Use of overpressure to assess the role of bubbles in focused ultrasound lesion shape in vitro

Overpressure--elevated hydrostatic pressure--was used to assess the role of gas or vapor bubbles in distorting the shape and position of a high-intensity focused ultrasound (HIFU) lesion in tissue. The shift from a cigar-shaped lesion to a tadpole-shaped lesion can mean that the wrong area is treated. Overpressure minimizes bubbles and bubble activity by dissolving gas bubbles, restricting bubble oscillation and raising the boiling temperature. Therefore, comparison with and without overpressure is a tool to assess the role of bubbles. Dissolution rates, bubble dynamics and boiling temperatures were determined as functions of pressure. Experiments were made first in a low-overpressure chamber (0.7 MPa maximum) that permitted imaging by B-mode ultrasound (US). Pieces of excised beef liver (8 cm thick) were treated in the chamber with 3.5 MHz for 1 to 7 s (50% duty cycle). In situ intensities (I(SP)) were 600 to 3000 W/cm(2). B-mode US imaging detected a hyperechoic region at the HIFU treatment site. The dissipation of this hyperechoic region following HIFU cessation corresponded well with calculated bubble dissolution rates; thus, suggesting that bubbles were present. Lesion shape was then tested in a high-pressure chamber. Intensities were 1300 and 1750 W/cm(2) ( +/- 20%) at 1 MHz for 30 s. Hydrostatic pressures were 0.1 or 5.6 MPa. At 1300 W/cm(2), lesions were cigar-shaped, and no difference was observed between lesions formed with or without overpressure. At 1750 W/cm(2), lesions formed with no overpressure were tadpole-shaped, but lesions formed with high overpressure (5.6 MPa) remained cigar-shaped. Data support the hypothesis that bubbles contribute to the lesion distortion.     

声通道的条状障碍物对高强度聚焦超声在离体牛肝内形成凝固性坏死的影响

目的 研究相同输出声功率下,声通道的条状障碍物对高强度聚焦超声(HIFU)在离体牛肝内形成凝固性坏死的影响.方法 在离体牛肝组织中进行HIFU辐照,首先在自由场进行HIFU辐照,然后将用条状障碍物模拟的肋骨置于声通道,引导声束轴线沿肋间隙正中、沿肋缘及正对肋骨,在每种情况下将肋骨分别置于距焦平面3 cm、6 cm、9 cm处在离体牛肝中进行HIFU辐照,辐照结束后观测凝固性坏死的形成率、体积、形态、位置与模拟肋骨表面的情况.结果 ①凝固性坏死形成率:自由场情况下,凝固性坏死形成率为100%.当肋骨距焦平面3 cm时,声束轴线沿肋间隙正中,凝固性坏死形成率为100%;声束轴线沿肋缘、正对肋骨时,凝固性坏死形成率急剧降低.分别为25%、8%;当肋骨距焦平面6 cm、9 cm,无论声束轴线与肋骨相对位置如何,形成率均在50%以上.②凝固性坏死体积:当声通道有肋骨存在,凝固性坏死体积比自由场明显降低,差异均具有统计学意义(P<0.05);肋骨与声束轴线的相对位置以及肋骨距焦平面的距离不同,凝固性坏死的体积、长度、宽度、形态、位置不同.③当肋骨置于接近焦平面的位置时,在形成凝固性坏死的同时,观察到肋骨表面被烫伤.当肋骨置于远离焦平面的位置时,在形成凝固性坏死的同时,不会对肋骨表面形成损伤.结论 声通道有肋骨存在时,HIFU在牛肝内形成的凝固性坏死体积明显减小.声通道的肋骨与声束轴线的相对位置以及肋骨距焦平面的距离均对凝固性坏死的形成率、体积、形态、位置有影响,声束透过肋间隙正中时在牛肝中形成的凝固性坏死体积较大,形态规则,易于控制.声通道的肋骨距离焦平面越近,越有可能对肋骨表面造成损伤.     

Hyperecho in ultrasound images of HIFU therapy: involvement of cavitation

High-intensity focused ultrasound (US), or HIFU, treatment of soft tissues has been shown to result in a hyperechoic region in B-mode US images. We report on detecting cavitation in vivo in correlation with the appearance of a hyperechoic region. The US system consisted of a HIFU transducer (3.3 MHz), a broadband A-mode transducer for active and passive cavitation detection and an US-imaging probe that were all confocal and synchronized. HIFU, at in situ intensities of 220 to 1710 W/cm(2), was applied for 10 s to pig muscles in vivo. Active and passive cavitation detection results showed a strong correlation between the onset of cavitation and the appearance of a hyperechoic region. Passive cavitation detection results showed that inertial cavitation typically occurred prior (within 0.5 s) to the appearance of a hyperechoic region. The observed cavitation activity confirms that bubbles are present during the formation of a hyperechoic region at the HIFU focus.     

Ablation of Leiomyomas Using a Combination of HIFU and Iodized Oil In Vitro

This study investigates the enhancement effects of iodized oil and the features of hyperechoic focus changes in a target region of high-intensity focused ultrasound (HIFU) ablated uterine fibroids. Leiomyomas in the experimental group of 20 randomly assigned patients were ablated by HIFU under certain parameters 30 min after 1 mL of iodized oil was injected into the center of the myomas. The value of the gray scale and its area were observed by B-mode ultrasound in the target region and were carefully recorded at 0, 2, 4, and 5 min, respectively, after HIFU ablation. The samples were sectioned successively in a thickness of 1～2 mm and stained by 2,3,5-triphenyltetrazolium chloride solution within 1 h after HIFU ablation. The TTC non-staining volumes were measured afterwards. All procedures in the control group of the other 20 randomly assigned patients were the same except that iodized oil was replaced by physiologic saline. The hyperechoic areas in the target region were observed in all fibroids of both groups. Compared with the control group, the gray scale values in the target region of the experimental group were higher 4 min after HIFU ablation. The enhanced gray scale area in the target region and the 2,3,5-triphenyltetrazolium chloride non-staining volumes in the experimental group were bigger. Based on our experience, HIFU sonication of leiomyomas injected with iodized oil produces hyperechoic foci that are greater than areas injected with saline. More studies are required to understand the clinical implications of these observations.     

Effects of fascia lata on HIFU lesioning in vitro

The effects of fascia lata on high intensity focused ultrasound (US), or HIFU,-induced lesions were demonstrated through comparison with and without fascia lata in bovine thigh muscle tissue. Experiments were conducted in an arrangement with a three-way multiscan ultrasonic inspection system and imaging done by B-mode US. Bovine thigh muscle (8-cm thick) was treated with 1.5 MHz for 8 s. Spatial peak intensity (ISP) was 3000 W/cm2. B-mode US imaging detected appearance at the HIFU treatment site. At a free-field intensity of 4000 W/cm2, the observed lesion length (along the axis) with fascia lata was 12 +/- 1.82 mm, compared with 4 +/- 1.54 mm for samples without fascia lata. At 3000 W/cm2, the values for samples with fascia lata and samples without fascia lata, respectively, were 13 +/- 1.50 mm and 2 +/- 1.42 mm. During a 30-s exposure, at ISP of 2000 W/cm2, the peak temperature reached 41 degrees C in samples without fascia lata and 70 degrees C in samples with fascia lata. At ISP of 3000 and 4000 W/cm2, the peak temperature reached, respectively, 73 degrees C and 84 degrees C in samples without fascia lata, compared with 102 degrees C and 104 degrees C, respectively, for samples with fascia lata. The results confirm that fascia lata contributes to increasing tissue necrosis, temperature elevation and echogenicity in US images.     

聚焦超声换能器焦距与直径比值对高强度聚焦超声透过条状障碍物的生物学焦域的影响

目的探讨聚焦超声换能器的物理参数换能器焦距与直径的比值（f-number）对高强度聚焦超声（HIFU）透过条状障碍物的生物学焦域的影响。方法分别选用f-number为0．65（1．0—200—130）和f-number为0．85（1．0—200—170）型换能器在离体牛肝中进行HIFU辐照。将肋骨置于换能器与焦平面之间,HIFU声束轴线在B超的引导下,分别沿肋间隙正中聚焦、肋骨边缘聚焦和正对肋骨聚焦。在每种聚焦方式下,肋骨置于距焦平面3、6、9cm处。HIFU声束透过肋骨聚焦,在牛肝中的辐照深度为20mm,辐照功率250W,辐照时间10S;辐照方式采用定点辐照。辐照结束后切开牛肝,观察和记录生物学焦域的形成率与体积。结果在其他辐照参数相同的情况下,f-number为0．65透过肋骨后生物学焦域的形成率和体积均较f-number为0．85时大（除f-number为0．85正对肋骨聚焦,肋骨距焦平面3cm外）。结论聚焦超声换能器f-number不同,HIFU透过肋骨所形成的生物学焦域形成率与体积不同。     

An adaptive spectral estimation technique to detect cavitation in HIFU with high spatial resolution

In ultrasound-guided high-intensity focused ultrasound (HIFU) therapy, the changes observed on tissue are subtle during treatment; some ultrasound-guided HIFU protocols rely on the observation of significant brightness changes as the indicator of tissue lesions. The occurrence of a distinct hyperechogenic region (“bright-up”) around the focus is often associated with acoustic cavitation resulting in microbubble formation, but it may indicate different physical events such as larger bubbles from boiling (known to alter acoustic impedance) or sometimes lesion formation. A reliable method to distinguish and spatially localize these causes within the tissue would assist the control of HIFU delivery, which is the subject of this paper. Spectral analysis of the radio frequency (RF) signal underlying the B-mode image provides more information on the physical cause, but the usual techniques that are methods on the Fourier transform require a long series for good spectral resolution and so they give poor spatial resolution. This paper introduces an active spectral cavitation detection method to attain high spatial resolution (0.15 × 0.15 mm per pixel) through a parametric statistical method (ARMA modeling) used on finite-length data sets, which enables local changes to be identified more easily. This technique uses the characteristics of the signal itself to optimize the model parameters and structure. Its performance is assessed using synthesized cavitation RF data, and it is then demonstrated in ex vivo bovine liver during and after HIFU exposure. The results suggest that good spatial and spectral resolution can be obtained by the design of suitable algorithms. In ultrasound-guided HIFU, the technique provides a useful supplement to B-mode analysis, with no additional time penalty in data acquisition.     

The feasibility of a noise elimination method using continuous wave response of therapeutic ultrasound signals for ultrasonic monitoring of high-intensity focused ultrasound treatment

Purpose

In this study, the robustness and feasibility of a noise elimination method using continuous wave response of therapeutic ultrasound signals were investigated when tissue samples were moved to simulate the respiration-induced movements of the different organs during actual high-intensity focused ultrasound (HIFU) treatment. In addition to that, the failure conditions of the proposed algorithm were also investigated.

Methods

The proposed method was applied to cases where tissue samples were moved along both the lateral and axial directions of the HIFU transducer to simulate respiration-induced motions during HIFU treatment, and the noise reduction level was investigated. In this experiment, the speed of movement was increased from 10 to 40 mm/s to simulate the actual movement of the tissue during HIFU exposure, with the intensity and driving frequency of HIFU set to 1.0–5.0 kW/cm² and 1.67 MHz, respectively. To investigate the failure conditions of the proposed algorithm, the proposed method was applied with the HIFU focus located at the boundary between the phantom and water to easily cause cavitation bubbles. The intensity of HIFU was set to 10 kW/cm².

Results

Almost all HIFU noise was constantly able to be eliminated using the proposed method when the phantom was moved along the lateral and axial directions during HIFU exposure. The noise reduction level (PRL in this study) at an intensity of 1.0, 3.0, and 5.0 kW/cm² was in the range of 28–32, 38–40, and 42–45 dB, respectively. On the other hand, HIFU noise was not basically eliminated during HIFU exposure after applying the proposed method in the case of cavitation generation at the HIFU focus.

Conclusions

The proposed method can be applicable even if homogeneous tissues or organs move axially or laterally to the direction of HIFU exposure because of breathing. A condition under which the proposed algorithm failed was when instantaneous tissue changes such as cavitation bubble generation occurred in the tissue, at which time the reflected continuous wave response became less steady.

     

声通道上的条状障碍物对高强度聚焦超声声场的影响

目的 研究高强度聚焦超声(HIFU)透过用条状障碍物模拟的肋骨后的声场分布.方法 引导HIFU声束透过肋间隙、肋缘及正对肋骨,在每种情况下将肋骨分别置于距焦平面3、6、9 cm处,采用水听器描绘出了每种情况下声场的分布.结果 轴向声压分布:在3种障碍物分布情况下,声通道上由于肋骨的存在,声压幅值较自由场下降了60%～80%,相对于自由场,-6 dB焦域的尺寸增大0.5～1.5 mm.焦点的位置相对于自由场,也发生了前移,焦点向换能器方向靠近0.1～2.3 mm.径向声压分布:在3种障碍物分布情况下,当肋骨距焦平面3、6 cm时,声压的分布出现多峰的现象,相对于自由场,声压幅值降低.当肋骨距焦平面9 cm时,在经过了肋骨之后,波束基本上还是保持了原来的形状,声压分布与自由场接近.结论 声通道上存在肋骨时HIFU焦域的声压幅值明显降低,声压幅值的降低与声束轴线与肋骨的相对位置以及肋骨距焦平面的距离有关.声通道上存在肋骨时对HIFU声焦域有影响,-6 dB焦域尺寸增加.     

脂肪及肌肉组织对高强度聚焦超声消融能量沉积及焦域形态的影响

目的比较高强度聚焦超声(HIFU)通过不同厚度脂肪及肌肉组织消融猪肉靶点时的能量衰减情况。方法在MRI监控下分别选择脂肪厚度为0 mm、20 mm、28 mm的猪肉靶点,以输出功率200 W、辐照时间10 s进行单点消融,比较相同辐照条件下所产生的凝固性坏死体积。进一步选择脂肪厚度25 mm带皮猪肉及无皮和脂肪猪肉,分别将焦点置于距肌肉表面深度20 mm、30 mm的靶点进行单点辐照。结果随着脂肪厚度的增加,相同辐照条件下产生凝固性坏死区体积逐渐缩小,长轴逐渐增加,短轴逐渐缩小。当辐照无脂肪的猪肉组织时,随肌肉厚度的增加,损伤体积缩小;但相同厚度的脂肪组织对超声能量的衰减大于肌肉组织。结论 HIFU通过离体脂肪组织、肌肉组织时均有能量衰减;脂肪组织对超声能量的衰减程度高于肌肉组织。     

Acoustic Radiation Force Impulse (ARFI) imaging-based needle visualization

Ultrasound-guided needle placement is widely used in the clinical setting, particularly for central venous catheter placement, tissue biopsy and regional anesthesia. Difficulties with ultrasound guidance in these areas often result from steep needle insertion angles and spatial offsets between the imaging plane and the needle. Acoustic Radiation Force Impulse (ARFI) imaging leads to improved needle visualization because it uses a standard diagnostic scanner to perform radiation force based elasticity imaging, creating a displacement map that displays tissue stiffness variations. The needle visualization in ARFI images is independent of needle-insertion angle and also extends needle visibility out of plane. Although ARFI images portray needles well, they often do not contain the usual B-mode landmarks. Therefore, a three-step segmentation algorithm has been developed to identify a needle in an ARFI image and overlay the needle prediction on a coregistered B-mode image. The steps are: (1) contrast enhancement by median filtration and Laplacian operator filtration, (2) noise suppression through displacement estimate correlation coefficient thresholding and (3) smoothing by removal of outliers and best-fit line prediction. The algorithm was applied to data sets from horizontal 18, 21 and 25 gauge needles between 0-4 mm offset in elevation from the transducer imaging plane and to 18G needles on the transducer axis (in plane) between 10 degrees and 35 degrees from the horizontal. Needle tips were visualized within 2 mm of their actual position for both horizontal needle orientations up to 1.5 mm offset in elevation from the transducer imaging plane and on-axis angled needles between 10 degrees-35 degrees above the horizontal orientation. We conclude that segmented ARFI images overlaid on matched B-mode images hold promise for improved needle visibility in many clinical applications.     

Can the Entire Ablative Hyperechoic Zone be Regarded as a Necrotic Lesion After Radiofrequency Ablation of the Liver?

Developments in image fusion technology made it possible to visualize the ablative margin on ultrasound (US). The purpose of the present study was to assess the ablative area of radiofrequency ablation for hepatocellular carcinoma and compare it with the ablative hyperechoic zone with a non-enhanced area on contrast-enhanced US/contrast-enhanced computed tomography (CEUS/CECT) in the same cross-section. This retrospective study included 25 patients with 27 hepatocellular carcinomas. The long and short dimensions of the ablative hyperechoic zone were measured using B-mode US, and those of the non-enhanced area were assessed with CEUS/CECT on the same cross-section measured with B-mode US, using image fusion techniques. The technical effectiveness of ablation with an adequate ablative margin in a single session was determined in all patients. The long and short dimensions of the ablative hyperechoic zone ranged between 15.0 and 40.7 mm (mean: 27.3 ± 6.9 mm) and between 14.0 and 33.0 mm (mean: 23.3 ± 5.8 mm), respectively. R values for the long and short dimensions were 0.99 and 0.98, respectively, between B-mode US and CEUS, and 0.96 and 0.92, respectively, between B-mode US and CECT. The ablative hyperechoic zone may be regarded as a necrotic lesion after radiofrequency ablation.     

Real-time visualization of high-intensity focused ultrasound treatment using ultrasound imaging

High-intensity focused ultrasound (HIFU) and conventional B-mode ultrasound (US) imaging were synchronized to develop a system for real-time visualization of HIFU treatment. The system was tested in vivo in pig liver. The HIFU application resulted in the appearance of a hyperechoic spot at the focus that faded gradually after cessation of HIFU exposure. The duration of HIFU exposure needed for a hyperechoic spot to appear, was inversely related to the HIFU intensity. The threshold intensity required to produce a hyperechoic spot in liver in < 1 s was 970 W/cm(2), in situ. At this HIFU dose, no immediate cellular damage was observed, providing a potential for pretreatment targeting. The real-time visualization method was used in hemostasis of actively bleeding internal pelvic vessels, allowing targeting and monitoring of successful treatment. Real-time US imaging may provide a useful tool for image-guided HIFU therapy.     

Gel phantom for use in high-intensity focused ultrasound dosimetry

An optically transparent phantom was developed for use in high-intensity focused ultrasound (US), or HIFU, dosimetry studies. The phantom is composed of polyacrylamide hydrogel, embedded with bovine serum albumin (BSA) that becomes optically opaque when denatured. Acoustic and optical properties of the phantom were characterized as a function of BSA concentration and temperature. The speed of sound (1544 m/s) and acoustic impedance (1.6 MRayls) were similar to the values in soft tissue. The attenuation coefficient was approximately 8 times lower than that of soft tissues (0.02 Np/cm/MHz for 9% BSA). The nonlinear (B/A) coefficient was similar to the value in water. HIFU lesions were readily seen during formation in the phantom. In US B-mode images, the HIFU lesions were observed as hyperechoic regions only if the cavitation activity was present. The phantom can be used for fast characterization and calibration of US-image guided HIFU devices before animal or clinical studies.     

HIFU联合脂质纳米氟碳液滴消融兔肝的实验研究

目的 探讨脂质纳米氟碳液滴增效HIFU消融兔肝脏的空化活动及术后病理学变化。方法 首先制备全氟戊烷脂质纳米氟碳液滴(L-PFP);然后将24只正常新西兰兔随机分为对照组(单纯HIFU组)和L-PFP组;在 B 超引导下进行HIFU定点消融兔肝脏(超声能量：900 J);通过被动空化检测系统(PCD)监控空化活动;分别将消融即刻、1d、3d、7d的兔肝脏标本取出进行H E染色,观察消融灶转归过程中的病理学变化。结果 经耳缘静脉注射L-PFP后HIFU 辐照兔肝脏所产生的空化泡群更明显,其灰度变化值为对照组的1.93倍,累积瞬态空化剂量为对照组的6.3倍,空化活动表现强烈;大体病理及H E结果显示L-PFP组造成的组织损伤严重,细胞变性更为彻底,炎性反应更为强烈;单纯HIFU组消融灶7d修复为正常组织,转归所需时间显著短于L-PFP组。结论 脂质纳米氟碳液滴通过增强空化效应有效提高HIFU消融效果,延长消融灶转归所需时间。     

Thermal Fixation of Swine Liver Tissue after Magnetic Resonance-Guided High-Intensity Focused Ultrasound Ablation

The aim of this study was to investigate experimental conditions for efficient and controlled in vivo liver tissue ablation by magnetic resonance (MR)-guided high-intensity focused ultrasound (HIFU) in a swine model, with the ultimate goal of improving clinical treatment outcome. Histological changes were examined both acutely (four animals) and 1 wk after treatment (five animals). Effects of acoustic power and multiple sonication cycles were investigated. There was good correlation between target size and observed ablation size by thermal dose calculation, post-procedural MR imaging and histopathology, when temperature at the focal point was kept below 90°C. Structural histopathology investigations revealed tissue thermal fixation in ablated regions. In the presence of cavitation, mechanical tissue destruction occurred, resulting in an ablation larger than the target. Complete extra-corporeal MR-guided HIFU ablation in the liver is feasible using high acoustic power. Nearby large vessels were preserved, which makes MR-guided HIFU promising for the ablation of liver tumors adjacent to large veins.     

Contrast-agent-enhanced ultrasound thermal ablation

The small thermal lesions induced when using high-intensity focused ultrasound (HIFU) to ablate tumors results in long treatment duration. In this study, the effect of using ultrasound contrast agent (UCA, Definity) to enhance the ultrasound (US) thermal effects and, thus to enlarge the lesion size, was studied in transparent tissue phantoms insonified by 1.85-MHz US with acoustical powers of 28.9 and 40.4 W. The experimental results show that the lesion size depended strongly on the electrical power and the concentration of UCA. UCA also reduced the power required to form a lesion of a certain size by about 30%. However, UCA moved the greatest heating position from the transducer focus, by 2.16 cm for 0.015% UCA at 40.4 W, and with lesions forming at the surface for UCA concentrations higher than 0.1%. An optimal result was obtained when using 0.001% UCA and 28.9-W US, which produced a lesion 12 times larger and an acceptable shift (less than half of the lesion length). UCA can effectively increase the size of the HIFU lesions, but lesion shift should be carefully considered while performing HIFU ablations.     

Selective reduction of multifetal pregnancy using high-intensity focused ultrasound in the rabbit model.

OBJECTIVE: High-order multifetal pregnancies carry a significant risk of obstetric complications and poor pregnancy outcome. Selective reduction has traditionally been performed using transabdominal and transvaginal ultrasound-guided intracardiac injection of potassium chloride. We have previously shown that high-intensity focused ultrasound (HIFU) can create a coagulative tissue necrosis in the sheep fetus. The objective of this study was to investigate the feasibility of non-invasive selective fetal reduction using HIFU in a rabbit model. METHODS: A protocol for HIFU-induced tissue coagulation was developed in the rabbit model. The fetal heart was targeted with ultrasound-guided tissue ablation by a HIFU beam. Five time-mated does between 20-29 days' gestation underwent transabdominal fetal cardiac ablation in a total of 11 fetuses. The HIFU system consisted of a 7-MHz high-power transducer, operated at 2000 W/cm2. The fetal heart rate was observed using real-time ultrasound with Doppler flow velocimetry. All lesions were assessed macroscopically and by histological analysis. RESULTS: Severe bradycardia leading to asystole was observed in all targeted fetuses with ultrasound examination. Dissection of fetuses demonstrated a necrotic intrathoracic lesion similar in size to the HIFU focus (approximately 1 x 9 mm). None of the surrounding fetuses was found to have bradycardia during the procedure or a macroscopic lesion on dissection. CONCLUSION: In this pilot study HIFU seems promising to ablate even highly vascularized tissue in the fetus.     

Evaluation of voxel-based registration of 3-D power Doppler ultrasound and 3-D magnetic resonance angiographic images of carotid arteries

Spatial registration and fusion of ultrasound (US) images with other modalities may aid clinical interpretation. We implemented and evaluated on patient data an automated retrospective registration of magnetic resonance angiography (MRA) carotid bifurcation images with 3-D power Doppler ultrasound (PD US) and indirectly with 3-D B-mode US. Volumes were initially thresholded to reduce the uncorrelated noise signals. The registration algorithm subsequently maximized the mutual information measure between the PD US and 3-D MRA via iterative simplex search to find best "rigid body" transformation. We rated the performance of the algorithm visually on (n = 5) clinical MRA and 3-D PD US datasets. We also evaluated quantitatively the effect of thresholding, initial misalignment of the paired volumes and the reproducibility registration. We investigated the effect of image artefacts by simulation experiments. Preregistration misalignments of up to 5 mm in the transaxial plane, up to 10 mm along the axis of the carotids and up to 40 degrees resulted in 107 of 110 successful registrations, with translational and rotational errors of 0.32 mm +/- 0.3 mm and 1.6 +/- 2.1 degrees. The algorithm was not affected by missing arterial segments of up to 8 mm in length. The average registration time was 4 min. We conclude that the algorithm could be applied to 3-D US PD and MRA data for automated multimodality registration of carotid vessels without the use of fiducials.     

Contrast-enhanced ultrasound assessment of tissue response to high-intensity focused ultrasound

We report the use of contrast-enhanced ultrasonography as an immediate means of assessing the clinical response to high-intensity focused ultrasound (US) or HIFU treatment of liver tumours. HIFU is a noninvasive transcutaneous technique for the ablation of tumours that has been shown to destroy tumour vasculature, as well as to cause coagulative necrosis of tumour cells. As a dynamic indicator of tissue perfusion, microbubble contrast agents have already been reported to increase the diagnostic sensitivity of ultrasonography in the detection of liver tumours. This report documents the ability of one i.v. microbubble contrast agent (SonoVue, Bracco, Italy) to delineate the extent of HIFU ablation by comparison of pre- and immediately posttreatment perfusion within the target tumour. Observed changes were seen to correlate well with the ablated volume on histologic evaluation of the treated volume. This is the first time that this imaging technique has been reported in this setting.