Currently, detection methods for blood clots are confined to scanning select areas of the body. However, a new technique developed at Massachusetts General Hospital promises to end that state of affairs by granting doctors the ability to survey the entire circulatory system in one scan. Greg Noone talks to Dr Peter Caravan, associate professor of radiology at the hospital, about the imaging potential of Cu-FBP8.
A stroke occurs when a section of the brain experiences catastrophic blood loss, depriving it of oxygen and causing cell death. Depending on the area affected, symptoms can range from impaired movement and slurred speech to vascular dementia or even death. In the UK, 85% of strokes are ascribed to constricted blood flow from clots. It is therefore vital that doctors treating stroke victims can easily pinpoint where these are forming to prevent secondary incidents.
Existing scanning techniques can accomplish this, albeit in select and distinct areas of the human body, and in ways that impair the swift identification of clots. Only now, at the Institute for Innovation in Imaging at Massachusetts General Hospital (MGH), has the potential emerged for a unified scanning technique.
In research published in the journal Arteriosclerosis, Thrombosis and Vascular Biology as a paper entitled ‘Multisite Thrombus Imaging and Fibrin Content Estimation With a Single Whole-Body PET Scan in Rats’, Dr Peter Caravan and his colleagues have outlined a new method of PET imaging that could allow radiologists to detect not only the location of clots and thrombi wherever they might be in the body, but also their age and composition.
Flow interrupted
Since receiving his PhD in chemistry from the University of British Columbia, Caravan has been immersed in the field of imaging agents, contributing seven book chapters and 15 review articles on their properties, composition and correct usage. At the Institute for Innovation in Imaging, he oversees a lab dedicated to the creation of new probes for PET and MR imaging, with a particular focus on the detection of cardiovascular and other chronic diseases.
A major factor in the direction his team’s research has taken has been to sidestep the natural limitations on time and efficiency imposed by traditional clot-imaging techniques, most of which can only accurately target a specific portion of the body at a time.
"To give you an example, radiologists use computed tomography [CT] to look for pulmonary emboli in the lungs," says Caravan. "The patient is given a dye that makes the blood vessels appear bright on the CT scan. There, you see areas where the dye doesn’t reach, and that’s assumed to be the embolus in the lung."
Ultrasound scanning, meanwhile, is used to map clots in the carotid arteries or to look for deep vein thrombosis (DVT) in the legs. "Again, you’re seeing changes in the vein’s structure, but the imaging test doesn’t actually give you a molecular readout," Caravan explains. Moreover, the technique can also suffer from very basic restrictions in access to the areas of the body it is effective in surveying. "Ultrasound works well in the areas where vessels are accessible by a probe. However, if the patient is wearing a cast on their leg – which in itself could increase their chances of DVT – an ultrasound cannot work because the transducer cannot penetrate the cast."
This is a minor limitation compared with the problems caused by the lack of a unified scanning technique for clot identification, embodied by the fact that a third of ischemic strokes are still diagnosed as cryptogenic in origin. A common denominator needed to be identified.
"What we did in our study was target a protein called fibrin," says Caravan. "It’s present only in blood clots and healing wounds, but not in circulating blood." The team found they could pinpoint fibrin deposits by injecting a peptide called Cu-FBP8 into the patient’s bloodstream. By conducting a PET scan, they were then able to map the location of blood clots across the entire circulatory system.
Seeing the difference
For Caravan’s team, the ability of Cu-FBP8 to swiftly latch onto fibrin deposits in the bloodstream is a crucial step to doing away with this uncertainty altogether. "We evaluated a number of other very similar compounds and, while many of them showed efficacy, this one was the best in terms of its uptake into clots and, importantly, its clearance from the rest of the body," he says. "What we found in most of the other compounds we tested was that they partially degraded, leading to a higher background signal and less signal in the clot."
"It also doesn’t associate with other proteins and doesn’t stay for a long time in the bloodstream," Caravan adds. "That makes it different from other approaches that were taken previously – for example, the use of an antibody to fibrin. Although it accumulates at the clot, its circulation time is too long."
To test the compound’s effectiveness, the study’s lead author, Dr Francesco Blasi, induced clots in the femoral veins and carotid arteries of 32 Sprague Dawley laboratory rats. After undergoing a PET and CT scan one, three and then seven days following clot inducement, clots were identified with an accuracy of 97%. Together, the team discovered that the strength of the signal being generated by the Cu-FBP8 peptide decreased with the age of the clot and the size of the fibrin deposit.
"What we were able to show is that we had much greater uptake of the compound into younger, fresher blood clots than in older, more stable ones," says Caravan. "This is based off the fact that the relative amount of fibrin present in the blood clots goes down with time."
Preventative care
The ability to differentiate clots in this way has the potential to transform secondary stroke prevention. Often, doctors are more concerned that fresher clots, rather than their older and more stable counterparts, are more culpable for cardiovascular events. Distinguishing one from the other could potentially lead to more accurate treatments in the future.
"One application could lie in the management of patients with atrial fibrillation," explains Caravan. "This is a highly prevalent disease among older people, affecting more than seven out every 100 people over the age of 65. The condition also increases the risk of stroke as, when your heart is out of rhythm, the risk of blood clots in the left atrium and left atrial appendage increases. They can then embolise to the brain."
Identifying the presence of these clots in the heart could also help doctors identify who would best benefit from anticoagulant therapy. "Anticoagulants increase the risk of bleeding, so there’s a trade-off in certain patients who may be at greater risks of falls or other minor injuries," says Caravan. "The ability to scan those patients at certain times and look for the presence of thrombi in the left atrium would be very valuable in managing their care."
Caravan is hopeful that the next rungs of the regulatory ladder will be easy to climb. "What we have to do now is look at it in healthy volunteers to show that it’s safe, which we have no reason to believe it shouldn’t be," he says. "After that, we would start to look at specific patients with known blood clots, particularly those with atrial fibrillation and DVT, and determine how well PET imaging of this type works in them compared with the established techniques."
Questions remain as to the efficacy of the imaging technique once human trials begin. Although the basic principles of the scan seem transferable to the human body, this is not a certainty, with Caravan and his colleagues admitting in the paper that the rat model used in their experiments can "only partially mimic… human pathology". Moreover, while use of the Cu-FBP8 probe may prove faster than other techniques, there is nonetheless enough of a delay between stroke diagnosis and the PET scan to eliminate its use in the emergency room.
Nevertheless, it is a start, and pointing a way towards the reduction in the number of scans from four to one is in itself a startling accomplishment for the team at MGH. Whatever their results, there can be no doubt that the course of future trials will be closely watched by the medical community at large.
