Single photon emission computed tomography (SPECT) is combined with MRI to form a hybrid technology used in preclinical applications mainly in tumor and neuroscience imaging. SPECT integrated with MRI enhances SPECT quantification. The use of hybrid imaging tools, such as SPECT/MRI, in preclinical small animal imaging has highly increased in the past decade due to advances in technology.
These hybrid tools offer considerable improvements in sensitivity as well as temporal and spatial resolution. They are non-invasive preclinical imaging modalities that enable molecular and physiological imaging of in vivo processes over a period of time. This removes the need for killing animals at various stages of these research studies.
SPECT/CT limitations such as radiation exposure and low soft tissue contrast prompted the integration of SPECT with MRI. Combined SPECT/MRI systems were first introduced in 2007. The initial systems used a low-strength MR field which was not well suited for preclinical research. Thus, systems with a high strength magnetic field and enhanced sensitivity were developed later.
Principles of SPECT/MR
Combined SPECT/MR imaging tools allow researchers to acquire in vivo tomographic images of small animals such as mice. While the SPECT tool offers functional information, the MR system gives anatomical data.
In earlier days, mutual interference was a big challenge to overcome when designing SPECT/MR systems. The latest SPECT technology fits well into traditional high-field MRI systems with a minimum bore size of 12cm. SPECT measures the intensity of γ-radiation emitted by radio-nuclei attached to compound molecules in a sample and provides corresponding tomographic images. MRI systems give tomographic images using signals coming from atomic nuclei that interact with the magnetic field.
Different combined SPECT/MR approaches include:
The 2 systems are placed side by side and small animals are transported from one device to another. It is also known as sequential SPECT-MR imaging
This approach of combined SPECT/MR involves mounting the two systems back to back on a single platform. Images from both the tools can be acquired one after another within a short interval. This enables performing simultaneous SPECT and MR imaging.
Advantages of combined SPECT /MR
Since both SPECT and MR have unique capabilities, the combined technique is highly powerful. Some of the advantages of SPECT/MR include the following:
- They have better temporal and spatial resolution
- Can measure morphology as well as function
- MRI offers high spatial resolution which is ideal for tissue phenotyping and anatomical imaging
- Both sequential and simultaneous imaging is possible
- Availability in various configurations and trans-axial field of views, which allows imaging a wide range of small animals
Weaknesses of SPECT/MR
- In sequential SPECT-MR, time is needed for specimen transfer between the SPECT and MR systems, which is not an issue with simultaneous SPECT-MR
- Apart from image timing, other challenges may include image registration and anesthesia times, which may influence the final results
- Researchers need to be well trained to interpret both SPECT and MR outputs
Applications of SPECT/MR
Several studies have been published regarding the preclinical applications of combined SPECT / MRI in regenerative medicine and cardiovascular research.
Some of the main applications include:
- Tumor imaging
- Imaging targets and probes
- Biodistribution studies/dosimetry/response assessment
- In neuroscience
- Preclinical studies in small animals with neurodegenerative diseases
- Preclinical neuro-oncology studies