DUBLIN - The combination of MRI with high-intensity focused ultrasound (HIFU) results in a noninvasive technique that precisely targets finely controlled amounts of energy to defined locations and is gaining acceptance, delegates learned today at the World Molecular Imaging Congress (WMIC).
The technique has received approval in two indications so far: ablation of uterine fibroids and palliative care of bone metastases in cancer. Clinical development efforts in the treatment of breast, liver, kidney, prostate, and brain cancers are under way, and MRI-HIFU also shows promise as a finely tuned system for highly localized delivery of drugs from temperature-sensitive liposome-based nanoparticles and for transporting molecules across the blood-brain barrier, according to Dr. Chrit Moonen, chair of the Image Sciences Institute at the University Medical Center in Utrecht, the Netherlands.
The WMIC opened today at the Convention Centre Dublin, known as the CCD. The venue claims to be Ireland's first purpose-built convention center, and is located in Spencer Dock in the heart of Dublin. Image courtesy of CCD."MR is the only method that allows in vivo temperature mapping," said Moonen, who was speaking at a WMIC spotlight session jointly organized by the European Society of Radiology (ESR) and the European Society of Molecular and Functional Imaging in Radiology (ESMOFIR). That attribute allows for the creation of a feedback control loop -- temperature data can be transmitted to a computer that controls the position and power of the ultrasound beam at the focal point.
"For the first time inside the body, you can really control the temperature, and that can be used in many different ways," he added.
In tumor ablation settings, motion of target organs -- due to the respiratory cycle, for example -- can be corrected for by the creation of a pretreatment temperature "atlas." Other precautions include the need to avoid burns caused by the presence of the ribcage in the HIFU beam path. Moonen and colleagues have developed an intercostal HIFU ablation method that is based on the pretreatment imaging of the "shadow" cast by the part of the rib cage responsible for blocking the beam. The corresponding transducer elements that are trained onto the ribs can be deactivated, with no loss of energy delivered to the focal point when the other transducer elements that contribute to the HIFU beam are turned on. The Utrecht team has obtained in vivo and ex vivo proof of principle in pig liver with this approach.
In drug delivery settings, focused ultrasound can be used to disrupt biological barriers, such as the blood-brain barrier or cell membranes, or to trigger the release of a drug payload from a nanoscale carrier. These approaches can help physicians to delivery drugs more precisely, ensuring that cytotoxic drugs, in particular, are trained on target tissues, while minimizing systemic exposure. The effects are not only mediated by temperature increase. The shear forces associated with an incident ultrasound beam can enhance the permeability of cells to drugs and can also cause leakage or extravasation from blood vessels, which can increase the diffusion of drugs or drug carriers into target tissues.
At the same session, Dr. Stephan Zangos, of the Institute for Diagnostic and Interventional Radiology at Johann Wolfgang Goethe-University in Frankfurt, Germany, described MRI-guided robotics for obtaining prostate tumor biopsies. Prototype robotic systems have now been developed for all three biopsy access routes -- transrectal, transperineal, and transgluteal.
Early clinical validation has been obtained for these systems, which offer improved accuracy with acceptable intervention times, he explained. Future directions include the development of interventional applications, such as laser ablation based on these systems.
