Toward MR-guided high intensity focused ultrasound for presurgical localization: focused ultrasound lesions in cadaveric breast tissue

Purpose:

To investigate magnetic resonance image‐guided high intensity focused ultrasound (MR‐HIFU) as a surgical guide for nonpalpable breast tumors by assessing the palpability of MR‐HIFU‐created lesions in ex vivo cadaveric breast tissue.

Materials and Methods:

MR‐HIFU ablations spaced 5 mm apart were made in 18 locations using the ExAblate2000 system. Ablations formed a square perimeter in mixed adipose and fibroglandular tissue. Ablation was monitored using T1‐weighted fast spin echo images. MR‐acoustic radiation force impulse (MR‐ARFI) was used to remotely palpate each ablation location, measuring tissue displacement before and after thermal sonications. Displacement profiles centered at each ablation spot were plotted for comparison. The cadaveric breast was manually palpated to assess stiffness of ablated lesions and dissected for gross examination. This study was repeated on three cadaveric breasts.

Results:

MR‐ARFI showed a collective postablation reduction in peak displacement of 54.8% ([4.41 ± 1.48] μm pre, [1.99 ± 0.82] μm post), and shear wave velocity increase of 65.5% ([10.69 ± 1.60] mm pre, [16.33 ± 3.10] mm post), suggesting tissue became stiffer after the ablation. Manual palpation and dissection of the breast showed increased palpability, a darkening of ablation perimeter, and individual ablations were visible in mixed adipose/fibroglandular tissue.

Conclusion:

The results of this preliminary study show MR‐HIFU has the ability to create palpable lesions in ex vivo cadaveric breast tissue, and may potentially be used to preoperatively localize nonpalpable breast tumors. J. Magn. Reson. Imaging 2012;35:1089‐1097. © 2011 Wiley Periodicals, Inc.     

Volumetric MRI‐guided high‐intensity focused ultrasound for noninvasive,in vivo determination of tissue thermal conductivity: Initial experience in a pig model

Purpose:

To estimate the local thermal conductivity of porcine thigh muscle at temperatures required for magnetic resonance imaging (MRI)‐guided high‐intensity focused ultrasound (MRgHIFU) surgery (60–90°C).

Materials and Methods:

Using MRgHIFU, we performed 40 volumetric ablations in the thigh muscles of four pigs. Thirty‐five of the sonications were successful. We used MRI to monitor the resulting temperature increase. We then determined local thermal conductivity by analyzing the spatiotemporal spread of temperature during the cooling period.

Results:

The thermal conductivity of MRgHIFU‐treated porcine thigh muscle fell within a narrow range (0.52 ± 0.05 W/[m*K]), which is within the range reported for porcine thigh muscle at temperatures of <40°C (0.52 to 0.62 W/[m*K]). Thus, there was little change in the thermal conductivity of porcine thigh muscle at temperatures required for MRgHIFU surgery compared to lower temperatures.

Conclusion:

Our MRgHIFU‐based approach allowed us to estimate, with good reproducibility, the local thermal conductivity of in vivo deep tissue in real time at temperatures of 60°C to 90°C. Therefore, our method provides a valuable tool for quantifying the influence of thermal conductivity on temperature distribution in tissues and for optimizing thermal dose delivery during thermal ablation with clinical MRgHIFU. J. Magn. Reson. Imaging 2013;37:950–957. © 2012 Wiley Periodicals, Inc.     

声脉冲辐射力成像技术对子宫腺肌病的诊断价值

目的观察子宫腺肌病（AM）患者月经周期中病变肌层声触诊组织量化VTQ值变化规律与特点,探讨应用声脉冲辐射力成像（ARFI）技术提高该病早期诊断率的可行性。方法应用超声声脉冲辐射力成像（ARFI）模式声触诊组织量化（VTQ）功能分别于月经周期的卵泡早期和黄体中期观测子宫腺肌病患者病变肌层声触诊组织量化VTQ值,分析对比观察组2个时期间及两组相应时期间VTQ值。结果黄体中期病变肌层VTQ值高于卵泡早期VTQ值,且差异有统计学意义（P〈0．05）。结论超声声脉冲辐射力成像（ARFI）技术能实时、敏感检测子宫腺肌病VTQ值,而且可重复性好．同时能体现子宫腺肌病肌层病变随月经周期性改变,有望成为子宫腺肌病诊断手段之一,从而为提高该病的早期诊断率提供有效依据。     

Design and development of a prototype endocavitary probe for high-intensity focused ultrasound delivery with integrated magnetic resonance imaging

Purpose

To integrate a high intensity focused ultrasound (HIFU) transducer with an MR receiver coil for endocavitary MR‐guided thermal ablation of localized pelvic lesions.

Materials and Methods

A hollow semicylindrical probe (diameter 3.2 cm) with a rectangular upper surface (7.2 cm × 3.2 cm) was designed to house a HIFU transducer and enable acoustic contact with an intraluminal wall. The probe was distally rounded to ease endocavitary insertion and was proximally tapered to a 1.5‐cm diameter cylindrical handle through which the irrigation tubes (for transducer cooling) and electrical connections were passed. MR compatibility of piezoceramic and piezocomposite transducers was assessed using gradient‐echo (GRE) sequences. The radiofrequency (RF) tuning of identical 6.5 cm × 2.5 cm rectangular receiver coils on the upper surface of the probe was adjusted to compensate for the presence of the conductive components of the HIFU transducers. A T1‐weighted (T1‐W) sliding window dual‐echo GRE sequence monitored phase changes in the focal zone of each transducer. High‐intensity (2400 W/cm^–2), short duration (<1.5 seconds) exposures produced subtherapeutic temperature rises.

Results

For T1‐W images, signal‐to‐noise ratio (SNR) improved by 40% as a result of quartering the conductive surface of the piezoceramic transducer. A piezocomposite transducer showed a further 28% improvement. SNRs for an endocavitary coil in the focal plane of the HIFU trans‐ducer (4 cm from its face) were three times greater than from a phased body array coil. Local shimming improved uniformity of phase images. Phase changes were detected at subtherapeutic exposures.

Conclusion

We combined a HIFU transducer with an MR receiver coil in an endocavitary probe. SNRs were improved by quartering the conductive surface of the piezoceramic. Further improvement was achieved with a piezocomposite transducer. A phase change was seen on MR images during both subtherapeutic and therapeutic HIFU exposures. J. Magn. Reson. Imaging 2007. © 2007 Wiley‐Liss, Inc.     

Renal ablation using magnetic resonance-guided high intensity focused ultrasound: Magnetic resonance imaging and histopathology assessment

AIM: To use magnetic resonance-guided high intensity focused ultrasound (MRg-HIFU), magnetic resonance imaging (MRI) and histopathology for noninvasively ablating, quantifying and characterizing ablated renal tissue.METHODS: Six anesthetized/mechanically-ventilated pigs underwent single/double renal sonication (n = 24) using a 3T-MRg-HIFU (1.1 MHz frequency and 3000J-4400J energies). T2-weighted fast spin echo (T2-W), perfusion saturation recovery gradient echo and contrast enhanced (CE) T1-weighted (T1-W) sequences were used for treatment planning, temperature monitoring, lesion visualization, characterization and quantification, respectively. Histopathology was conducted in excised kidneys to quantify and characterize cellular and vascular changes. Paired Student’s t-test was used and a P-value < 0.05 was considered statistically significant.RESULTS: Ablated renal parenchyma could not be differentiated from normal parenchyma on T2-W or non-CE T1-W sequences. Ablated renal lesions were visible as hypoenhanced regions on perfusion and CE T1-W MRI sequences, suggesting perfusion deficits and necrosis. Volumes of ablated parenchyma on CE T1-W images in vivo (0.12-0.36 cm³ for single sonication 3000J, 0.50-0.84 cm³, for double 3000J, 0.75-0.78 cm³ for single 4400J and 0.12-2.65 cm³ for double 4400J) and at postmortem (0.23-0.52 cm³, 0.25-0.82 cm³, 0.45-0.68 cm³ and 0.29-1.80 cm³, respectively) were comparable. The ablated volumes on 3000J and 4400J double sonication were significantly larger than single (P < 0.01), thus, the volume and depth of ablated tissue depends on the applied energy and number of sonication. Macroscopic and microscopic examinations confirmed the locations and presence of coagulation necrosis, vascular damage and interstitial hemorrhage, respectively.CONCLUSION: Contrast enhanced MRI provides assessment of MRg-HIFU renal ablation. Histopathology demonstrated coagulation necrosis, vascular damage and confirmed the volume of damage seen on MRI.