SPECT Imaging Technique
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Direct answer
Single photon emission computed tomography rebuilds a three-dimensional functional image from planar gamma-camera projections: the detector head orbits the patient through 180 or 360 degrees, pausing at 32-120 projections while counting gamma photons (Tc-99m's 140 keV the workhorse), and reconstruction — filtered back-projection with a Butterworth pre-filter or iterative OSEM — turns the angular series into tomographic slices. Cardiac perfusion SPECT uses a 180-degree arc from the 45-degree right anterior oblique to the left posterior oblique position with a 64 x 64 matrix and gated acquisition of typically eight frames per cardiac cycle; general purpose SPECT spans the full 360 degrees in a 128 x 128 matrix. Attenuation correction (Chang's method or CT-based) and a quarterly centre-of-rotation check are the technique's non-negotiable quality anchors.
What you must remember
- Acquisition arithmetic: 360-degree brain or bone SPECT takes 64-120 projections at 15-40 seconds each; cardiac SPECT takes 32-64 stops over 180 degrees; matrix 64 x 64 for cardiac, 128 x 128 otherwise.
- Collimator and energy: low-energy high-resolution collimator for Tc-99m with a 15-20% window at 140 keV; medium-energy for gallium-67 and indium-111; high-energy for iodine-131.
- Orbit discipline: circular or non-circular (body-contoured) orbit; keep the head as close to the patient as possible because spatial resolution degrades roughly linearly with source-to-collimator distance.
- Cardiac conventions: 180-degree acquisition from RAO 45 to LPO 45 (start either direction), gated to the ECG with 8 frames per cycle, stress and rest studies — commonly a same-day low-dose 250-300 MBq stress plus higher-dose 750-900 MBq rest Tc-99m protocol.
- Reconstruction filters: Butterworth (order 5-8, cut-off 0.3-0.5 cycles/pixel typical for cardiac) controls noise-streak trade-off; ramp filter alone amplifies noise; OSEM iterative reconstruction is the modern default.
- Corrections: uniformity (flood) correction from a daily 30 million-count flood, centre-of-rotation correction verified against a point source (drift beyond about half a pixel causes ring or blur), and attenuation correction (Chang's uniform method or CT-based hybrid).
- Clinical anchors: myocardial perfusion (sestamibi/tetrofosmin stress-rest), brain perfusion (HMPAO or ECD), dopamine transporter imaging for parkinsonism, bone SPECT for focal lesions, and parathyroid or sentinel node localisation.
Working through a gated cardiac perfusion study
Take a 58-year-old with exertional chest pain booked for a Tc-99m sestamibi stress-rest study on a same-day protocol. During stress the first dose (roughly 250-300 MBq) is injected at peak, and imaging follows 15-60 minutes later: the patient's left arm is raised, ECG electrodes attached, and the head sweeps 180 degrees from RAO 45 over 32-64 stops. Acquisition is gated into eight frames per cardiac cycle, so each projection is eight images stratified by cardiac phase — reject extrasystoles by beat-length windowing rather than aborting. Four hours later, the rest injection of about 750-900 MBq follows, and the second acquisition uses identical geometry, because the interpreter subtracts stress from rest frame for frame.
Reconstruction then decides diagnostic quality. Apply the Butterworth pre-filter before ramp back-projection, reorient the reconstructed slices along the heart's long axis into short-axis, vertical and horizontal long-axis views, and hand over for polar-map scoring and ejection fraction from the gated data. One artefact deserves ritual checking before reporting a fixed inferior defect: diaphragmatic or breast attenuation. A prone repeat shifts the heart off the diaphragm, and hybrid CT attenuation correction addresses the same — if the defect disappears, it was anatomy.
Where students slip
The examination trap is treating SPECT as "a gamma camera that rotates" with no concept of what rotation buys and costs. Candidates must say why resolution depends on radius — a head parked 10 cm away delivers mush — and why 180 degrees suffices for the heart (an anterior organ; the posterior arc adds spine-dominated noise) while the brain demands the full circle. The second classic confusion is uniformity versus centre-of-rotation: a daily flood corrects detector non-uniformity, but a ring artefact that survives a perfect flood points to centre-of-rotation drift, checked quarterly with a point source and corrected in software. Examiners also love asking which correction uses Chang's method — uniform attenuation correction of brain SPECT.
Frequently asked questions
Why does cardiac SPECT acquire over only 180 degrees?
The heart sits anteriorly, so the 180-degree RAO-to-LPO arc captures most cardiac photons while the posterior arc would add spine-dominated noise and scan time.
What matrix and frame count are standard for gated cardiac SPECT?
A 64 x 64 matrix with eight gated frames per cardiac cycle and 32-64 projection stops over 180 degrees is the conventional acquisition.
How does source-to-detector distance affect SPECT image quality?
Gamma-camera spatial resolution worsens approximately linearly with distance from the collimator, so the smallest comfortable orbit radius — ideally body-contoured — preserves resolution.
What is the centre-of-rotation check and how often is it done?
A point source imaged at opposite projections verifies the mechanical and electronic rotation centres align; drift beyond about half a pixel blurs or rings the reconstruction, so it is checked quarterly.
Why is a prone repeat acquisition performed after an inferior wall defect?
Prone positioning shifts the heart away from the diaphragm, distinguishing true inferior ischaemia from diaphragmatic attenuation artefact before any clinical conclusion is drawn.