Single photon emission computed tomography

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.

Principles
In the similar way as a plain X-ray is a 2-dimensional view of a 3-dimensional structure, the image obtained by a gamma camera image is a 2D view of 3D distribution of a radionuclide.

SPECT imaging is performed by using a gamma camera to acquire multiple images (also called projections) from multiple angles. A computer can then be used to apply a tomographic reconstruction algorithm to the multiple projections, yielding a 3D dataset.

Because SPECT acquisition is very similar to planar gamma camera imaging, the same radiopharmaceuticals may used. If a patient attends for a nuclear medicine scan, but the images are non-diagnostic, it may be possible to proceed straight to SPECT by simply reconfiguring the camera 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 degrees. In most cases, a full 360 degree rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15 – 20 seconds is typical. This gives a total scan time of 15-20 minutes.

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

Gated acquisitions are possible with SPECT, just as with planar imaging techniques such as MUGA. Cardiac gated myocardial SPECT can be used to obtain quantitative information about myocardial perfusion during the cardiac cycle, thickness and contractility of the myocardium and allow calculation of left ventricular ejection fraction, stroke volume, and cardiac output.

Application
SPECT can be used to complement any gamma imaging study, where a true 3D representation can be helpful. E.g. tumor imaging, infection (leukocyte) imaging, thyroid imaging or bone imaging.

Because SPECT permits accurate localisation in 3D space, it can be used to provide information about localised function in internal organs. E.g. functional cardiac or brain imaging.

Myocardial perfusion imaging
Myocardial perfusion imaging (MPI) is a form of functional cardiac imaging, used for the diagnosis of ischemic heart disease. The underlying principle is that under conditions of stress, diseased myocardium receives less blood flow than normal myocardium. MPI is one of several types of cardiac stress test.

A cardiac specific radiopharmaceutical is administered. E.g. 99mTc-tetrofosmin (Myoview™, GE healthcare), 99mTc-sestamibi (Cardiolite®, DuPont). Following this, the heart rate is raised to induce myocardial stress, either by exercise or pharmacologically with adenosine or dobutamine.

SPECT imaging performed after stress reveals the distribution of the radiopharmaceutical, and therefore the relative blood flow to the different regions of the myocardium. Diagnosis is made by comparing stress images to a further set of images obtained at rest. As the radionuclide redistributes slowly, it is not usually possible to perform both sets of images on the same day, hence a second attendance is required 1-7 days later. However, if stress imaging is normal, it is unnecessary to perform rest imaging, as it too will be normal – thus stress imaging is normally performed first.

MPI has been demonstrated to have an overall accuracy of about 83% (sensitivity: 85%; specificity: 72%), and is comparable (or better) than other non-invasive tests for ischemic heart disease, including stress echocardiography. The gold-standard test, however, remains invasive cardiac catheterization.

Reconstruction
Reconstructed images typically have resolutions of 64x64 or 128x128 pixels, with the pixel sizes ranging from 3-6 mm. The number of projections acquires 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.

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 artefact. A very intense area of activity (e.g. the bladder) can cause extensive streaking of the images, and obscuration of neighboring areas of activity. (This is a limitation of the filtered back projection reconstruction algorithm. Iterative reconstruction is an alternative algorithm which is growing in importance as it is less sensitive to artefacts and can also correct for attenuation).

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. 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 be incorporated into the SPECT reconstruction to correct for attenuation. It also provides a precisely registered CT image which can provide additional anatomical information.