Thermometry

Since these phantoms are intended to be used in heating studies with MRgFUS, it is important that they perform consistently through multiple heating cycles. Thermal repeatability testing was carried out on the three bloom-valued phantoms used for the MR-property determination by heating each phantom with a sequence of ultrasound exposures, interleaving lower power sonications with higher power exposures. During each exposure, 3D temperature maps were obtained with the proton resonant frequency (PRF)-shift MR-thermometry technique [19 (link)]. A 1-MHz 256-element phased-array transducer (Imasonic, Voray-sur-l’Ognon, France) with a focal distance of 13 cm (aperture diameter 14.5 cm, f-number 0.90) driven by electronics developed by Image Guided Therapy (Pessac, France) was employed for this testing. The heating parameters and order of each sonication for all blooms are provided in the next section. All powers provided were converted from electrical input watts to acoustic output watts using a calibration factor obtained with a radiation-force balance technique to measure the efficiency of the transducer. The 125-bloom and 175-bloom phantoms were exposed to fewer sonications based upon the experience with the 250-bloom phantom. Our initial thermal testing was performed on the 250-bloom phantom. We started at an initial low-power sonication of 6.6 W, then increased the power in small increments while interleaving with the 6.6-W low-power heating. By establishing the low- and high-power values and thereby setting the medium-power value, this allowed us to select fewer sonication powers for the 125-bloom and 175-bloom phantoms.
All heating was done with the geometric focus positioned 3 cm into the phantoms. A fiber optic temperature probe (Neoptix, Quebec, Canada) was inserted 4 cm into the other side of the phantom to measure the bulk temperature of the gelatin, approximately 8 cm away from the beam focus; all phantoms started at the MR suite’s ambient temperature (~24 °C).
The 3D MRI temperatures in the phantoms were obtained using a segmented GRE echo planar imaging (EPI) pulse sequence with TR 25 ms, TE 13 ms, FoV 192 × 96 × 32 mm, Res 1.2 × 1.2 × 2.0 mm (zero-filled interpolated to 0.5-mm isotropic voxel spacing), number of slices 16, FA 20°, BW 744 Hz/pixel, EPI factor 9, echo spacing 1.59 ms, acquisition time 3.625 s, with no fat saturation pulse applied. The start of each 18.125 s ultrasound-heating exposure was synchronized with the beginning of the sixth MR measurement using a fiber optic trigger pulse emitted by the pulse sequence. Between each heating exposure was a 10-min cooling period. Based on our previous experience with MR-temperature measurements in similar phantoms, the 10-min minimum-cooling interval was found sufficient to allow the heated region of the phantoms to return to a baseline-temperature value.

We used a focused ultrasound device consisting of 1024 individual transducers with a frequency of 220 kHz (ExAblate Neuro; InSightec Haifa). The device integrates intraoperative imaging, which was used for interim evaluations of the patient, and real-time acoustic monitoring to support decision-making on sonication parameters. On the day of the procedure, a Cosman-Roberts-Wells (CRW) stereotactic frame was fixed to the patient’s head under local anesthetic. The frame was then coupled to the helmet transducer array, with the patient entering the MRI supine and awake. A safety switch was given to the patient to abort the procedure in case of discomfort or pain. The patient was examined and questioned for adverse events after each sonication.
A 3-Tesla MRI (Signa MR750; GE Healthcare, Milwaukee, Wis.) was used to obtain T1, T2 (fast spin echo), and T2* (gradient echo) weighted images for surgical planning. A region in the right frontal lobe was then selected for BBB opening. To minimize the risk, we avoided areas containing sulci and vessels within two contiguous MRI slices in each of the axial, sagittal, and coronal planes. Once the target region was identified, patients received a weight-based intravenous injection of microbubble contrast (Definity®) (4 μl/kg), followed shortly by the application of low-frequency focused ultrasound to the target. MR thermometry was used to monitor tissue temperature at the sonicated region in real time. The sonication parameters were limited by the clinical device hardware and software, and corresponded to those previously tested in large animal models^{36 (link)}.
At each new target, a power ramp test was performed with the first microbubble injection. This test involves applying short sonications with increasing power in 5% increments until the device hydrophones detect a sub-harmonic acoustic feedback from the target, indicating a cavitation. Subsequent sonications are then performed at 50% of this ‘cavitation threshold’ power. The ramp test was developed from preclinical studies to determine the optimal power required for safe opening of the BBB^{36 (link),37 (link)}. Sonication volumes covered a rectangular spot approximately 9 mm by 9 mm, comprised of 3-by-3 grid of spots, each 3 mm in diameter. For the last three patients, given the extent of atrophy on their MRI, a 2-by-2 grid was utilized, yielding a spot approximately 5 mm by 5 mm. The device electronically steered the ultrasound through each grid for 50 s total, sonicating each spot with 2 ms on and 28 ms off bursts for 300 ms, with a repetition interval of 2.7 s (duty cycle 0.74%). For stage 2, performed approximately 1 month following stage 1, the procedure was repeated, opening the BBB at the original location as well as at an adjacent area, following the same protocol, but doubling the volume of tissue opened.
After completion of the sonication protocol, a gadolinium-enhanced T1 sequence was performed to verify definitive evidence of BBB opening. Contrast enhancement at the targeted region signified the end of the procedure. The patient was then taken out of the scanner, the stereotactic frame removed, and additional high-resolution MRI sequences obtained. Patients were admitted to the surgical short stay unit for overnight observation.

The phantom described in the previous section was used in the experimental setup, shown schematically in Figure 1C. For clarity we photographed several key elements of the setup in Figures 1D–H. The phantom was filled with water, in total around 2.6 L. A roller pump (Label 1) was used (WatsonMarlow 530S/R2, Falmouth, United Kingdom) to circulate the fluid from the outflow (Label 2) into two heat exchangers (Label 3) placed in separate water baths(Lauda aqualine AL12, Beun–De Ronde BV, Abcoude, the Netherlands) and then on to the inflow (Label 4). The heat exchangers consisted of two hollow copper coils through which the water was able to flow. The two coils were connected in series to ensure rapid heating and a stable inflow temperature. Both inflow and outflow temperatures were monitored. In the phantom we placed 9 multi-sensor type T thermocouple probes (Ella-CS, Hradec Kralove, Czech Republic), each having 7 measurement points, separated by1 centimeter with an accuracy of 0.01-0.1°C and an accuracy < 0.1°C. The probe locations are shown at Label 5. The thermocouple probes were placed such that the temperature was measured over a length of 7 centimeter on each location. In total, the temperature was measured at 7 × 9 = 63 locations in the phantom. In this way, we were able to capture an adequate representation of the thermal gradients in that region. The temperature was measured by a 196 channel thermometry system (Label 6) and monitored on a computer (Label 7). The thermocouples were linearly calibrated between 25-45°C using an Isotech TTI-10 thermometer with a probe (Isotech 935–14–61) that has an accuracy < 0.05°C. For some setups, the inflow tube was split into several inflow catheters. The radius of the circular inflow and outflow catheters was 3.5 mm and featured one single hole at the end of the catheter. The tips of the catheters were solid and fixed into position (see Figure 1D) during the experiments to prevent changes in position and/or orientation to occur due to the pulsatile nature of the roller-pump.

Magnetic resonance (MR) thermometry was used to noninvasively quantify the temperature rise associated with FUS stimulation at the motor cortex. Wild-type, untreated C57/BL6 mice (10–12 weeks) were used in this experiment. The age of these mice was slightly older to match the age of the mice used in the locomotor behavior test. The FUS transducer base plate was attached to the skull (as previously described), and the mice were allowed to recover for at least a week.
At the time of MR imaging, mice were anesthetized with isoflurane (2% for induction, 1.5% for maintenance). Similar to the FUS stimulation during behavior recording, the base plate and wearable ultrasound transducer were both sufficiently filled with degassed ultrasound gel, and the wearable transducer was plugged into the base plate of the mouse. Mice remained under anesthesia for the duration of MR imaging at 1.5% isoflurane. Mice were fixed in a small animal cradle, coupled to an MRI saddle coil (Image Guided Therapy). The cradle was then inserted into a 4.7 T MRI system (Agilent/Varian DirectDrive Console). Temperature maps were generated with a continuously applied gradient-echo imaging sequence with a flip angle of 20 degrees, T_R of 10 ms, T_E of 4 ms, slice thickness of 1.5 mm, and a matrix size of 128 × 128 for 60 × 60 mm field of view. Phase images were processed using ThermoGuide software (Image Guided Therapy). During MR imaging, the mouse rectal temperature was monitored throughout the duration of the experiment. The mouse body temperature was maintained at approximately 37 °C using warm air. During the ultrasound stimulation, six ultrasound stimuli were applied to the motor cortex with the same parameters used in the locomotor behavior assay.