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.

Keywords: CT; SDR; ZTE; focused ultrasound; thalamotomy.