SPECT

Principle

Single photon emission computed tomography (SPECT) is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a Gamma camera. However, it is able to provide true 3D information. This information is typically presented as cross-sectional slices through the patient, but can be freely reformatted or manipulated as required.

In the same way that a plain X-ray is a 2-dimensional (2-D) view of a 3-dimensional structure, the image obtained by a gamma camera image is a 2-D view of 3-D distribution of a radionuclide. This is often ^99mTc, a metastable (indicated by “m”) nuclear isomer radionuclide (radioisotope, see Gamma camera) which emits gamma rays which can be detected by a gamma camera. It is coupled to an organ specific tracer molecule.

Fig. 1 SPECT imaging machine

SPECT imaging is performed by using a gamma camera to acquire multiple 2-D images (also called projections), from multiple angles. A computer is then used to apply a tomographic reconstruction algorithm to the multiple projections, yielding a 3-D dataset. This dataset may then be manipulated to show thin slices along any chosen axis of the body, similar to those obtained from other tomographic techniques, such as MRI, CT (see CT scan (dual energy)), and PET.

Because SPECT acquisition is very similar to planar gamma camera imaging, the same radiopharmaceuticals may be used. If a patient is examined in another type of nuclear medicine scan but the images are non-diagnostic, it may be possible to proceed straight to SPECT by moving the patient to a SPECT instrument, or even by simply reconfiguring the camera for SPECT image acquisition while the patient remains on the table.

To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every 3-6^o. In most cases, a full 360^o rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15 – 20 s is typical. This gives a total scan time of 15-20 min.

Multi-headed gamma cameras can provide accelerated acquisition. For example, a dual headed camera can be used with heads spaced 180^o apart, allowing 2 projections to be acquired simultaneously, with each head requiring 180^o of rotation. Triple-head cameras with 120^o spacing are also used.

Application

SPECT can be used to complement any gamma imaging study, where a true 3D representation can be helpful (tumor, infection, thyroid, bone). With accurate 3D localization, it can be used for functional imaging, e.g. myocardial perfusion imaging

Myocardial perfusion imaging (MPI) MPI, used for the diagnosis of ischemic heart disease, is a type of cardiac stress test (restricted myocardium blood flow). A cardiac specific radiopharmaceutical is e.g. ^99mTc-tetrofosmin.

Gated myocardial SPECT Triggered by ECG to obtain differential information about the heart in various parts of its cycle, gated myocardial SPECT can be used to obtain quantitative information about myocardial perfusion, thickness, and contractility of the myocardium during various parts of the cardiac cycle; and also to allow calculation of left ventricular ejection fraction, stroke volume, and cardiac output. The Technique is similar as a MUGA scan.

Functional brain imaging Usually the gamma-emitting tracer used in functional brain imaging is ^99mTc-HMPAO (hexamethylpropylene amine oxime). When it is attached to HMPAO, this allows ^99mTc to be taken up by brain tissue in a manner proportional to brain blood flow, in turn allowing brain blood flow to be assessed with the nuclear gamma camera.

Because blood flow in the brain is tightly coupled to local brain metabolism and energy use, the ^99mTc-HMPAO tracer (as well as the similar ^99mTc-EC tracer) is used to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of dementia. SPECT is able to differentiate Alzheimer's disease from vascular dementias. ^99mTc-HMPAO SPECT scanning competes with FDG (2-fluoro-2-deoxy-D-glucose) PET scanning of the brain, which works to assess regional brain glucose metabolism, to provide very similar information about local brain damage from many processes.

SPECT is more widely available, for the basic reason that the radioisotope generation technology is longer-lasting and far less expensive in SPECT, and the gamma scanning equipment is less expensive as well. The reason for this is that ^99mTc (technetium-99m) is extracted from relatively simple technetium-99m generators which are delivered to hospitals and scanning centers weekly, to supply fresh radioisotope.

Ictal-interictal SPECT Analysis by SPM (ISAS) is a specific application of functional brain imaging with SPECT. The goal of ISAS is to localize the region of seizure onset for epilepsy surgery planning. ISAS is an objective tool for analyzing ictal versus interictal SPECT scans. SPM is Statistical parametric mapping, a statistical technique for examining differences in brain activity recorded during functional neuro-imaging experiments using neuro-imaging technologies such as SPECT, fMRI or PET.

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Limitations

Scanning is time consuming, and it is essential that there is no patient movement during the scan time. Movement can cause significant degradation of the reconstructed images, although movement compensation reconstruction techniques can help with this. A highly uneven distribution of radiopharmaceutical also has the potential to cause artifacts. A very intense area of activity (e.g. the bladder) can cause extensive streaking of the images and obscure neighboring areas of activity.

Attenuation of the gamma rays within the patient can lead to significant underestimation of activity in deep tissues, compared to superficial tissues. Approximate correction is possible, based on relative position of the activity. However, optimal correction is obtained with measured attenuation values.

Reconstruction

Reconstructed images typically have resolutions of 64x64 or 128x128 pixels, with the pixel sizes ranging from 3-6 mm. The number of projections acquired is chosen to be approximately equal to the width of the resulting images. In general, the resulting reconstructed images will be of lower resolution, have increased noise than planar images, and be susceptible to artifacts.

Combined imaging

Modern SPECT equipment is available with an integrated X-ray CT scanner. As X-ray CT images are an attenuation map of the tissues, this data can correct the SPECT map for attenuation. It also provides a precisely registered CT image which can provide additional anatomical information.

Typical SPECT acquisition protocols

Study	Radio-isotope	Emission energy (keV)	Half-life (h)	Radio- pharma- ceutical	Activity (MBq)	Rotation (^o)	Pro-jections	Image resolution	Time per projection (s)
Bone	^99mTc	140	6	Phosphon- ates / Bisphos phonates	800	360	120	128 x 128	-
Myo-cardial perfusion	^99mTc	140	6	tetrofosmin; Sestamibi	700	180	60	128 x 128	30
Brain	^99mTc	140	6	HMPAO; ECD	555-1110	360	64	128 x 128	30
Tumor	^123mI	159	13	MIBG	400	360	60	64 x 64	30
White cell	^111mIn	171 & 245	67	in vitro labelled leucocytes	18	360	60	64 x 64	30

Literature

Elhendy et al., Dobutamine Stress Myocardial Perfusion Imaging in Coronary Artery Disease, J Nucl Med 2002 43: 1634-1646 (review).

W. Gordon et al. (2005). "Neuroreceptor Imaging in Psychiatry: Theory and Applications". International Review of Neurobiology, 67: 385-44.