Single Photon Emission Computed Tomography (SPECT) is a cornerstone of nuclear medicine imaging, providing valuable information about organ function, blood flow, and tissue metabolism. While PET has become increasingly prominent in oncology, SPECT remains widely used because of its accessibility, availability of tracers, and versatility in cardiology, neurology, and orthopaedics.
This training scenario introduces the workflow of SPECT imaging through a patient case at Westbrook Medical Centre. By exploring isotope properties, acquisition techniques, collimation, and reconstruction, you will see how image quality and diagnostic accuracy depend on both physics and clinical choices.
At the end of this scenario, you will complete a Knowledge Check Quiz to reinforce your understanding of SPECT principles.
Scenario: Maria’s Cardiac SPECT Study
Referral
Maria, a 63-year-old woman with hypertension and chest discomfort, was referred for a SPECT myocardial perfusion study. Her cardiologist wanted to evaluate blood flow to the heart muscle, especially under stress conditions.
SPECT is ideal for such cases, since it provides functional imaging of perfusion using widely available tracers such as Technetium-99m (Tc-99m) and Iodine-123 (I-123).
Radiopharmaceuticals and energies
Maria’s test used Tc-99m sestamibi, one of the most common isotopes in SPECT. Tc-99m has a primary photon energy of about 140 keV, which is well suited for gamma camera detection. Its half-life of around 6 hours makes it practical for diagnostic use.
Other isotopes sometimes used include thallium-201 or I-123 (with a half-life of about 13 hours). However, some radionuclides are not common in SPECT — and careful selection of energy and half-life is essential for balancing image quality and patient safety.
Gamma cameras and collimators
SPECT relies on gamma cameras to detect emitted photons. Each camera head is fitted with a collimator, typically made of lead, which channels photons travelling in specific directions.
Collimators define image resolution and sensitivity, but also cause trade-offs. For example:
- High-resolution collimators improve spatial detail but reduce sensitivity.
- High-sensitivity collimators capture more counts but blur small structures.
- High-energy collimators are necessary when imaging isotopes that emit higher-energy photons, to reduce septal penetration artefacts.
Maria’s scan used a low-energy high-resolution collimator optimised for Tc-99m.
Multi-head SPECT cameras
Westbrook used a dual-head SPECT camera. Compared with single-head systems, multi-head cameras improve sensitivity, reduce scan time, and enhance patient comfort. In some centres, triple-head cameras are used, particularly for dynamic studies requiring rapid acquisition.
Dynamic SPECT
Although Maria’s case involved static myocardial perfusion, the technologist mentioned that dynamic SPECT is sometimes performed to measure time-dependent processes such as tracer uptake and washout. This technique is particularly useful in assessing organ function, for example in renal imaging.
Spatial resolution and sensitivity
Maria was positioned carefully, since spatial resolution in SPECT depends strongly on the distance between the patient and the collimator. The closer the organ is to the detector, the sharper the image. Sensitivity, on the other hand, is most affected by collimator design and detector efficiency.
A balance between resolution and sensitivity is always required, guided by the clinical indication.
Artefacts and corrections
During the acquisition, Maria was asked to remain as still as possible. Patient movement is a common cause of artefacts in SPECT, leading to blurring or false perfusion defects.
Another issue in SPECT is septal penetration artefacts, where high-energy photons penetrate the thin septa of a collimator and create image distortion. These artefacts can be reduced by using appropriate high-energy collimators.
Attenuation correction was also performed, using the integrated CT component of the hybrid SPECT-CT scanner. This correction compensates for tissue absorption of photons, especially important in the chest, where breast tissue or the diaphragm may reduce apparent tracer uptake.
Reconstruction methods
After data collection, the images were reconstructed using iterative reconstruction techniques, which have largely replaced traditional filtered back projection. Iterative methods account for system geometry and physics, improving contrast resolution and reducing noise.
Frequency and resolution concepts
The physicist explained to Maria’s trainees that the Nyquist frequency is relevant in digital SPECT, defining the maximum spatial frequency that can be sampled without aliasing. Adequate sampling is crucial to accurately reconstruct fine details.
Contrast resolution in SPECT can be improved by scatter correction, collimator choice, and iterative reconstruction methods.
Effect of energy selection
One question often discussed in teaching is: What happens when photon energy settings are increased? Increasing energy can reduce scatter, potentially improving image quality, but it also decreases sensitivity because fewer photons fall within the detection window. Optimising energy windows is therefore critical.
Clinical outcome
Maria’s reconstructed SPECT-CT images revealed a perfusion defect in the lateral wall of the left ventricle, consistent with coronary artery disease. The findings helped guide her cardiologist in planning further management, including possible revascularisation.
Key Concepts Reinforced
Through Maria’s case, the following SPECT principles were illustrated:
- SPECT stands for Single Photon Emission Computed Tomography.
- Tc-99m is the most common isotope used, with 140 keV photons.
- I-123 has a half-life of about 13 hours.
- Collimators are made of lead and determine spatial resolution and sensitivity.
- Multi-head cameras improve efficiency and patient throughput.
- Dynamic SPECT can assess tracer kinetics and organ function.
- Patient motion is a common cause of artefacts.
- High-energy collimators reduce septal penetration artefacts.
- Attenuation correction is commonly performed using SPECT-CT.
- Iterative reconstruction is standard in modern SPECT, improving image quality.
- Nyquist frequency relates to sampling requirements in digital imaging.
- Increasing energy reduces scatter but decreases sensitivity.
- Spatial resolution is most affected by the collimator and patient distance.
Conclusion
Maria’s case demonstrates how isotope selection, collimator design, patient preparation, and reconstruction methods determine the quality and reliability of SPECT imaging. While artefacts and limitations exist, careful optimisation of acquisition parameters ensures accurate assessment of cardiac perfusion and other functional processes.
SPECT remains an essential tool in clinical practice, complementing PET and anatomical imaging modalities by providing unique functional insights.
Knowledge Check
You have now reviewed a clinical scenario introducing the principles of SPECT imaging. The following knowledge check quiz will test your understanding of isotopes, collimators, artefacts, sensitivity, reconstruction, and clinical applications.
Instruction: Select the best answer from the options provided. Refer back to the scenario if needed, and use it to guide your responses. Completing the quiz will help consolidate your knowledge and prepare you for applying SPECT principles in clinical practice.
Disclaimer
This training scenario has been created for educational purposes only. It is designed to illustrate the physics, technical principles, and clinical workflow of SPECT imaging through a fictional patient case.
The content does not replace professional medical training, institutional protocols, or official regulatory guidance. Qualified healthcare professionals must always make clinical decisions regarding nuclear medicine imaging and patient management in accordance with established safety standards and local regulations.
All patients, hospitals, and case details described in this scenario are fictional and included solely to support learning objectives. Learners should always apply the ALARA principle, follow institutional policies, and consult accredited references when working with ionising radiation in clinical practice.
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