M7 SimPet™

Aspect Imaging and Brightonix Imaging have partnered to provide the first truly simultaneous PET/MRI system utilizing the compact, permanent magnet, technology from Aspect Imaging. This system combines all of the benefits of the M-Series high-performance MRI systems with the state-of-the-art SimPET insert from Brightonix Imaging.

The M7 system from Aspect Imaging is a self-shielded, cryogen free, MRI system which can be installed right next to other imaging equipment or within an animal facility. This system provides optimized imaging protocols for a variety of anatomical targets, thus providing complimentary 3D anatomical images to those acquired using the SimPET insert.

The SimPET-S and L model insert, from Brightonix, have been optimized for use within the M7 system. Brightonix has created an advanced silicon photomultiplier (SiPM) based PET insert for truly simultaneous PET/MR imaging. The engineering team at Brightonix has developed a state-of-the-art, compact, insert with low power consumption and excellent PET detector stability.

Aspect Imaging and Brightonix Imaging have partnered to provide the first truly simultaneous PET/MRI system utilizing the compact, permanent magnet, technology from Aspect Imaging.

The M7 system from Aspect Imaging is a self-shielded, cryogen free, MRI system which can be installed right next to other imaging equipment or within an animal facility. This system provides optimized imaging protocols for a variety of anatomical targets, thus providing complimentary 3D anatomical images to those acquired using the SimPET insert.

The M7 system is ideally suited for MR imaging of mice, rat, and similarly sized animals, however when the SimPET insert is installed the imaging volume is limited to that of mice an similarly sized animals. The SimPET is loaded from one side of the magnet, while the M7 animal handling system with RF coil is loaded from the other. Aspect Imaging has created an RF coil optimized for use with the SimPET insert to ensure true compatibility of the two imaging modalities.

a
In the video above the M7 animal handling system enters the bore from the right, with the RF coil in place, while the SimPET insert enters from the left to incircle the RF coil.

The SimPET-S and L model insert, from Brightonix, have been optimized for use within the M7 system. Brightonix has created an advanced silicon photomultiplier (SiPM) based PET insert for truly simultaneous PET/MR imaging. The engineering team at Brightonix has developed a state-of-the-art, compact, insert with low power consumption and excellent PET detector stability.

There are four (4) different models of SimPET inserts available, they vary in inner/outer diameter and axial field of view. As mentioned above the SimPET-S and SimPET-L models have been optimized for use with the M7 MRI system from Aspect Imaging; the other models are optimized for use with some of the most commonly used high-field (>3T) superconducting MRI systems from preclinical imaging. Please refer to the Brightonix Imaging Technical Specifications for complete details.

The PET workstation software is intuitive and easy to use, as is the workstation for the M7 system. The images are acquired simultaneously by both systems, and co-registered within InviCROs VivoQuant visualization software provided with the systems.

On the PET workstation the user is provided with real-time count rate monitoring, FastTomo reconstruction, along with in-line image reconstruction, all with flexible list-mode data acquisition. Calibration procedures for quality control, PET/MR geometry, as well as count rate/activity are all included within the software. Quantification is provided in Bq/ml or standardized uptake value (SUV).

The highly sensitive LSO (lutetium oxyorthosilicate) detectors provide a best-in-class sub-millimeter spatial resolution while maintaining count-rate performance to allow for both low and high-dose imaging applications.

Simultaneous acquisition of both the PET and MR images on the M7 SimPETTM allows for both spatial and temporal understanding of the PET signal with regards to the anatomical image. MRI is the gold standard in soft tissue imaging, providing not only anatomical reference for the PET signal, but may also in itself provide some additional information with regards to the tissues being imaged. Various image weighting protocol exist to help identify abnormal or pathological tissues, along with the added benefit of using specific and perhaps multi-modal contrast agents to further elucidate the molecular pathway or imaging target of interest.

There are several key benefits of the M7 SimPETTM system:

  • SimPET Insert from Brightonix Imaging:
    • Exceptional PET performance, optimized for standalone or simultaneous with MR imaging
    • Highly sensitive detectors provide sub-millimeter spatial resolution
    • State-of-the-art silicon photomultiplier technology allows for compact design with low power consumption
    • Acquisition and reconstruction software are intuitive and simple to use
  • M7 High-Performance MRI system from Aspect Imaging:
    • Compact, permanent magnet, which is self-shielded making it easy to install right next to other imaging or laboratory equipment
    • No cryogens, or liquid of any kind for cooling, simple fans are used as needed to pull room air over the gradients, making ongoing running and maintenance costs minimal
    • Optimized image acquisition for numerous imaging targets, allowing biologists without a background in MR imaging to acquire high quality images on their imaging target
    • Simultaneous PET/MR imaging or standalone use
  • M7 SimPETTM System:
    • Fully integrated system with no compromise to MR or PET image quality or homogeneity
    • MRI-based attenuation correction of PET images
    • Complete system installed in one day
Oncology is one of the most common applications of PET/MRI in preclinical imaging. Used clinically to detect small metastatic lesions throughout the body, preclinical applications of PET imaging are diverse. PET tracers have been developed to study:

  • Cell proliferation
  • Apoptosis
  • Angiogenesis
  • Metastasis
  • Gene expression of specific molecular targets
  • Receptor-ligand interactions
  • Substrate transportation
  • Nutrient metabolism

PET imaging may be used on any type of tumor model, as the radiotracers are typically injected intravenously for distribution to all areas of the body. Specifically, PET imaging may be used on orthotopic and transgenic/spontaneous tumors, as well as xenografts or metastatic lesions.

The simultaneously acquired MR image may provide additional insights about the tumors themselves, or surrounding anatomical

The image above shows the effect of tumor associated macrophages on tumor hypoxia and aerobic glycolysis, displayed in values of standard uptake values. A T2-weighted fast spin echo (FSE) image was acquired on the M7, compact, permanent magnet MRI system from Aspect Imaging; PET images were acquired simultaneously with the SimPET-S model.

Neurological applications of PET/MRI are commonplace both clinically and preclinically. Preclinical PET tracers have been developed to study:

  • Biodistribution of a specific target
  • Cerebral blood flow
  • Cerebral metabolic rate
  • Availability of specific receptors in the brain
  • Dopamine transmission
  • Plasma membrane transporters
  • Receptor binding sites

PET imaging has been used to study the pathological processes, response to therapeutics, and resolution of many disease models, including Parkinson’s Disease, Alzheimer’s Disease, Huntington’s Disease, Stroke, Epilepsy, Traumatic Brain Injury, and Neuropsychiatric disorders.

The co-registration of the PET and MR images provides even greater information to the research with regards to the location of the PET tracer within the various structures of the brain, as well as additional anatomical information which may only be available on the MR image. Multi-modal contrast agents may also help elucidate further the various questions being investigated with these powerful imaging modalities.

Cardiology applications of PET are commonplace both clinically and preclinically. Preclinical PET tracers have been developed to study:

  • Myocardial perfusion to examine the extent of stenosis and severity of obstruction
  • Myocardial metabolism
  • Myocardial viability
  • Infarct assessment
  • Calcium scoring in coronary artery disease
  • Inflammation and plaque development for risk stratification

Many different cardiovascular diseases may be investigated using PET imaging, including coronary artery disease, myocardia infarction, or heart failure.

Co-registration of the PET and MR images provides additional information about the heart and surrounding vasculature, to further elucidate the answers to the questions being asked during these imaging studies.

The recent COVID-19 pandemic has brought to the forefront the importance of PET imaging in immunological and infectious disease research. A survey of the literature shows that preclinical PET imaging has been shown to be invaluable in studying the following:

  • Understanding disease progression and pathogenesis
  • Diagnosis of disease, by targeting the specific pathogenic agent
  • Studying therapeutic efficacy of target compounds
  • Optimizing treatment regimes and studying the resolution of infections
  • Understanding the host’s immune response to infection
  • Studying the efficacy of vaccines

Infectious diseases and immunology are not limited to the study of viruses, but rather go well beyond these specific agents to look at models of bacterial, viral, parasite, and prion infections. The only limitation to this type of work is in the PET tracers available to investigate both the infectious agent as well as the host’s response.

The anatomical information provided by the simultaneously acquired MR image, in these specific studies, may help researchers to better understand the exact location of the infectious agent being studied, or further elucidate the hosts response to the infection or response to the therapy being studied.

Dynamic PET imaging is often used to study the kinetics of a newly developed PET tracer, or to investigate target concentrations, or biodistribution. Time-activity curves are often prepared along with assessments of radiotracer accumulation and biodistribution kinetic modeling.

Simultaneously acquired MR images help to provide the anatomical context necessary to fully understand the dynamic PET images in these types of studies.

The above set of images shows a dynamic scan acquired with the SimPET-X insert, having an axial field of view of 11cm. A mouse was injected, via the tail vein, with 331µCi [18F]-fluorodeoxygluocose (FDG).

A variety of bone diseases may be studied using PET imaging, these include but are not limited to osteoporosis, osteomalacia, rickets, or rheumatoid arthritis.

A wide variety of other diseases, including metabolic disorders, can also be studied using PET imaging.

There is truly no limit on the application of PET in preclinical imaging, it is merely dependent on the development of specific and relevant PET tracers.

The anatomical information provided by the MR images provide the context for the PET signal, and may help further elucidate the answers to the biological questions being asked in these types of studies.

A wide variety of other diseases, including metabolic disorders, can also be studied using PET imaging.

There is truly no limit on the application of PET in preclinical imaging, it is merely dependent on the development of specific and relevant PET tracers.

The above image shows increased metabolic activity in the rear paws in the mouse model of arthritis. These images were acquired using the SimPET-S insert along with the M7, compact, permanent magnet from Aspect Imaging, following an i.v. injection of  300µCi of [18F]- fluorodeoxygluocose (FDG). The K/BxN model is a commonly used mouse model to study arthritic disease. The gradient echo (GRE) MR image, acquired on the M7 system, provides the anatomical context for the PET images.

A recent publication in Molecular Imaging and Biology provides the full details of the system performance, a summary of some key outcomes is provided here.

This authors followed the NEMA NU 4-2008 standard to assess the performance of the SimPET-S insert, along with it’s performance within the M7, compact, permanent magnet MRI system from Aspect Imaging.

The spatial resolution, at the center of the field of view, based on the 3D OSEM reconstruction was found to be 0.7mm without a warm background and 1.45mm with a warm background.

In the hot rod phantom image, seen below, the 0.75mm diameter rods were resolved with incorporation of the point spread function.

In the image above a hot rod phantom was imaged, rods with diameters of 0.75, 1.0, 1.35, 2.0, and 2.4mm were included. Reconstruction was completed using a 3D OSEM algorithm. The point spread function has been incorporated in a, while it was not incorporated in b.

The sensitivity of the SimPET-S was found to be 4.21% (energy window = 250-750 keV) in this study.

Above, is the NEMA sensitivity plots, measured at the transaxial center, for the SimPET-S at various energy windows.

The NEC rate of the SimPET-S was found to be 151 kcps at 38.4 MBq, using the same energy window as the sensitivity testing (250-750 keV);

The above is the NEMA Count Rate Performance measurements made for the SimPET-S model insert

There were no remarkable differences in signal to noise ratio (SNR) and uniformity of the MR images and PET count rates when using different PET conditions and MRI pulse sequences.

The image above demonstrates the compatibility of the SimPET insert with the M7 system from Aspect Imaging. Various MRI pulse sequences including T2-weigheted Fast Spin Echo (T2WFSE), T1-weighted Spin Echo (T1WSE), and a 3D Gradient Echo (3DGRE).
When used with the M7, the compact permanent magnet from Aspect Imaging, the SimPET-S insert is placed within the bore of the magnet from one side, while the animal handling system with the RF coil is inserted from the other side of the magnet fitting within the PET insert.
The image above shows the SimPET-S insert positioned to enter into the M7 MRI system from Aspect Imaging. The animal handling system with RF coil enters the M7 from the opposite side.
The diagram above shows the configuration of the SimPET-S insert positioned within the bore of the M7 MRI system from Aspect imaging.