Molecular Imaging: fast and reliable stroke detection

After a stroke, a patient’s life depends on getting acute care at a hospital. Vital monitoring systems ensure safe and effective treatment. An innovative tomographic imaging system is designed to help prevent the patient’s risky journey to radiology and to enable bedside monitoring of cerebral blood flow.

In this MEDICA.de interview, Dr. Matthias Gräser talks about the innovative tomography imaging solution for the acute stroke unit, explains its functions and advantages and highlights how it reduces the workload for medical staff.

What prompted the idea to develop the diagnostic tomographic imaging system?

Dr. Matthias Gräser: Our team has worked at the Institute for Biomedical Imaging (IBI) for years to integrate Magnetic Particle Imaging (MPI) into clinical practice. Until now, the devices were still in the preclinical stage, meaning they are designed for small-animal experiments. Yet scaling for human application comes with its own set of added challenges. Not only do stringent medical safety standards have to be taken into account, but you must also ensure that the magnetic fields will not stimulate nerves and cause muscle twitching, for example. The research objective is to make the technology available for human application. To accomplish this goal, we chose a system that bridges a care gap in the acute stroke unit with simple technical means. Unlike the ribcage - an area where most imaging methods focus on -, the head is much smaller in size, allowing us to create a more compact system, which reduces the required power to achieve the magnetic fields. Plain image resolution suffices to detect a stroke, because the interrupted blood flow in the head usually affects large areas of several milliliters of brain volume.

How does the new MPI solution differ from conventional imaging systems such as CT scans or MRIs?

Gräser: Like an MRI, the MPI solution uses magnetic fields and likewise doesn’t expose the patient to ionizing radiation. Unlike the MRI, MPI does not depict the anatomy, but a contrast medium. The MPI solution, therefore, has a superior contrast to noise ratio, allowing the imaging system to detect even small traces of the contrast agent in the body without causing disruptive overexposure caused by the anatomy. The MPI technique also has a high refresh rate of up to 42 volumes per second. This enables it to map the quantitative cerebral blood flow in short intervals. The MPI system also has low magnetic fields of less than 100 millitesla compared to the strength of an MRI machine with up to 3 Tesla. This allows us to make our devices more flexible and mobile as is the case with MRI technology.

What does the MPI solution measure and how does it work?

Gräser: The system measures the spatial distribution of iron oxide nanoparticles, which serve as contrast agents. These particles want to align like tiny molecular magnets in the magnetic field. We can measure this alignment electronically in receive coils. By superpositioning static and variable magnetic fields, we assign a different alignment behavior at every position in the selection field. If the contrast agent is then injected into the bloodstream as a pharmaceutical, it disperses in the body. The nanoparticles are excited from the outside by several magnetic fields. We receive the sum of the signals from different positions in the receive coils. Prior calibration tells us how the contrast agent behaves at various points in the selection field. Subsequently, if an image is measured on the patient, we receive an overlay of all signals in the selection field and – based on our prior knowledge - use a system of linear equations to reconstruct the origin and distribution of signals.  We display this information as a medical grayscale image that looks somewhat similar to an MRI image.

What are the challenges of this system?

Gräser: The biggest challenge is the dual use of contrast agent and instrumentation. There are some MRI contrast agents that also work for MPI, but the ones that are best suited for this setting still have to be approved, which is a time-consuming and cost-intensive process. Another challenge lies in the prototype’s advancement into an electrically and medically safe system. Once that has been achieved, we can expect to create the first images pertaining to human beings.

Which aspects bridge the care gap in the acute stroke unit?

Gräser: Right now, acute stroke units don’t feature an imaging system. Having a system right at a patient’s bedside would be a notable benefit for physicians. Patient and physician wouldn’t have to make the risky journey through the hospital to the nearest CT scan room, while the physician would get the required information about the patient's condition directly on site.

At the intensive care unit, multiple life support and monitoring devices have to be attached to the stroke patient’s bed. What are the benefits of the new tomographic imaging system when it comes to the setup and installation?

Gräser: Our system only needs about 1.5 square meters of floor space. Currently, we also need a cabinet right next to it to accommodate additional technology. However, the technology will be integrated into the system down the road. Once the system will take up less space, it will be introduced to the acute care unit.

How does the new imaging system device reduce the workload for medical staff?

Gräser: Staff in the intensive care unit regularly checks the stroke patient’s brain functions. This includes monitoring the papillary reflex, and asking patients general questions if they are conscious. A CT scan is ordered if the brain function deteriorates – which is reflected in symptoms of memory loss or lack of response to sensory stimuli. The MPI system can monitor blood flow to the brain right in the intensive care unit and detect perfusion reductions of 30 percent in imaging phantoms. In a phantom, the patient typically doesn’t show any perfusion defect signs. However, since the system is highly sensitive, it detects brain function decline during neurological checks that are repeated every 30 minutes and immediately notifies the staff. Clinical trials must first confirm the reliability as it pertains to patients. That being said, this system has great potential.