Zero TE MRI applications to transcranial MR‐guided focused ultrasound: Patient screening and treatment efficiency estimation

Abstract

Background The high acoustic impedance of the skull limits the performance of transcranial magnetic resonance‐guided focused ultrasound (tcMRgFUS) therapy. Subject suitability screening is based on skull parameters estimated from computed tomography (CT) scans. Purpose To assess the feasibility of screening for tcMRgFUS based on zero echo time (ZTE) MRI, and to explore the influence of measurable skull parameters in treatment performance. Study Type Retrospective. Population Sixteen patients treated with tcMRgFUS thalamotomy for tremor. Sequence ZTE on a 3.0T GE scanner. Assessment Baseline CT and ZTE images were processed to extract skull measures associated with treatment success: skull density ratio (SDR), skull thickness, and angle of incidence. Eight new metrics were proposed. CT and ZTE‐based measures were compared. Each subject's energy–temperature curve was processed to extract a global estimate of efficiency and a measure of nonlinearity. These parameters were then correlated with the skull measures. Statistical Tests Linear regression analysis to compare ZTE vs. CT‐based measures, measures vs. efficiency, and measures vs. nonlinearity. Paired t‐test to assess nonlinearity. Results CT and ZTE‐based measures were significantly correlated (P < 0.01). In particular, classical metrics were robustly replicated (P < 0.001). The energy–temperature curves showed a nonlinear (logarithmic) relationship (P < 0.01). This nonlinearity was greater for thicker skulls (P < 0.01). Efficiency was correlated with skull thickness (P < 0.001) and SDR (P < 0.05). Data Conclusion The feasibility of ZTE‐based screening has been proven, potentially making it possible to avoid ionizing radiation and the extra imaging session required for CT. The characterization of the influence that skull properties have on tcMRgFUS may serve to develop patient‐specific heating models, potentially improving control over the treatment outcome. The relationship of skull thickness with efficiency and nonlinearity empowers the role of this metric in the definition of such models. In addition, the lower association of SDR with the energy–temperature curves emphasizes the need of revisiting this metric. Level of Evidence: 3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:1583–1592.

Patient‐specific effects on sonication heating efficiency during magnetic resonance‐guided focused ultrasound thalamotomy

“…13 Consistent with these observations, subsequent studies have demonstrated a negative correlation between SDR and HE on univariate analysis. 8,[14][15][16][17][18] We expand upon these results, by demonstrating on multivariate regression that SDR is an independent negative predictor of HE (Table 1). Skull thickness has also been proposed to be a useful metric, whereby greater thickness is associated with decreased HE.…”

“…21 However, the literature on the relationship between individual patient skull morphology and the rate of HE decline is limited to one study. 18 Based on the data from a relatively small cohort of sixteen patients, univariate analysis showed that thicker skulls were associated with greater HE loss, whereas no correlation was observed with SDR. In contrast, we provide compelling evidence from a much larger cohort that SDR, but not thickness or area, is an independent predictor of the rate of HE decline (Table 2).…”

“…In addition, HE has been reported to decline across successive sonications throughout the course of treatment 5,18,21 , 22 . Interestingly, this phenomena is not observed in skull/tissue‐mimicking phantoms, 23 and is not explained by classical models of heat transfer to biological tissue 21,24 .…”

Transcranial Magnetic Resonance Imaging-Guided Focused Ultrasound Treatment at 1.5 T: A Retrospective Study on Treatment- and Patient-Related Parameters Obtained From 52 Procedures

“…Accordingly, evaluating this aspect will definitely be interesting, specifically for other currently investigated FUS applications in the field of neuroscience, such as brain tumor ablation [12,29,30], blood-brain barrier opening [13,14], and other non-thermal FUS effects [31], in which a much wider region could be considered for treatment and the ability to attain predefined temperature levels may vary based on the location of the target and the effect to achieve. Lastly, there are currently under investigation MRI-based methods to replicate CT-based measurements of skull properties associated with the success of tcMRgFUS procedures that may deserve attention in the next future [32].…”

Sonoselective transfection of cerebral vasculature without blood–brain barrier disruption

“…As the field of FUS-mediated therapeutic delivery to the brain gains momentum and moves closer to regulatory approval in human patients, there remains an ongoing concern over the potential dangers of neuroinflammation and petechiae due to damage caused by oscillating MBs. Additionally, variability in skull shape and thickness can result in variability in the bioeffects and treatment efficacy of FUS with MBs across and within test subjects (93)(94)(95). The concern over inflammation and treatment variability led to the development of monitoring systems to assess MB activity within the brain and the use of these observations to inform treatments.…”