Scanners and Advances in Technology

Four types of scanners are most commonly used in nuclear cardiac imaging:

• 1.
  

  Conventional SPECT scanners, based on the Anger gamma camera

  
  
• 2.
  

  Newer scanners with solid-state and semiconductor detectors

  
  
• 3.
  

  Conventional PET scanners

  
  
• 4.
  

  Hybrid SPECT or PET scanners combined with computed tomography (CT) scanners

A conventional SPECT system or a scintillation scanner (Anger gamma camera) consists of the following elements: a collimator, with lead septa to localize the source of the emitted gamma rays; a sodium iodide crystal, which scintillates when gamma rays interact with it and produces visible light; photomultiplier tubes, which amplify the emitted light signal and convert energy from visible light into an electric signal; pulse height analyzers and positioning circuitry to localize the signal; and an analog or digital computer designed to determine the location and energy of a photon striking the crystal. Patients are imaged in the supine or prone position by using a step and shoot acquisition mode, in which the scanner starts imaging in the right anterior oblique position, moves to the next position (3 degrees) and takes another projection image, and so on until a 180-degree rotation is completed (left posterior oblique) and tomographic images are calculated. The images obtained are processed using postprocessing filters and reconstructed into three cardiac views (short-axis, vertical long-axis, and horizontal long-axis views). The spatial resolution of the SPECT system is approximately 9 to 12 mm.

Advances in scintillation scanner technology have resulted in the development of solid-state detectors using cesium iodide coupled to photodiodes or novel semiconductor-based detectors using cadmium zinc telluride (CZT). The latest of these newer technologies is the CZT detector, which directly converts gamma radiation to an electronic pulse and thereby eliminates the need for a scintillating crystal and photomultiplier tubes. Some of the detector systems are open and L-shaped or C-shaped with an array of detector elements, as opposed to the circular gantry ( Fig. 5-1 ). Some of the newer scanners also offer imaging with the heart at the focal point of the detector array (cardiofocal imaging), thereby improving image resolution. Because the heart volume is imaged simultaneously, rather than in a step and shoot mode, dynamic images can be acquired in a tomographic mode. With some of these systems, imaging is performed with the patient in an upright (sitting) position in a chair, rather than in the supine position in bed. In addition, high-speed imaging is possible such that acquisition of the stress and rest data takes only 4 and 2 minutes, respectively, and the entire stress-rest SPECT protocol can be completed within 30 minutes.

An example of a newer-generation dedicated ultrafast acquisition scanner containing multiple (cadmium zinc telluride) detector columns (inset on the right) arranged in an L-shape configuration (left) that simultaneously image the heart. Each detector column uses a tungsten parallel-hole collimator that fans back and forth and hence allows the object of interest to be viewed from hundreds to thousands of different “viewing” angles. These scanners have the potential for a five- to tenfold increase in count sensitivity with gain in resolution. The patient is also able to sit in an upright position during image acquisition.

Myocardial count rates and image quality are higher than in conventional SPECT systems. Studies are currently under way to determine whether this improvement in image quality may translate into more accurate assessment of patients with CAD. A multicenter trial by Maddahi et al validated a half-time acquisition protocol and demonstrated that the rapid gated rest-stress upright system acquired perfusion and functional information comparable to that obtained with the conventional system. Clinical validation of the D-SPECT system (Spectrum Dynamics, Haifa, Israel) was performed by comparing the newer camera with a conventional SPECT camera. The newer high-speed camera produced images with higher counts and excellent linear correlation between the extent of perfusion abnormality at stress and rest compared with the conventional system. Taking into account the more linear relationship between myocardial uptake and blood flow at high flow rates of thallium-201 (Tl-201) than technetium-99m (Tc-99m), a fast sequential stress Tl-201 and rest Tc-99m sestamibi dual isotope protocol was accomplished within 20 minutes with image quality and dosimetry similar to those of a rest-stress Tc-99m protocol. Furthermore, CZT detectors have higher spatial and energy resolution when compared with sodium iodide systems. This superiority in energy resolution allows discrimination of photons of different energies and hence provides the opportunity to perform simultaneous acquisition of Tl-201 and Tc-99m. A comparison of simultaneous dual tracer rest Tl-201 and stress Tc-99m sestamibi on a solid-state D-SPECT scanner with a conventional SPECT camera by Ben-Haim et al showed that fast and high-quality imaging is feasible and diagnostically comparable to conventional SPECT and separate rest Tl-201 and stress Tc-99m acquisition.

PET scanners have higher spatial (4 to 6 mm) and temporal resolution, and they detect paired photons of 511 keV energy produced by annihilation of a positron. A positron is an unstable particle with a positive charge that can travel some distance (positron range) before it interacts with an electron and is annihilated, thus releasing two 511-keV photons at 180-degrees angle from each other. Photons striking diametrically opposing detectors within a short time interval (coincidence interval of approximately 20 nsec) are considered true coincidences and are imaged by the scanner. Higher-energy positrons have a longer positron range, which can limit image resolution ( Table 5-1 ). PET imaging is performed in a two-dimensional (2-D) mode with septa or in a three-dimensional (3-D) mode without septa. Because the 3-D mode offers higher count sensitivity, including more scatter and randoms, it requires more efficient crystals with shorter dead time and high computing capabilities. PET detectors are made up of bismuth germanate (BGO), lutetium oxyorthosilicate (LSO), gadolinium oxyorthosilicate (GSO), or lutetium yttrium orthosilicate (LYSO). BGO has limited energy and timing resolution compared with LSO, LYSO, and GSO crystals. Hence, BGO is more commonly used in 2-D PET systems, which have lower count sensitivity, and LSO, LYSO, and GSO are more commonly used in 3-D PET systems.

Most PET scanners have built-in attenuation correction. Attenuation from soft tissues is estimated using a transmission scan based on a radionuclide source (germanium-68, gallium-68, or cesium-137) or a CT image. Typically, the transmission and emission scans are obtained sequentially while the patient is kept in the same position. The CT in PET-CT systems allows for a rapid transmission scan (10 seconds versus 3 to 6 minutes for a radionuclide transmission scan), that permits improved laboratory throughput and reduced patient motion. Time of flight PET technology may improve the noise quality in the images by better delineating the origin of the photons, but it is not yet widely used in cardiac applications.

Software Advances

Software techniques have evolved to improve image quality, shorten image acquisition time and thus improve patient comfort and minimize motion, and reduce radiation dose to the patient. The major advances in software and reconstruction techniques include resolution recovery, noise reduction, wide beam reconstruction, and improved scatter and attenuation correction algorithms. For example, resolution recovery methods are used to improve spatial resolution and reduce noise. By using recovery algorithms from a database of known detectors and collimator characteristics, resolution recovery mathematically corrects for resolution degradation that occurs inherently with parallel-hole collimators and increasing distance from the scintillation crystal.

Commercial manufacturers have developed different versions of resolution recovery algorithms that include not only improved resolution but also reduced image noise. For noise reduction, filters are applied either to the original projection or after reconstruction. In addition, these newer algorithms also include Compton scatter and attenuation correction techniques. Using these newer software techniques, SPECT myocardial perfusion imaging (MPI) may be performed using half the conventional scan time with preserved or even improved image quality. In several studies, half-time processing (Astonish [Philips, San Jose, Calif.]) has been associated with superior image quality and interpretative certainty compared with full-time ordered subset expectation maximization (OSEM). This has the advantage of reducing scan acquisition time and, potentially, patient radiation exposure with half-dose full-time wide beam reconstruction.

Current methods for normal limits-based quantification of myocardial perfusion on SPECT use ungated normal limits and derive perfusion information from summed images. However, image blurring caused by motion may have a significant effect on quantification. Initial evaluation of a novel motion-frozen display and quantification method for gated MPI that potentially eliminates the problem of image blurring by cardiac motion has been performed. In this proposed method, gated perfusion images are analyzed after cardiac motion tracking and 3-D motion correction. These motion-frozen images visually resemble end-diastolic frames, but they contain counts from all cardiac cycles and hence are less noisy and of higher resolution than summed images. Initial results with this method show an improvement in image quality and good diagnostic performance.

Radiotracers

The typical clinically radiotracers used in nuclear cardiac imaging can be characterized broadly as tracers to study myocardial blood flow (perfusion tracers) and tracers to study metabolic processes in the heart (see Table 5-1 ).

Practical Points

Technetium-99m versus Thallium-201

The higher energy of Tc-99m gamma rays and the shorter half-life translate into higher image resolution and lower radiation dose to patients when compared with Tl-201. With Tl-201, the low-energy mercury x-rays make them more susceptible to scattering, attenuation, and limited gated images. In addition, the longer physical half-life of Tl-201 (half-life, 73-hours) limits the ability to administer higher doses and hence results in lower image quality. However, Tl-201 has a high first-pass extraction (approximately 85%) compared with Tc-99m agents (approximately 65%). Therefore, Tl-201 images can provide a better measure of hyperemic myocardial blood flow and may be more sensitive than Tc-99m images for the detection of milder degrees of stenosis. This property has been used, and newer protocols using Tl-201 for stress and Tc-99m agents for rest have been developed for some of the newer higher-sensitivity cardiac SPECT scanners. Although the first-pass extraction of Tc-99m agents is lower than that of Tl-201, the shorter half-life of Tc-99m allows for the use of a higher radiotracer dose that emits higher-energy gamma rays (less prone to attenuation) and results in better image quality compared with Tl-201.

Technetium-99m Agents

Because of the high hepatobiliary uptake of Tc-99m sestamibi, one must wait 45 to 60 minutes for optimal myocardial images and good clearance of hepatic uptake. The more rapid liver clearance of tetrofosmin compared with sestamibi significantly improves the ratios of cardiac to digestive activity, especially after exercise or at rest, and images can be obtained sooner after injection (10 to 15 minutes). Moreover, although tetrofosmin is reconstituted with Tc-99m, it can be allowed to stand at room temperature, unlike sestamibi, which requires boiling. This property, in addition to more rapid liver clearance and earlier image acquisition after tetrofosmin injection, is useful for performing studies in patients with acute chest pain, with higher patient throughput and image quality comparable to that with sestamibi.

Positron Emission Tomography Radiotracers

The main advantage of Rb-82 is the ability to perform PET MPI without an on-site cyclotron. However, the necessary strontium-83 generator is very expensive, and this factor should be considered, especially if patient volumes are limited. The ultrashort half-life of Rb-82 of 76 seconds requires that the generator be directly connected to the intravenous line of the patient so that dose can be delivered directly to the patient during PET imaging. Repeat imaging is uncomplicated, and preintervention and postintervention studies are simple to perform without a time lag. The short physical half-life of N-13 ammonia of just less than 10 minutes makes an on-site cyclotron necessary for its use. N-13 ammonia produces better-quality perfusion images with high signal-to-noise ratio, low background, and low liver activity. At high flow rates (>3 mL/g/minute), N-13 ammonia may result in some underestimation of myocardial blood flow. Because O-15 water is freely diffusible and metabolically inert, it is an ideal perfusion agent and does not underestimate flow even at high flow rates. However, its freely diffusible state can lead to poor image contrast resolution and requires correction of blood pool activity or myocardial activity with a separate scan to improve signal-to-noise ratio. F-18 flurpiridaz, the investigational PET perfusion tracer, would be suitable for exercise stress imaging because of its half-life of 109 minutes. Also because of its long half-life, this radiotracer can be transported to remote (but still accessible) locations and does not need an on-site cyclotron or a generator. The lower-energy positron to F-18 compared with Rb-82 and N-13 ammonia results in sharper images related to shorter positron range. For viability imaging, myocardial F-18 FDG uptake is imaged ( Fig. 5-2 , I ). Myocardial uptake of F-18 FDG highly depends on the substrate milieu. Therefore, good patient preparation is paramount to obtaining interpretable diagnostic images ( Tables 5-2 and 5-3 ; see Fig. 5-3 ) and may be time-consuming and difficult to implement without protocols in place. For more details, readers are referred to the excellent review on this topic by Machac et al.

Imaging protocols for single-photon emission computed tomography (SPECT) and positron emission tomography (PET). Protocols using technetium-99m (Tc-99m) or thallium-201 (Tl-201) myocardial perfusion imaging (MPI), as well as fluorine-18 fluorodeoxyglucose (FDG) PET, are shown. A to G, various SPECT protocols. H, Conventional PET MPI protocol using rubidium-82 (Rb-82). Patients are imaged supine with arms raised above their shoulders. A scout scan is a simple topographic image of the body obtained to ensure appropriate patient positioning. A scout image is obtained using a low-energy computed tomography (CT) scan (10 mA, 120 keV), with the upper border defined by the tracheal bifurcation and the lower border defined by the lower margin of the heart. A transmission scan obtained by CT to provide anatomically specific density maps of the thorax is used to correct PET emission data for photon attenuation (CT attenuation correction; CTAC). The dose of radiotracer for Rb-82 is 40 to 60 mCi and 15 to 25 mCi for two-dimensional (2-D) and three-dimensional (3-D) modes, respectively. Stress testing using pharmacologic and exercise stress is followed by radiotracer injection. Image acquisition in the list mode is preferred whenever feasible. The images are acquired (as gated or static images) or reconstructed (when acquired as a list mode file) with a prescan delay of 70 to 90 seconds for Rb-82. If longer circulation time is anticipated, a higher value of pre-scan delay is used. ^∗ Optional CTAC, calcium score, or CT coronary angiography. For nitrogen-13 (N-13) ammonia studies, the dose of radiotracer is 20 to 30 mCi for 2-D imaging (15 to 25 mCi for 3-D), with a prescan delay of 180 to 240 seconds. A minimum of 50 minutes should elapse between the first and second injection of N-13 ammonia (to allow for decay of the first injection). I, PET viability protocol with F-18-FDG in conjunction with resting MPI or stress MPI. Following rest or stress MPI, glucose manipulation is performed (outside the gantry; see text), and F-18-FDG is injected. Hence, scout and CTAC images are obtained again, followed by gated FDG images. J, FDG PET protocol for cardiac sarcoid. This protocol requires special dietary preparation with a low-carbohydrate, high-fat diet (with or without intravenous heparin to promote lipolysis). This dietary preparation allows for suppression or minimization of glucose use by normal myocytes, and areas of FDG uptake are considered abnormal. Perfusion imaging is also performed in conjunction with FDG images, with PET perfusion tracers such as Rb-82 or N-13 ammonia.

(Data from Yamagishi H, Shirai N, Takagi M, et al. Identification of cardiac sarcoidosis with [13]N-NH[3]/[18]F-FDG PET. J Nucl Med. 2003;44:1030-1036.)

Table 5-2

Guidelines for Performing Fluorine-18 Fluorodeoxyglucose Cardiac Positron Emission Tomography

Adapted with permission from Machac J, Bacharach SL, Bateman TM, et al. Positron emission tomography myocardial perfusion and glucose metabolism imaging. J Nucl Cardiol . 2006;13:e121-e151.

Procedure	Technique
Fast patient	6-12 hr
Load oral glucose	If fasting BG <~250 mg/dL (13.9 mmol/L), give 25-100 g glucose orally; then monitor BG based on Table 5-3 If fasting BG >~250 mg/dL, notify physician
Inject FDG	After monitoring BG (see Table 5-3 ), when BG is ~100-140 mg/dL (5.55-7.77 mmol/L), inject FDG intravenously
Begin PET imaging	Start PET imaging ~90 min after FDG injection

 

BG, Blood glucose, FDG, fluorine-18 fluorodeoxyglucose; PET, positron emission tomography.

Table 5-3

Guidelines for Blood Glucose Maintenance After Oral Glucose Administration for Optimal Fluorodeoxyglucose Cardiac Uptake ^∗

Adapted with permission from Machac J, Bacharach SL, Bateman TM, et al. Positron emission tomography myocardial perfusion and glucose metabolism imaging. J Nucl Cardiol . 2006;13:e121-e151.

Blood Glucose 45 to 60 Min After Administration	Possible Restorative Measure (Intravenous)
130-140 mg/dL (7.22-7.78 mmol/L)	1 unit regular insulin
140-160 mg/dL (7.78-8.89 mmol/L)	2 units regular insulin
160-180 mg/dL (8.89-10 mmol/L)	3 units regular insulin
180-200 mg/dL (10-11.11 mmol/L)	5 units regular insulin
>200 mg/dL (>11.11 mmol/L)	Notify physician

 

∗ The aim is to achieve a blood glucose concentration of approximately 100 to 140 mg/dL (5.55 to 7.77 mmol/L) at the time of fluorine-18 fluorodeoxyglucose injection.

Algorithm for selection of an appropriate myocardial perfusion study (single-photon emission computed tomography [SPECT] versus positron emission tomography [PET]). Asterisk, Pharmacologic PET preferred when available; N-13, nitrogen-13.

In addition, F-18 FDG PET can be used to image cardiac sarcoid (see Fig. 5-2 , J ). F-18 FDG PET is emerging as a promising diagnostic modality in identifying cardiac involvement of sarcoidosis. In these studies, localized uptake of F-18 FDG is a hallmark of active inflammatory change and potentially may also prove to be a useful indicator of response to steroid therapy (see Chapter 29 for more information).

The higher energy of PET radiotracers compared with SPECT radiotracers can result in greater penetration through the subject and can thereby lower the radiation dose to the patient. However, the radiation burden to the staff may be higher because more gamma rays are emitted from the patient and may reach the staff. The shorter the half-life of the radiotracer, the faster the protocols and lower the radiation dose to the patient will be. Extremely short half-life radiotracers such as O-15–labeled water (and sometimes Rb-82) are challenging to use for exercise stress testing because of decay of the radiotracer before the patient can be transferred from the treadmill to the scanner. Therefore, pharmacologic stress testing is preferable with short half-life radiotracers, and exercise or pharmacologic stress testing can be performed with longer half-life radiotracers. The short half-life radiotracers have a significant advantage in that rest and stress imaging can be performed using equal doses of radiotracer injection. In addition, if the dose has inadvertently infiltrated subcutaneously, rather than being injected intravenously, another scan can be obtained immediately after reinjection of another dose (rest imaging with repeat stress imaging is determined based on the half-life of the stress agent). Finally, for short half-life radiotracers produced by a cyclotron, the clinical schedule must be closely coupled with the cyclotron production schedule, thus making it less suitable for high-volume imaging. High-volume imaging using PET MPI is best performed with generator-produced radiotracers such as Rb-82 or with unit dose tracers such as the novel F-18 flurpiridaz (when it is approved by the FDA).

How to Decide Which Radioisotope to Use

This decision is based on a balance of several factors:

• 1.
  

  The type of stress protocol

  
  
• 2.
  

  The clinical question: ischemia assessment, viability assessment, assessment of ischemic memory

  
  
• 3.
  

  The availability for routine clinical use

  
  
• 4.
  

  The physical property of the radioisotope

  
  
• 5.
  

  Image quality

The decision to use SPECT or PET can be based on a simple algorithm ( Fig. 5-3 ). Exercise is the preferred modality for stress imaging. Hence, any patient able to exercise is scheduled for an exercise MPI study (SPECT study for normal weight and N-13 ammonia PET when available, for obese individuals). When resources are not constrained and both SPECT and PET imaging techniques are available, PET is preferred for all pharmacologic studies. For SPECT imaging, Tc-99m agents are the most widely used; less than 10% of laboratories use Tl-201. Tc-99m sestamibi and Tc-99m tetrofosmin appear to be equally efficient, except for the differences mentioned earlier. Tl-201 has superior extraction characteristics and can be used to improve the test sensitivity for detection of ischemia, particularly with the high-sensitivity scanners and improved image quality. Moreover, Tl-201 has less hepatic uptake and can occasionally be used in patients with excessive subdiaphragmatic activity from Tc-99m tracers that cannot be resolved with delayed imaging, feeding, or prone imaging. When the clinical question involves assessment of myocardial viability when PET is not available, Tl-201 perfusion imaging with delayed imaging can be performed.

Imaging of ischemic memory is being explored with I-123 BMIPP. This technique enables identification of ischemic episodes approximately 24 hours before rest imaging with I-123 BMIPP and may play a role in imaging of patients with acute chest pain and a nondiagnostic electrocardiogram (ECG).

When to Consider Positron Emission Tomography Myocardial Perfusion Imaging

PET offers several advantages compared with SPECT ( Box 5-1 ). PET yields higher-resolution images than SPECT. The unique opportunity to measure peak stress left ventricular ejection fraction (LVEF) during PET is also an advantage over current SPECT protocols, which acquire poststress LVEF. LVEF reserve (peak stress minus rest LVEF) has been shown to be an important diagnostic tool for identifying multivessel disease, and a low LVEF reserve appears to have adverse prognostic implications. Images are acquired simultaneously in multiple projections that allow quantification of absolute blood flow at rest, and peak stress is a distinct advantage of PET. Because conventional SPECT and PET techniques rely on the concept of flow heterogeneity and compare relative uptake of radiotracer within the myocardium, they may potentially underdiagnose multivessel disease.

If all segments of the myocardium are ischemic to similar degrees, a situation known as balanced ischemia can result. In PET, perfusion tracers such as Rb-82, N-13 ammonia, and oxygen-15 water allow for absolute quantification of myocardial blood flow in which resting and peak stress myocardial blood flow is measured. This approach may be useful for detecting the presence of multivessel CAD. In addition, absolute quantification of PET tracer uptake permits the detection of abnormal flow reserve in preclinical states before the development of significant epicardial coronary stenoses in which abnormal microvascular or endothelial function causes vascular dysfunction. Quantitative blood flow estimation can allow the study of progression or regression of atherosclerosis and appears to add incremental prognostic value to relative perfusion imaging. The indications and clinical applications for PET MPI and quantitative PET MPI are discussed in Chapter 29 .

How to Choose Among Perfusion Imaging Protocols

The choice of imaging protocol depends on the following:

• 1.
  

  The clinical question

  
  
• 2.
  

  Considerations for the appropriate radiation dose for each patient

  
  
• 3.
  

  The availability of radiopharmaceutical agents

  
  
• 4.
  

  The logistics for patients and laboratory personnel

  
  
• 5.
  

  The throughput of the laboratory

Gated Myocardial Perfusion Imaging

ECG-gated SPECT is an accurate and reproducible technique used to measure LVEF, volumes, and regional wall motion. At each step of a conventional Anger scanner, several images (8 to 16 images) are acquired in relation to the surface ECG ( Fig. 5-4 ). The newer SPECT scanners and PET scanner allow for list mode image acquisition in which image frames are acquired in relation to the ECG, as well as a time signal (see Fig. 5-4 ). The list mode files can be unlisted into a static or a gated image file. Gating with a higher frame rate improves temporal resolution for assessment of regional wall motion, and the overall LVEF may be comparable or slightly higher. The gated images are reconstructed using parameters similar to those used for the static images, and they are reviewed using commercial software packages for computation of LVEF and volumes. In patients who have irregular heart rates, gated information may be inaccurate, and acceptance windows can be specified to reject cycle lengths that do not fall within the predetermined R-R interval range.

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related posts:

Coronary Angiography: Technique Cardiac Computed Tomography for the Evaluation of Acute Coronary Syndrome in the Emergency Department Radiation Issues Myocardial Ischemic Disease: Computed Tomography Bronchial Arteries Deep Venous Thrombosis

Stay updated, free articles. Join our Telegram channel

Join