Continuing Education Activity

Single-photon emission computed tomography (SPECT) is a nuclear imaging modality used frequently in diagnostic medicine. It allows the clinician to assess the perfusion and functionality of specific tissues. This activity reviews the basics of single-photon emission computed tomography imaging, including the underlying mechanism of imaging, indications, and contraindications, the technique utilized and personnel required, complications, and the clinical significance of this imaging modality in medicine.

Objectives:

• Identify the indications of SPECT imaging.
• Describe the equipment and personnel required for SPECT imaging.
• Outline the potential complications and their clinical significance of SPECT imaging.
• Summarize interprofessional team strategies for improving test performance and outcomes in SPECT imaging.

Introduction

Single-photon emission computed tomography (SPECT) is a nuclear imaging modality used frequently in diagnostic medicine. At its most basic level, SPECT produces a 3-dimensional image of the distribution of a radioactive tracer (sometimes called a probe) injected into the bloodstream and subsequently taken up by certain tissues.[1] This is accomplished via the use of specialized nuclear medicine cameras.

Thus, SPECT allows the clinician to assess the perfusion and functionality of specific tissues. The ability to assess tissue functionality and physiology is what sets SPECT imaging apart from purely anatomic imaging modalities such as computed tomography, magnetic resonance imaging, and roentgenography.[1]

Anatomy and Physiology

The utility of the SPECT scan lies in its ability to provide detailed physiological information about the tissues. This is accomplished via the production and administration of radioactive tracer compounds, also known as probes. These probes generally consist of a detectable (via gamma camera) radioactive isotope coupled with a biologically active ligand specific to the tissue being imaged.[2] The most common isotopes used are technetium-99m (Tc), iodine-123, and to a lesser extent, thallium-201. When selecting an isotope, the clinician must consider the goal of the scan, the risk to the patient, and the expense of the isotope itself. Tc has a half-life of 6 hours and high photon flux, leading to shorter scan times and lesser radiation exposure to the target organ. This allows for the administration of higher tracer doses, providing clear images without significantly increasing radiation exposure to the patient.[2]

On the other hand, iodine-123 has a much longer half-life of 13 hours, leading to a greater concentration of detectable iodine in the target organ compared to the background. However, the longer half-life and greater localized concentration of iodine-123 increase the dose of radiation to the organ. Because of the short scan time, clear images, and low associated radiation dose, Tc is often the preferred isotope for most clinical applications. However, iodine isotopes are highly preferred for studies of iodine-consuming tissues such as the thyroid gland.[3]

While the isotope chosen by the clinician is important, SPECT imaging depends on more than just the selection of the correct radioisotope. The success of the SPECT scan requires the isotope to be successfully bound to a biologically active ligand, which will interact with the body tissues to deliver the isotope to the desired location. For example, Tc can be linked to a compound known as hexamethylpropyleneamine oxime (HMPAO), which is able to cross the blood-brain barrier and is taken up as a metabolic substrate by brain tissue. Since HMPAO uptake is directly correlated with metabolic activity, cerebral SPECT studies can show areas of increased brain activity as well as areas of anatomically normal but metabolically deranged brain tissue, which would otherwise not be seen on conventional imaging. This makes SPECT a useful modality in the workup of cerebral perfusion disorders and dementia.[4][5]

SPECT imaging has also found great utility in the field of cardiovascular disease, particularly in the assessment of myocardial perfusion during stress testing. For this purpose, Tc is bound via coordination complex to 6 methoxyisobutylisonitrile (MIBI) groups to form a tracer compound known as sestamibi. The lipophilic MIBI groups are able to cross myocardial capillary membranes and deliver Tc into the cells of the myocardium at a rate proportional to blood flow. This allows the clinician to diagnose, assess, and evaluate perfusion defects of the heart. The scan may also be linked to the patient’s heartbeat via electrocardiography, and this type of study is known as a multi-gated acquisition scan (MUGA). MUGA produces detailed information about the perfusion and functionality of the heart at different points of the cardiac cycle. Thallium was commonly used for myocardial SPECT in the past but has been largely phased out in favor of Tc.

Indications

Indications for SPECT imaging are developed by imaging societies, and some important ones are listed below.[3] These indications include:

1. Evaluating patients with suspected dementia
2. Localizing epileptic foci preoperatively
3. Diagnosing encephalitis
4. Monitoring and assessing vascular spasm following subarachnoid hemorrhage
5. Mapping of brain perfusion during surgical interventions
6. Detecting and evaluating cerebrovascular disease
7. Predicting the prognosis of patients with cerebrovascular accidents
8. Corroborating the clinical impression of brain death
9. Several indications in the field of oncology are listed in this article.[1]

In addition, the American Society of Nuclear Cardiology has developed an extensive list of indications for cardiac SPECT studies.[6][7] Some notable indications include:

1. Evaluating patients for coronary artery disease.
2. Assessing treatment response and guidance of future therapy in patients with coronary artery disease, cardiomyopathy, and heart failure.
3. Diagnosing coronary artery disease in patients unable to perform a standard exercise stress test.
4. Pre-surgical evaluation of patients with suspected or confirmed coronary artery disease.

SPECT scans are also indicated for non-cardiac and non-neurological conditions such as osteomyelitis, spondylolysis, parathyroid disease, pulmonary embolism, and abscess localization.

Contraindications

SPECT imaging alone has no absolute contraindications; however, patients may rarely experience allergic reactions to the tracer compound. Most often, contraindications for SPECT imaging is related to the underlying procedure rather than the SPECT scan itself. For example, a cardiac stress test augmented with SPECT carries the same risks and contraindications as a routine stress test. Clinicians should also carefully consider the risks of radiation exposure when referring patients that are pregnant for SPECT, and radioactive iodine isotopes should be avoided in these patients as well due to fetal iodine uptake.[8] Finally, some obese patients may exceed the weight limit of the scanner apparatus.

Equipment

3D-SPECT imaging requires a rotating multi-headed gamma camera, a collimator, and a radio-labeled tracer specific to the target tissue. The basic function of the camera is the detection of photons produced by gamma decay of the tracer compound, which is emitted in all directions from the patient. The collimator functions to filter these photons, allowing only particles that are parallel to the detector through. The collimator, therefore, focuses the radiation, and collimator selection influences the sensitivity and resolution of the final image.

The number of gamma camera heads increases the resolution of the final image and decreases the amount of time required for the scan. However, high-quality images can be produced with single-headed cameras by skilled medical physicists and nuclear radiologists.[3][9]

Personnel

The American College of Radiology offers technical standards for the roles and responsibilities for personnel involved in the safe and effective administration of a SPECT study. The personnel required for a safe and effective SPECT scan generally include:

1. Clinician
2. Nuclear medicine technologist
3. Nuclear pharmacist
4. Medical physicist
5. Radiation safety officer

A board-certified nuclear cardiologist or nuclear radiologist should be present to supervise the study.

Preparation

Patients should be instructed to avoid caffeinated beverages for at least 12 hours before the study. This is because caffeine interferes with the action of vasodilatory medications that may be administered during the study. Caffeine also affects cerebral blood flow and can negatively influence cerebral SPECT imaging. The patient should also avoid dipyridamole and other phosphodiesterase-3 inhibitors for at least 48 hours to avoid a synergistic vasodilatory effect, which may produce an unsafe drop in blood pressure.[10] Finally, all patients should be made nil per oral for at least 3 hours before the procedure and asked to void urine beforehand to maximize comfort.

Technique or Treatment

First, the patient is injected with a radio-labeled tracer compound selected by the nuclear pharmacist. For cerebral studies, the patient will first be seated in a quiet, dimly lit room and asked not to read or speak for at least 10 minutes prior to tracer injection. If the patient requires sedation for the procedure, it should be administered after the tracer, if possible. Following injection, there is a variable waiting period to allow the tracer to circulate and be taken up by the target tissues. This waiting period can be as long as 90 minutes for cerebral studies or as short as 15 minutes for cardiac stress tests. Wait periods for a particular study will also vary depending on tracer selection and dose.

Once the waiting period has elapsed, the patient will then be moved into the detector apparatus. If the patient is undergoing cardiac stress testing, they will be given cardiac stimulants such as atropine in accordance with standard stress testing protocols. Both cerebral and cardiac studies may utilize vasodilatory medications to assess tissue perfusion. Once the patient is in place and the necessary medications have been administered, the detector will rotate around the patient taking planar scans every 3 to 6 degrees, which will be combined to produce the final 3D image. Specific imaging protocols vary and may require a single scan, as in brain imaging, or several scans may be taken at set timing intervals, as in stress/rest imaging for cardiac SPECT.[11] While SPECT is able to produce detailed information about the tissues, it is not without limitations. Small metabolic abnormalities that may be detected on SPECT are often difficult to localize without corresponding anatomical imaging. To avoid these difficulties, a combined SPECT/computed tomography protocol has been developed, wherein functional and anatomical abnormalities detected by the SPECT study are also imaged simultaneously on computed tomography.

Certain SPECT protocols that can be employed to reduce radiation exposure and hazards include the following:[12][13][14]

• Appropriate patient selection for SPECT imaging based on clear indication will reduce unnecessary radiation exposure to patients as well as prevent the possibility of an error occurring.
• Technetium-99m based SPECT protocols (sestamibi and tetrofosmin) offer lower patient radiation exposure than thallium-201 (stress/redistribution and stress/reinjection) protocols. For the evaluation of chest pain and the diagnosis of ischemia, technetium-99m based protocols are safer and preferred.
• Radiotracer dose should be weight-based to optimize the radioactivity dose required.
• Cadmium zinc telluride detectors in the imaging camera are more sensitive to the incident ionizing radiation, and these detectors have better energy and spatial resolution. Utilization of cadmium zinc telluride cameras and Anger cameras can optimize radiation techniques.
• SPECT stress-only protocols using technetium-99m labeled radiotracers may reduce radiation exposure by 25% in comparison to typical rest/stress studies. Stress-first imaging is advisable in subjects who are good imaging subjects and who do not have a high pretest probability of an abnormal study. This is particularly important and feasible in young patients, and especially those with low to moderate pretest probability of coronary artery disease.
• The 2-day rest/stress technetium-99m (14 mSv) can be optimized to either stress only (7 mSv) or single-day rest/stress protocols (10 mSv), thereby minimizing radiation exposure.
• Image acquisition practices can be optimized to lower the amount of required radioactivity. In cooperative patients, the radiotracer dose can be reduced by lengthening acquisition times. It is important to position the camera as close as possible to the patient throughout the acquisition to lower the required radioactivity dose.
• Newly developed reconstruction algorithms are able to maintain image quality from SPECT studies, despite reducing the radiotracer dose. Using a novel wide-beam reconstruction algorithm, superior image quality can be acquired with a 50% reduction in radiation dose.
• If a scanner utilizes a CT scan for attenuation correction, the acquisition protocol can be optimized to use the lowest dose available for attenuation correction (rod-source).
• Software developments like resolution-recovery techniques are able to significantly reduce radiation exposure.

Complications

The most common complications from the procedure are mild reactions to the vasodilators and other medications used during the test. These side effects include flushing, headache, GI distress, and lightheadedness. More severe side effects such as hypotension, arrhythmias, chest discomfort, or AV blockade may also be seen. Rarely, an allergic reaction to the tracer compound or study medications may occur. The healthcare team should also consider the risks of radiation exposure to the patient, particularly if the patient is pregnant or planning to become pregnant. A Tc stress/rest cardiac scan carries the greatest effective dose of radiation at 11.8 mSv, while a Tc brain scan carries an effective dose of 5.7 mSv. The effective dose for most SPECT scans, excluding cardiac stress/rest studies, is usually below 10 mSv. By comparison, the effective radiation doses for head computed tomography, chest computed tomography, and computed tomography coronary angiography are 2.0, 7.0, and 16.0 mSv, respectively.[15]

Clinical Significance

In the setting of myocardial perfusion testing, SPECT has been shown to have a sensitivity of 82% and a specificity of 76% for the diagnosis of coronary artery disease. In addition, those patients with normal myocardial SPECT imaging have less than 1%  yearly risk of adverse cardiac events. Regarding cerebral imaging for the diagnosis of Alzheimer's dementia, SPECT has a sensitivity of 92%, a specificity of 100%, a positive predictive value of 92%, and a negative predictive value of 57%.[10][16][17]

Enhancing Healthcare Team Outcomes

To optimize outcomes, the nurse and/or technician should confirm the patient has not consumed caffeine or phosphodiesterase-5 inhibitor medications within 12 to 24 hours of the test, respectively. The patient’s vital signs and oxygen saturation should be monitored, recorded, and reported to the team at regular intervals by a dedicated nurse. This nurse should also be responsible for ensuring all necessary resuscitative equipment is in the imaging suite and fully functional before the study begins. For cardiac stress testing, the patient should be on a cardiac monitor with venous access established prior to the administration of atropine and other cardiac stimulants. The risk of radiation exposure to staff is low; however, radiological safety protocols should be made clear to all involved staff and strictly enforced by the designated radiation safety officer.[10]