Temperature-Sensitive Frozen-Tissue Imaging for Cryoablation Monitoring Using STIR-UTE MRI
Junichi Tokuda1, Qun Wang, Kemal Tuncali1
1From the Department of Radiology, Brigham and Women's Hospital and Harvard Medical School, Boston, MA.
Purpose:
The aim of this study was to develop a method to delineate the lethally frozen-tissue region (temperature less than -40°C) arising from interventional cryoablation procedures using a short tau inversion-recovery ultrashort echo-time (STIR-UTE) magnetic resonance (MR) imaging sequence. This method could serve as an intraprocedural validation of the completion of tumor ablation, reducing the number of local recurrences after cryoablation procedures.
Materials And Methods:
The method relies on the short T1 and T2* relaxation times of frozen soft tissue. Pointwise Encoding Time with Radial Acquisition, a 3-dimensional UTE sequence with TE = 70 microseconds, was optimized with STIR to null tissues with a T1 of approximately 271 milliseconds, the threshold T1. Because the T1 relaxation time of frozen tissue in the temperature range of -40°C < temperature < -8°C is shorter than the threshold T1 at the 3-tesla magnetic field, tissues in this range should appear hyperintense. The sequence was evaluated in ex vivo frozen tissue, where image intensity and actual tissue temperatures, measured by thermocouples, were correlated. Thereafter, the sequence was evaluated clinically in 12 MR-guided prostate cancer cryoablations, where MR-compatible cryoprobes were used to destroy cancerous tissue and preserve surrounding normal tissue.
Results:
The ex vivo experiment using a bovine muscle demonstrated that STIR-UTE images showed regions approximately between -40°C and -8°C as hyperintense, with tissues at lower and higher temperatures appearing dark, making it possible to identify the region likely to be above the lethal temperature inside the frozen tissue. In the clinical cases, the STIR-UTE images showed a dark volume centered on the cryoprobe shaft, Vinner, where the temperature is likely below -40°C, surrounded by a doughnut-shaped hyperintense volume, where the temperature is likely between -40°C and -8°C. The hyperintense region was itself surrounded by a dark volume, where the temperature is likely above -8°C, permitting calculation of Vouter. The STIR-UTE frozen-tissue volumes, Vinner and Vouter, appeared significantly smaller than signal voids on turbo spin echo images (P < 1.0 × 10), which are currently used to quantify the frozen-tissue volume ("the iceball"). The ratios of the Vinner and Vouter volumes to the iceball were 0.92 ± 0.08 and 0.29 ± 0.07, respectively. In a single postablation follow-up case, a strong correlation was seen between Vinner and the necrotic volume.
Conclusions:
Short tau inversion-recovery ultrashort echo-time MR imaging successfully delineated the area approximately between -40°C and -8°C isotherms in the frozen tissue, demonstrating its potential to monitor the lethal ablation volume during MR-guided cryoablation.
Insights
This study developed a novel short tau inversion-recovery ultrashort echo-time (STIR-UTE) MR imaging method to precisely map lethal cryoablation zones. This technique enhances intraprocedural validation and improves tumor ablation outcomes.
Area of Science:
- Medical Imaging
- Interventional Radiology
- Biophysics
Background:
- Cryoablation is a minimally invasive cancer treatment that uses extreme cold to destroy tumor tissue.
- Accurate intraprocedural monitoring of the lethal temperature zone is crucial for effective cryoablation and preventing recurrence.
- Current imaging methods for assessing the frozen tissue volume (iceball) have limitations in precision.
Purpose of the Study:
- To develop and validate a magnetic resonance (MR) imaging technique for delineating the lethally frozen-tissue region (below -40°C) during interventional cryoablation.
- To enable intraprocedural assessment of ablation completeness.
- To potentially reduce local tumor recurrence rates after cryoablation.
Main Methods:
- Utilized a short tau inversion-recovery ultrashort echo-time (STIR-UTE) MR imaging sequence, optimized for short T1 and T2* relaxation times of frozen tissue.
- Employed a 3D UTE sequence with radial acquisition and a specific echo time (TE = 70 microseconds).
- Evaluated the sequence in ex vivo frozen tissue and clinically in 12 MR-guided prostate cancer cryoablations.
Main Results:
- STIR-UTE imaging successfully identified tissue regions between -40°C and -8°C as hyperintense, distinguishing them from lethal (below -40°C) and non-lethal (above -8°C) zones.
- Clinical STIR-UTE images revealed distinct inner (dark, < -40°C) and outer (hyperintense, -40°C to -8°C) frozen volumes.
- The delineated STIR-UTE frozen volumes (Vinner, Vouter) were significantly smaller and more precise than traditional "iceball" measurements from turbo spin echo images.
Conclusions:
- Short tau inversion-recovery ultrashort echo-time (STIR-UTE) MR imaging effectively delineates the -40°C to -8°C isotherms within frozen tissue.
- This technique shows significant potential for real-time monitoring of lethal ablation volumes during MR-guided cryoablation.
- The findings suggest STIR-UTE can improve the accuracy of assessing cryoablation effectiveness.
