Transcranial cavitation detection in primates during blood-brain barrier opening--a performance assessment study

Abstract

Focused ultrasound (FUS) has been shown promise in treating the brain locally and noninvasively. Transcranial passive cavitation detection (PCD) provides methodology for monitoring the treatment in real time, but the skull effects remain a major challenge for its translation to the clinic. In this study, we investigated the sensitivity, reliability, and limitations of PCD through primate (macaque and human) skulls in vitro. The results were further correlated with the in vivo macaque studies including the transcranial PCD calibration and real-time monitoring of blood-brain barrier (BBB) opening, with magnetic resonance imaging assessing the opening and safety. The stable cavitation doses using harmonics (SCDh) and ultraharmonics (SCDu), the inertial cavitation dose (ICD), and the cavitation SNR were quantified based on the PCD signals. Results showed that through the macaque skull, the pressure threshold for detecting the SCDh remained the same as without the skull in place, whereas it increased for the SCDu and ICD; through the human skull, it increased for all cavitation doses. The transcranial PCD was found to be reliable both in vitro and in vivo when the transcranial cavitation SNR exceeded the 1-dB detection limit through the in vitro macaque (attenuation: 4.92 dB/mm) and human (attenuation: 7.33 dB/ mm) skull. In addition, using long pulses enabled reliable PCD monitoring and facilitate BBB opening at low pressures. The in vivo results showed that the SCDh became detectable at pressures as low as 100 kPa; the ICD became detectable at 250 kPa, although it could occur at lower pressures; and the SCDu became detectable at 700 kPa and was less reliable at lower pressures. Real-time monitoring of PCD was further implemented during BBB opening, with successful and safe opening achieved at 250 to 600 kPa in both the thalamus and the putamen. In conclusion, this study shows that transcranial PCD in macaques in vitro and in vivo, and in humans in vitro, is reliable by improving the cavitation SNR beyond the 1-dB detection limit.

Fig. 1

In vitro experimental setup. The cranial part of the macaque skull (including frontal bone, parietal bones, and occipital bone) was 3.09-mm thick in average of the beam-path region, and the human skull (including the frontal and the parietal bones) was 4.65-mm thick.

Fig. 2

In vitro cavitation monitoring: spectrograms. (a) Sonicating water without the skull in place. (b) Sonicating microbubbles without the skull in place. (c) Sonicating microbubbles with the macaque skull in place. (d) Sonicating microbubbles with the human skull in place. (i), (ii), (iii), and (iv) represents 50 kPa, 150 kPa, 200 kPa, and 450 kPa, respectively. The colorbar shows the intensity of the spectra, with a dynamic range of 25 dB and 15 dB for the macaque and human skull experiments, respectively, based on the preamplification (macaque: 20 dB, human: 10 dB).

Fig. 3

In vitro cavitation monitoring: B-mode images in transverse plane after the sonication. (a) Without the skull in place using 100 cycles. (b) With the macaque skull in place using 100 cycles. (c) With the human skull in place using 100 cycles. (d) Without the skull in place using 5000 cycles. (i), (ii), (iii), and (iv) represents 50 kPa, 150 kPa, 200 kPa, and 450 kPa, respectively. The arrows indicate the spot losing echogenicity at the pressure threshold (200 kPa). The images showed good focal alignment to the channel and the bubbles lost the property of contrast enhancement at or above 200 kPa. The shape to the hypoechogenitic area was roughly a circle with an averaged diameter of 1.3 mm at 200 kPa and 4 mm at 450 kPa.

Fig. 4

In vitro cavitation doses. (a) SCD_h, (b) SCD_u, and (c) ICD for the macaque skull experiments using 100-cycle pulses. (d) SCD_h, (e) SCD_u, and (f) ICD for the human skull experiments using 100-cycle pulses. (g) SCD_h, (h) SCD_u, and (i) ICD without the skull in place using 100- and 5000-cycle pulses. The error bar shows the standard deviation. *: p<0.05. Green *: comparison made in the cases without the skull in place. Red *: comparison made in the cases with the skull in place. All of the comparisons in (g)–(i) showed statistical significance. All of the cavitation doses became detectable at 50 kPa, while this detectable pressure threshold may change after placing the skull. The nonlinear effect of the skull was seen after placing the human skull at high pressures as the SCD_h increased significantly. Applying long pulses (5000 cycles) was effective in generating high cavitation doses at low pressures when compared with applying short pulses (100 cycles).

Fig. 5

In vitro cavitation SNR (a) without the skull in place using 100-cycle pulses, (b) without the skull in place using 5000-cycle pulses, (c) with the macaque skull in place using 100 cycles, and (d) with the human skull in place using 100 cycles. The error bar shows the standard deviation. The dash lines in (a) represent the SNR threshold for surpassing the skull attenuation (macaque: 15.2 dB, human: 34.1 dB). For SNR higher than 1 dB, the detected cavitation doses were significantly higher than that of control. This 1 dB was deemed as the detection threshold with and without the skull.

Fig. 6

In vivo cavitation doses using 100- and 5000-cycle pulses. (a) SCD_h. (b) SCD_u. (c) ICD. *: p<0.05. The error bar shows the standard deviation. When using 5000-cycle pulses, the SCD_h became detectable transcranially at 100 kPa; for the ICD, 250 kPa; while the SCD_u was unrealiable and could be detected at high pressures. When using 100-cycle pulses, the pressure threshold in detecion increased.

Fig. 7

In vivo cavitation SNR using (a) 100-cycle and (b) 5000-cycle pulses. The error bar shows the standard deviation. The cavitation SNR using 100-cycle pulses increased with pressure. When using 5000-cycle pulses, the SNR of the SCD_u and ICD increased with pressure, while it for the SCD_h reached plateau due to the nonlinear effect of the skull at high pressures.

Fig. 8

In vivo BBB opening at (a) 275 kPa, (b) 350 kPa, (c) 450 kPa, and (d) 600 kPa in the thalamus (orange arrow) or the putamen (green arrow). The upper and middle rows show the post-contrast T1 weighted images with calculated enhancement (with colorbar) in axial and coronal view, respectively. The opening volume of (a) to (d) was 494.7, 230.9, 112.9, and 299.2 mm³, respectively. The bottom row shows the real-time monitoring of the SCD_h, SCD_u, and ICD for sonicating the thalamus. Note that in (d) the microbubbles were injected before the sonication was started. Case (b)–(d) were performed in the same macaque.

Fig. 9

Safety assessment using MRI at (a) 275 kPa, (b) 350 kPa, (c) 450 kPa, and (d) 600 kPa. The upper row shows the T2-weighted images (coronal view) for detecting the edema, which is lighter if occurred. The lower row shows the SWI (coronal view) for detecting the hemorrhage, which is darker if occurred. In all cases, no hemorrhage and edema was detected.