Nuclear Cardiac Imaging



Nuclear Cardiac Imaging


Santosh S. Oommen



I. INTRODUCTION.

Nuclear cardiology has an integral role in the noninvasive detection of coronary artery disease (CAD), assessment of myocardial viability, and stratification of risk. It imparts improved sensitivity and specificity over standard exercise stress testing. For example, the average sensitivity and specificity of single-photon emission computed tomography (SPECT) with technetium 99m have been reported to be 90% and 74%, respectively—though the exact performance characteristics depend on the prevalence of the disease in the population being studied. Nuclear imaging can provide functional and prognostic information that is quantifiable, reproducible, and readily obtainable in diverse patient populations.


II. INDICATIONS (Table 48.1)



A. Diagnosis of CAD.

Nuclear perfusion studies are performed to establish noninvasively the diagnosis of CAD in the following situations: history of stable angina; chest pain of unclear causation; unstable angina after stabilization; abnormal exercise test result without symptoms; risk stratification in the setting of multiple factors thought to confer a high likelihood of subclinical CAD; scheduled standard exercise testing in the setting of an abnormal electrocardiogram (ECG; due to left ventricular hypertrophy with associated repolarization changes, ST depression >1 mm, manifest pre-excitation pattern on ECG, digoxin use, left bundle branch block, or ventricularpaced rhythm); and previously nondiagnostic graded exercise test.


B. Assessment of the physiologic importance of known CAD.

Perfusion imaging can assist in the determination of the functional significance of a coronary stenosis that is in the “moderate-to-severe” (50% to 70%) range on angiographic evaluation. It can therefore be useful to evaluate a specific coronary lesion before proceeding to percutaneous intervention. This remains an accepted indication for nuclear perfusion imaging, although its use for this purpose is being supplanted by other modalities that can assess the functional significance of coronary lesions at the time of angiography (e.g., fractional flow reserve).


C. Assessment after therapeutic intervention.

In the past, perfusion imaging was often performed as a routine follow-up procedure after percutaneous intervention and coronary artery bypass grafting (CABG). More recent recommendations on appropriate use of this modality suggest that routine screening in asymptomatic patients who have been successfully revascularized by either method is not necessarily warranted, except in the evaluation of patients more than 5 years after CABG. On the other hand, radionuclide perfusion imaging is certainly appropriate in patients who have undergone prior revascularization and are presenting with recurrent symptoms consistent with coronary ischemia.


D. Risk stratification.

With nuclear imaging, it is possible to stratify risk among patients with stable angina or unstable angina, those who have had myocardial infarction (MI), and those about to undergo noncardiac operations.












TABLE 48.1 Appropriate Indications for Myocardial Perfusion Imaging—Based on the ACCF/ASNC/ACR/AHA/ASE/SC/T/SCMR/SNM 2009 Appropriate Use Criteria for Cardiac Radionuclide Imaging

























































































































































Patient group

Condition

Imaging technique


ER patient with chest pain

For risk stratification in pt with possible ACS. Initial serum markers and enzymes. ECG is nondiagnostic.

Rest perfusion imaging (with ECG gating, if possible).

For CAD diagnosis in pt with possible ACS and nondiagnostic ECG. Negative serum markers and enzymes or normal rest perfusion scan.

Same-day rest/stress (ECG-gated) myocardial perfusion imaging.


Acute MI/unstable angina

Assessment of LV function.

Rest myocardial perfusion imaging with ECG gating (rest gated radionuclide angiography is alternative option).


ST-elevation MI

Measurement of infarct size and residual viable myocardium, in an unrevascularized asymptomatic stable patient after completion of the infarct.

Rest myocardial perfusion imaging with ECG gating or with stress perfusion imaging with ECG gating.

Thrombolysis without coronary angiogram, to identify inducible ischemia and myocardium at risk.

Rest and stress myocardial perfusion imaging, with ECG gating whenever possible.


Non-ST-elevation MI/unstable angina

In an unrevascularized stable asymptomatic patient after completion of the infarct, to determine the extent and severity of inducible ischemia, either in the distribution of the “culprit” vessel or in remote myocardium.

Rest and stress myocardial perfusion imaging, with ECG gating whenever possible.

In individuals whose angina is stabilized on medical therapy or in whom the diagnosis is uncertain, to identify the extent and severity of inducible ischemia.

Rest and stress myocardial perfusion imaging, with ECG gating whenever possible.

To assess the functional significance of a coronary stenosis on angiography.

Rest and stress myocardial perfusion imaging.


CAD diagnosis in an individual with an intermediate probability of disease and/or risk stratification in someone with an intermediate or high likelihood of disease and able to exercise to 85% MPHR or more

Those with pre-excitation, LVH, on digoxin, or >1 mm ST-segment depression on resting ECG.

Rest and exercise stress myocardial perfusion imaging, with ECG gating whenever possible.

Individuals with left bundle branch block or ventricular-paced rhythm.

Rest and vasodilator stress myocardial perfusion imaging, with ECG gating whenever possible.

Patients with an intermediate- or high-risk Duke treadmill score.

Rest and exercise stress myocardial perfusion imaging, with ECG gating whenever possible.

In an individual with prior abnormal myocardial perfusion scan and new or worsening symptoms.

Repeat rest and exercise stress myocardial perfusion imaging, with ECG gating whenever possible.


CAD diagnosis in an individual with an intermediate probability of disease and/or risk stratification in someone with an intermediate or high likelihood of disease and not able to exercise

To identify the extent, severity, and location of inducible ischemia.

Rest and vasodilator stress myocardial perfusion imaging, with ECG gating whenever possible.


Detection of CAD in patients with ventricular tachycardia

Patients without known CAD or ischemic equivalent.

Rest and stress myocardial perfusion imaging, preferably exercise stress, with ECG gating whenever possible.


Detection of CAD in patients with syncope.

Patients with intermediate and high risk for CHD and no ischemic equivalent.

Rest and stress myocardial perfusion imaging, preferably exercise stress, with ECG gating whenever possible.


Prior to intermediate- and high-risk noncardiac surgery

Initial diagnosis of CAD in those with at least one clinical risk factor for adverse perioperative CV events, and poor (<4 METSs) or unknown functional capacity.

In those able to exercise, rest and exercise stress myocardial perfusion imaging, with ECG gating whenever possible


or


In those unable to exercise, rest and vasodilator stress myocardial perfusion imaging, with ECG gating whenever possible.

In individuals with established or suspected CAD and poor (<4 METS) or unknown functional capacity.

In those able to exercise, rest and exercise stress myocardial perfusion imaging, with ECG gating whenever possible


or


In those unable to exercise, rest and vasodilator stress myocardial perfusion imaging, with ECG gating whenever possible.

Diagnosis of CAD in patients with left bundle branch block or ventricular-paced rhythm and at least one risk factor for adverse perioperative CV events.

Rest and vasodilator stress myocardial perfusion imaging, with ECG gating whenever possible.

In suspected or established CAD, prognostic assessment of those with left bundle branch block or ventricular-paced rhythm on rest ECG.

Rest and vasodilator stress myocardial perfusion imaging, with ECG gating whenever possible.


Equivocal SPECT myocardial perfusion scan

Clinically indicated SPECT perfusion study is equivocal for CAD diagnosis or risk stratification purposes.

Rest and adenosine or dipyridamole stress PET myocardial perfusion study.


CAD patient with systolic dysfunction and CHF, with little or no angina

Prediction of improvement in regional/global LV function following revascularization.

Stress/redistribution/reinjection thallium 201 SPECT perfusion imaging


or


Rest/redistribution SPECT perfusion imaging


or


Myocardial perfusion plus FDG PET metabolic imaging


or


Resting sestamibi SPECT perfusion imaging.

Prediction of improvement in natural history following revascularization.

Stress/redistribution/reinjection thallium 201 SPECT perfusion imaging


or


Rest/redistribution thallium 201 SPECT perfusion imaging


or


Myocardial perfusion plus FDG PET metabolic imaging.


ACS, acute coronary syndrome; CAD, coronary artery disease; CHF, congestive heart failure; CV, cardiovascular; ECG, electrocardiogram; ER, emergency room; FDG, [18F]fluoro-2-deoxyglucose; LV, left ventricular; LVH, left ventricular hypertrophy; MI, myocardial infarction; MPHR, maximal age-predicted heart rate; PET, positron emission tomography; pt, patient; SPECT, single-photon emission computed tomography.




E. Identification of prior MI

among patients with angiographically normal coronary arteries is afforded by nuclear imaging.


F. Assessment of left ventricular (LV) function.

Although nuclear imaging is used less often for this purpose than in the past—due to the desire to reduce patients’ radiation exposure when possible—gated blood pool imaging remains an accurate method of determining the ejection fraction.


III. CONTRAINDICATIONS.

In addition to standard contraindications to exercise stress testing, specific considerations apply uniquely to nuclear imaging in general and the subgroup of dipyridamole stress perfusion studies.


A. General contraindications to nuclear studies.

Nuclear imaging is contraindicated for patients who have had iodine 131 therapy within 12 weeks; technetium 99m studies within 48 hours, including bone, lung, multigated acquisition (MUGA), liver, tagged red blood cell (to evaluate gastrointestinal bleeding), and renal scans; indium 111 scans within 30 days; gallium 67 scans within 30 days; and oral intake within 4 hours (except for water).


B. Contraindications to dipyridamole, adenosine, or regadenoson

administration include allergy to any of these agents, allergy to aminophylline, ongoing theophylline therapy (must be discontinued for 36 hours), history of uncontrolled asthma or reactive airway disease, significant atrioventricular nodal block, and caffeine consumption within 12 to 24 hours.


IV. EQUIPMENT.

The most basic tool in nuclear imaging is the gamma or scintillation camera, which is used to detect gamma rays (i.e., x-ray photons) produced by the chosen radionuclide. Three types of gamma camera exist.

A. A single-crystal camera consists of one large sodium iodide crystal. Other essential elements of this camera include the collimator, a lead device that screens out background or scattered photons, and the photomultiplier, an electronic processor that translates photon interactions with the crystal into electric energy.

1. Electric signals from the photomultiplier are processed by the pulse height analyzer before reaching a final form. Only signals in a specified energy range are incorporated into the interpreted images. The range recognized by the pulse height analyzer is adjustable and is established on the basis of the radiopharmaceutical used.

2. Digitalization of the single-crystal camera has greatly enhanced its performance.

B. A multicrystal camera works with an array of crystals with increased count detection capability. Because of the availability of an individual crystal to detect scintillation at any given time, this type of camera can be used to detect many more counts than can a single-crystal camera.

C. In the case of positron emission tomography (PET) scanning, a positron camera is a gamma camera used to detect the photon products of positron annihilation. Interaction between a positron and an electron causes annihilation, with the generation of two high-energy photons (511 keV) that travel in opposite directions.

1. An array of multiple concentric rings of crystals constitute a positron camera. Each crystal is linked optically to multiple photomultipliers. The crystals are oriented in diametric pairs in such a way that each pair of crystals must be struck simultaneously by annihilation photons to record activity. Background interference and stray photon energy are automatically accounted for, and artifact is limited.

2. Most positron cameras contain bismuth germanate for annihilation photon detection. The clinical utility and radiopharmaceuticals for PET are discussed in Section X.


V. MECHANICS AND TECHNIQUES


A. Image acquisition.

Basic perfusion imaging can be performed by means of planar and tomographic techniques. The tomographic, or SPECT, method is the most commonly used today.







FIGURE 48.1 Standard planar views and vascular territories. Circ: circumflex artery; LAO, left anterior oblique; LAD: left anterior descending artery; RCA: right coronary artery.

1. Planar images are acquired in three views: anterior, left anterior oblique (LAO), and steep LAO or left lateral (LLAT) orientation (Fig. 48.1). The patient is supine for anterior and LAO views but is placed in the lateral decubitus position for LLAT image acquisition. Although it allows examination of specific myocardial segments, planar imaging superimposes vascular distributions and therefore can compromise the ability to implicate a specific vascular supply when a defect is present. For example, normally perfused myocardial segments may overlap perfusion defects in a separate distribution.

2. Using SPECT, a series of planar images are usually obtained over a 180° arc to reconstruct a three-dimensional representation of the heart. The arc typically extends from the 45° right anterior oblique plane to the 45° left posterior oblique plane, with the patient in the supine position.

a. Three orientations are analyzed in the final representation: short axis, vertical long axis, and horizontal long axis. A computer-generated display, the polar map, is also analyzed as a quantifiable representation of count density.

b. Unlike planar imaging, SPECT can be used to separate vascular territories and improve image interpretation. SPECT, however, also increases the time needed for image acquisition and requires close attention to quality control issues.


B. Radiopharmaceuticals

available for nuclear imaging include thallium 201, technetium 99m, and several positron imaging agents. Each possesses specific energy characteristics, kinetic profiles, and biodistribution (see below as well as Table 48.2 and Section X for further details).


1. Thallium 201

a. General characteristics. Thallium 201 (i.e., thallous chloride) is a metallic element in group IIIA of the periodic table; it is produced in a cyclotron. Thallium emits gamma rays at an energy range of 69 to 83 keV and has a half-life of 73 hours

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Jun 7, 2016 | Posted by in CARDIOLOGY | Comments Off on Nuclear Cardiac Imaging

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