X-ray tube output during systole – only data acquired during diastole are used for evaluation of coronary arteries.⁸

2. Volume of contrast medium – 95–100 ml LOCM 350–400 mgI/ml followed by saline chase of 50 ml through cannula in right antecubital vein. (The saline chase reduces streak artifact from dense contrast in the right side of the heart, whilst the right-sided cannulation reduces streak artifact from the left subclavian artery or internal mammary coronary graft.) An initial unenhanced scan is sometimes used to assess the coronary artery calcium score, provide landmarks for planning of the contrast run and allow the patient to become accustomed to the scan.

3. Delay:

a Preset delay of 18–20 s

OR, preferably,

b Bolus tracking using software supplied with the multidetector scanner. A ROI (region of interest) is positioned over the ascending aorta. After commencing the injection a ‘tracker scan’ monitors the hounsfield level at the ROI and the scan is then triggered when the density at the ROI reaches a preset value, e.g.150 HU

4. Rate of injection – LOCM 400 at 5 ml s⁻¹ or LOCM 350 at 6 ml s⁻¹

5. Collimator/reconstruction width (mm) – 0.6 to 0.75/0.75 to 1 as per available CT technology ⁹

6. Rotation time/tube voltage – fastest possible with available scanner technology/120 kV

7. Scan direction and extent – cranio-caudal from tracheal bifurcation to 1 cm below heart

8. Image reconstruction, review and post-processing – images can be reconstructed at various times through the cardiac cycle from the R–R peak, usually expressed as percentage through the length of the cycle. Image review using axial scans, multiplanar reconstructions (MPR), curved MPRs, volume rendering and surface shaded display all useful to analyse the vessel pathway, wall and caliber.

References

1 Weinreb J.C., Larson P.A., Woodard P.K., et al. American College of Radiology clinical statement on noninvasive cardiac imaging. Radiology. 2005;235:723-727.

2 Preis S.R., O’Donnell C.J. Coronary heart disease risk assessment by traditional risk factors and newer subclinical disease imaging: is a ‘one-size-fits-all’ approach the best option? Editorial. Arch. Intern. Med.. 2007;167(22):2399-2401.

3 Mahesh M., Cody D.D. AAPM/RSNA physics tutorial for residents: physics of cardiac imaging with multiple-row detector CT. RadioGraphics. 2007;27:1495-1509.

4 Coles D.R., Smail M.A., Negus I.S., et al. Comparison of radiation doses from multislice computed tomography coronary angiography and conventional diagnostic angiography. J. Am. Coll. Cardiol.. 2006;47:1840-1845.

5 Abdulla J., Abildstrom S.Z., Gotzsche O., et al. 64-multislice detector computed tomography coronary angiography as potential alternative to conventional coronary angiography: a systematic review and meta-analysis. Eur. Heart J.. 2007;28(24):3042-3050.

6 Nieman K., Cademartiri F., Lemos P.A., et al. Reliable noninvasive coronary angiography with fast submillimeter multislice spiral computed tomography. Circulation. 2002;106:2051-2054.

7 Schoepf U.J., Zwerner P.L., Savino G., et al. Coronary CT angiography: how I do it. Radiology. 2007;244:48-63.

8 Earls J.P., Berman E.L., Urban B.A., et al. Prospectively gated transverse coronary CT angiography versus retrospectively gated helical technique: improved image quality and reduced radiation dose. Radiology. 2008;246:742-753.

9 Pugliese F., Mollet N.R., Hunink M.G., et al. Diagnostic performance of coronary CT angiography by using different generations of multisection scanners: single center experience. Radiology. 2008;246:384-393.

Contraindications

Significant arrhythmias may make gated blood-pool imaging impossible.

Radiopharmaceuticals

^99mTechnetium-in-vivo-labelled red blood cells, 800 MBq max (8 mSv ED).

Before radiolabelling with ^99mTechnetium (^99mTc)-pertechnetate, the red blood cells are ‘primed’ with an injection of stannous pyrophosphate. The stannous ions reduce the pertechnetate and allow it to bind to the pyrophosphate which adsorbs on to the red blood cells.

Equipment

1. Gamma-camera: equipped with low-energy general purpose collimator

2. Imaging computer: list-mode or multi-gated acquisition (MUGA)

3. ECG monitor with gating pulse output.

Patient preparation

1. An i.v. injection of ‘cold’ stannous pyrophosphate (20 μg kg⁻¹) is given directly into a vein 20–30 min before the pertechnetate injection. (Avoid injection via long plastic Teflon coated cannula which may result in a poor label.)

2. Three ECG electrodes are placed in standard positions to give a gating signal.

Technique

Gated blood-pool study

1. Patient supine

2. The ECG trigger signal is connected

3. I.v. injection of ^99mTc pertechnetate

4. One minute is allowed for the bolus to equilibrate before computer acquisition is commenced (see ‘Films’ below).

List mode is best, where individual events are stored as their x, y coordinates along with timing and gating pulses. This allows maximum flexibility for later manipulation and framing of data. Around 5 million counts should be acquired. However, MUGA mode is adequate, and is still the most commonly used. In this, the start of an acquisition cycle is usually triggered by the R wave of the patient’s ECG. A series of 16–32 fast frames are recorded before the next R wave occurs. Each of these has very few counts in from a single cycle, so every time the R wave trigger arrives another set of frames is recorded and summed with the first. The sequence continues until 100–200 kilo-counts per frame have been acquired in about 10 min. Some degree of arrhythmia can be tolerated using the technique of ‘buffered bad beat rejection’, where cardiac cycles of irregular length are rejected and the data not included in the images. The length of time to acquire an image set increases as the proportion of rejected beats rises.

First-pass radionuclide angiography

This provides information on right ventricular function and intra-cardiac shunts, although only from one view unless a multi-headed camera or bi-planar collimator is available:

1. Positioning depends upon the clinical question. For ventricular function evaluation, the patient lies supine with the camera against the chest in the RAO 30° position for best visualization of the right atrium and ventricle, or the LAO 35–45° position for best visualization of the left ventricle. A caudal tilt of 15–30° may improve separation of the ventricles. For assessment of shunting, the camera is positioned anteriorly.

2. The ECG trigger signal is connected.

3. The validity of the first-pass study is dependent on the quality of the bolus injection. For RVEF assessment, the ^99mTc-pertechnetate is injected over approximately 3 s (too tight a bolus will not provide sufficient cardiac cycles for evaluation; too slow an injection will result in poor image statistics before circulation through the lung capillary bed and left side of the heart). However, for shunt quantification, ^99mTc-pertechnetate is injected in as tight a bolus as possible by using the Oldendorf or similar technique (see Chapter 1).

4. Gated list-mode computer acquisition is started as the bolus is released. Although the first pass usually takes a maximum of 10–15 s, it is advisable to continue data acquisition for up to 50 s in case of slow bolus arrival. (If list mode is not available, a short-frame dynamic study with 20 frames s⁻¹ or faster on a 64×64 matrix may be used, although it is unlikely that gating information will be able to be stored, so calculation of RVEF will be sub-optimal. For shunt assessment, a rate of 2 frames s⁻¹ is reasonable.)

5. A gated blood-pool study follows, as above.

Images

Gated blood-pool study

A number of views may be recorded, depending on the clinical problem:

1. LAO 35–45° with a 15–30° caudal tilt, chosen to give best separation between left and right ventricles. Patient supine. This view is sufficient if only LVEF is required

2. Anterior, patient supine

3. LPO, chosen to give best separation between atria and ventricles. Patient in right lateral decubitus position.

Additional techniques

1. Gated blood-pool imaging can be carried out during controlled exercise with appropriate precautions to assess ventricular functional reserve. Leg exercise using a bed-mounted bicycle ergometer is the method of choice. Shoulder restraints and hand grips help to reduce upper body movement during imaging. For patients unable to exercise effectively, stress with dobutamine infusion is used.⁴ Under continuous monitoring, the dose is incrementally increased from 5 to 20 μg kg⁻¹ min⁻¹, infusing each dose for 3 min. The infusion is stopped when S-T segment depression of >3 mm, any ventricular arrhythmia, systolic blood pressure >220 mmHg, attainment of maximum heart rate, or any side-effects occur

2. Gated single photon emission computed tomography (SPECT) blood pool acquisition can be used for measurement of left as well as the right ventricular function using special software (Autoquant+) which calculates the left and right ventricular volumes

3. With gated SPECT using the myocardial perfusion imaging agents ⁹⁹Tc^m-MIBI and ⁹⁹Tc^m-tetrofosmin (see ‘Radionuclide myocardial perfusion imaging’), it is possible to combine ventriculography and perfusion scans in a single study.^5,⁶

When acquiring gated SPECT studies ventricular parameters are usually measured during rest study only because of the length of the examination (20 min).

Aftercare

1. Monitor recovery from exercise

2. Normal radiation safety precautions (see Chapter 1).

Complications

Complications are related to the exercise test and include induction of angina, cardiac arrhythmias and cardiac arrest.

References

1 Metcalfe M.J. Radionuclide ventriculography. In: Sharp P.F., Gemmell H.G., Murray A.D., editors. Practical Nuclear Medicine. London: Springer, 2005.

2 Friedman J.D., Berman D.S., Borges-Neto S., et al. First-pass radionuclide angiography. J. Nucl. Cardiol.. 2006;13(6):e42-e55.

3 Williams K.A. Measurement of ventricular function with scintigraphic techniques: part 1 imaging hardware, radiopharmaceuticals, and first-pass radionuclide angiography. J. Nucl. Cardiol.. 2005;12(1):86-95.

4 Konishi T., Koyama T., Aoki T., et al. Radionuclide assessment of left ventricular function during dobutamine infusion in patients with coronary artery disease: comparison with ergometer exercise. Clin. Cardiol.. 1990;13:183-188.

5 Fukuoka S., Maeno M., Nakagawa S., et al. Feasibility of myocardial dual-isotope perfusion imaging combined with gated single photon emission tomography for assessing coronary artery disease. Nucl. Med. Commun.. 2002;23(1):19-29.

6 Anagnostopoulos C., Underwood S.R. Simultaneous assessment of myocardial perfusion and function: how and when? Eur. J. Nucl. Med.. 1998;25:555-558.

Radionuclide Myocardial Perfusion Imaging

Indications

1. Diagnosis and assessment of extent and severity of myocardial ischaemia or infarction

2. Assessment of myocardial viability

3. Evaluation of prognosis

4. Evaluation of effects of angioplasty and bypass surgery on myocardial perfusion with pre- and post-intervention imaging.

Contraindications

1. Unstable angina

2. Frequent ventricular arrhythmias at rest

3. Contraindications to pharmacological stress agent

4. Second-degree heart block

5. Severe valvular disease, especially aortic valve stenosis.

Radiopharmaceuticals ^1,²

1. ^99mTc-methoxyisobutylisonitrile (MIBI or sestamibi), up to 800 MBq (8 mSv ED) for planar imaging, 800 MBq (8 mSv ED) for SPECT (or 1600 MBq max. for the total of two injections in single day rest/exercise protocols). Cationic complex with myocardial uptake in near proportion to coronary blood flow but minimal redistribution. There is also, normally, liver uptake and biliary excretion, which can cause inferior wall artifacts on SPECT if care is not taken. Separate injections are required for stress/rest studies, but image timing is flexible due to minimal redistribution

2. ^99mTc-tetrofosmin (Myoview) (activity and radiation dose as for MIBI). Similar uptake characteristics and diagnostic efficacy to MIBI, but with easier preparation

3. ²⁰¹Tl-thallous chloride, 80 MBq max. (18 mSv ED). Thallium is a potassium analogue with initial rapid myocardial uptake in near proportion to coronary blood flow, and subsequent washout and redistribution. Hence, unlike the ⁹⁹Tc^m agents, same-day stress/rest redistribution studies can be performed with a single injection. With principal photon energies of 68–72 and 167 keV and T_1/2 of 73 h, it is not ideal for imaging and gives a higher radiation dose than the newer ^99mTc alternatives. It is increasingly being replaced by ^99mTc agents. However, many still consider ²⁰¹Tl for assessment of myocardial viability and hibernation, with either re-injection at rest or a separate day rest-redistribution study giving the greatest sensitivity.

4. ¹⁸FDG + blood flow PET. The radio-isotope gold standard for viability assessment, but not widely available ³

5. Rubidium-82 PET. This study can be used to assess myocardial perfusion. Not widely available.

Patient preparation

1. Nil by mouth or light breakfast 4–6 h prior to test

2. Cessation of cardiac medication on the day of the test if possible. ß-blockers can be continued and adenosine stress used. Avoid caffeine for 24 h if using dipyridamole or adenosine.

Technique

The principal of the technique is to compare myocardial perfusion under conditions of pharmacological stress or physical exercise, with perfusion at rest. Diseased but patent arterial territories will show lower perfusion under stress conditions than healthy arteries, but will show relatively improved perfusion at rest. Infarcted tissue will show no improvement at rest. Hence, prognostic information on the likelihood of adverse cardiac events and the benefits of revascularization can be gained.⁴

Stress regime

Pharmacological stress has become increasingly widely used instead of physical exercise.⁵ The optimal stress technique aims to maximize coronary arterial flow. The preferred pharmacological stressing agent is adenosine infusion (0.14 mg kg⁻¹ min⁻¹ for 6 min).⁶ Adenosine is a potent coronary vasodilator. It reproducibly increases coronary artery flow by more than maximal physical exercise (which often cannot be achieved in this group of patients). It has a short biological half-life of 8–10 s, so most side-effects are reversed simply by discontinuing infusion. Stressing with adenosine has now largely replaced dipyridamole, which will not be discussed here.

There are circumstances where adenosine is contraindicated, e.g. asthma, second-degree heart block or systolic blood pressure <100 mmHg. Dobutamine stress may be employed in these circumstances.⁷ Dobutamine acts as a ß₁ receptor agonist, increasing contractility and heart rate. Under continuous monitoring, the dose is incrementally increased from 5 to 20 μg kg⁻¹ min⁻¹, infusing each dose for 3 min. The infusion is terminated when S-T segment depression of >3 mm, any ventricular arrhythmia, systolic blood pressure >220 mmHg, attainment of maximum heart rate, or any side-effects occur. Dobutamine is contraindicated in patients with aortic aneurysm. New, more specific targeted agents such as regadenoson (A_2A adenosine receptor agonist) are being developed which could be used in asthmatic patients rather than dobutamine.⁸

^99mTc-MIBI or tetrofosmin rest/stress test

Because MIBI and tetrofosmin have minimal redistribution, separate injections are needed for stress and rest studies. Two-day protocols are optimal, but it is often more convenient to perform both studies on the same day. A number of groups have shown that this is possible without significantly degrading the results, most effectively when the resting study is performed first.⁹

2-Day protocol (stress/rest)

1. Initiate pharmacological stress or exercise

2. Up to 800 MBq MIBI or tetrofosmin is administered i.v. at maximal stress, continuing the stress protocol for 2 min post injection to allow uptake in the myocardium

3. 10–30 min post injection, a milky drink or similar is given to promote biliary clearance; high fluid intake will dilute bowel contents

4. Images are acquired 15–30 min after tetrofosmin or 60–120 min after sestamibi injection. If there is excessive liver uptake or activity in small bowel close to the heart, imaging should be delayed by a further 60 min

5. Depending on the clinical situation, if the stress scan is completely normal the patient may not need to return for the rest scan ¹⁰

6. Preferably 2–7 days later, the patient returns for a resting scan

7. Glyceryl trinitrate (GTN) (two 0.3 mg tablets) or equivalent sublingual spray may be given to improve blood flow to ischaemic, but viable segments ¹¹

8. Immediately, up to 800 MBq MIBI or tetrofosmin i.v. is administered and proceed as for stress imaging.

1-Day protocol (stress/rest)

1. Initiate pharmacological stress or exercise; up to 400 MBq MIBI or tetrofosmin i.v. is administered at maximum stress

2. Image as per 2-day protocol

3. A minimum of 2 h after first tetrofosmin injection and minimum of 4 h after sestamibi injection, GTN is administered as above, followed by up to 1200 MBq MIBI or tetrofosmin (with longer period between injections, the activity of the second injection may be reduced)

4. Image as per 2-day protocol.

²⁰¹Tl stress/rest test

Since ²⁰¹Tl redistributes after injection, stress/rest studies can be performed with a single injection. However, stress image timing is more critical:

1. Initiate pharmacological stress or exercise

2. Administer 80 MBq ²⁰¹Tl i.v. at maximal stress, continuing the stress protocol for 1 min post injection to allow uptake in the myocardium

3. Image immediately (<5 min)

4. Image at rest 3–4 h after redistribution period, during which time patients should not eat

5. If fixed defects are present in exercise and rest images and assessment of viability and hibernation is required, either:

a administer a second smaller dose of 40 MBq ²⁰¹Tl, giving GTN pre injection, and image 20–30 min later. If defects still persist, image again at 18–24 h

b perform a separate day rest-redistribution study with 80 MBq ²⁰¹Tl after GTN, and image at 20–30 min and 3–4 h.

Images

Spect

1. Position patient as comfortably as possible with their arms above their head (or at least the left arm) if possible. SPECT images may be severely degraded by patient movement, so attention should be paid to keeping the patient very still

2. For the ⁹⁹Tc^m agents, check for activity in bowel loops close to the inferior wall of the heart. This can cause artifacts in the reconstructed images, so if significant activity is seen, delay the imaging to give greater time for clearance

3. 180° orbit from RAO 45° to LPO 45°, elliptical if possible. With modern dual-headed systems, this can be achieved with the heads at 90° to each other to minimize the amount of camera rotation required

4. Matrix size and zoom to give a pixel size of 6 mm

5. 60 projections, with a total imaging time of about 30–40 min for single and 15–25 min for dual-head systems

6. View the projections as a cine before the patient leaves the department. If available, perform software motion correction. If there is significant movement that cannot be corrected, repeat imaging. Beware ‘diaphragmatic creep’, particularly on ²⁰¹Tl patients breathless after exercise, where the average position of the diaphragm changes as they recover.

Analysis

1. Short, vertical long and horizontal long axis views are reconstructed, taking care to use the same orientation for stress and rest image data sets (modern systems have automatic alignment software).

2. Two-dimensional circumferential profile or ‘bull’s-eye’ polar maps may be generated.

Additional techniques

1. Gated SPECT is now recommended,¹² and with special software (e.g. Cedars Sinai QGS package, Emory cardiac tool box) can provide additional information on ventricular wall motion, ejection fraction and chamber volume. It can also improve specificity by reducing artifactual defects caused by regional myocardial motion and wall thickening.

2. Bull’s-eye maps can be compared to normal databases and displayed quantitatively in terms of severity and extent of relative underperfusion.¹³

3. Attenuation and scatter correction using scanning transmission line sources or CT are now available on most modern dual-headed systems. This technique can improve diagnostic specificity by correcting attenuation artifacts and thereby increasing the normalcy rate. However, algorithms are still being improved, so at present it is wise to view the attenuation-corrected and uncorrected images together to identify possible introduced artifacts.¹⁴

4. The combination of rest ²⁰¹Tl and exercise ^99mTc MIBI or tetrofosmin can be used to assess both ischaemia and viability.¹⁵