[Show abstract][Hide abstract] ABSTRACT: Purpose:
The recent clinical emergence of minimally invasive image-guided therapy has demonstrated promise in the management of brain metastasis, although control over the spatial pattern of heating currently remains limited. Based on experience in other organs, the delivery of high-intensity contact ultrasound energy from minimally invasive applicators can enable accurate spatial control of energy deposition, large treatment volumes, and high treatment rate. In this acute study, the feasibility of active MR-Temperature feedback control of dynamic ultrasound heat deposition for interstitial thermal ablation in brain was evaluatedin vivo.
A four-element linear ultrasound transducer (f=8.2 MHz) originally developed for transurethral ultrasound therapy was used in a porcine model for generating thermal ablations in brain interstitially. First, the feasibility of treating and retreating preciselyin vivo brain tissues using stationary (non-rotating device) ultrasound exposures was studied in two pigs. Experimental results were compared to numerical simulations for maximum surface acoustic intensities ranging from 5 to 20 W cm(-2). Second, active MRT feedback-controlled ultrasound treatments were performed in three pigs with a rotating device to coagulate target volumes of various shapes. The acoustic power and rotation rate of the device were adjusted in real-time based on MR-thermometry feedback control to optimize heat deposition at the target boundary. Modeling of in vivo treatments were performed and compared to observed experimental results.
Overall, the time-space evolution of the temperature profiles observedin vivo could be well estimated from numerical simulations for both stationary and dynamic interstitial ultrasound exposures. Dynamic exposures performed under closed-loop temperature control enabled accurate elevation of the brain tissues within the targeted region above the 55 °C threshold necessary for the creation of irreversible thermal damage. Treatment volumes ranging from 1 to 9 cm3 were completed within 8±3 min with a radial targeting error<2 mm on average (treatment rate: 0.7±0.5 cm3/min). Tissue changes were visible on T1-weighted contrast-enhanced (T1w-CE) images immediately after treatment. These changes were also evident on T2-weighted (T2w) images acquired 2 h after the 1st treatment and correlated well with the MR-thermometry measurements.
These results support the feasibility of active MRT feedback control of dynamic interstitial ultrasound therapy ofin vivo brain tissues and confirm the feasibility of using simulations to predict spatial heating patterns in the brain.
[Show abstract][Hide abstract] ABSTRACT: The recent emergence at the clinical level of minimally-invasive focal
therapy such as laser-induced thermal therapy (LITT) has demonstrated
promise in the management of brain metastasis , although control over
the spatial pattern of heating is limited. Delivery of HIFU from
minimally-invasive applicators enables high spatial control of the heat
deposition in biological tissues, large treatment volumes and high
treatment rate in well chosen conditions [2,3]. In this study, the
feasibility of MRI-guided interstitial ultrasound therapy in brain was
studies in-vivo in a porcine model. A prototype system originally
developed for transurethral ultrasound therapy [4,5,6] was used in this
study. Two burr holes of 12 mm in diameter were created in the animal's
skull to allow the insertion of the therapeutic ultrasound applicator
(probe) into the brain at two locations (right and left frontal lobe). A
4-element linear ultrasound transducer (f = 8 MHz) was mounted at the
tip of a 25-cm linear probe (6 mm in diameter). The target boundary was
traced to cover in 2D a surface compatible with the treatment of a 2 cm
brain tumor. Acoustic power of each element and rotation rate of the
device were adjusted in real-time based on MR-thermometry feedback
control to optimize heat deposition at the target boundary [2,4,5]. Two
MRT-controlled ultrasound brain treatments per animal have been
performed using a maximal surface acoustic power of 10W.cm-2. In all
cases, it was possible to increase accurately the temperature of the
brain tissues in the targeted region over the 55°C threshold
necessary for the creation of irreversible thermal lesion. Tissue
changes were visible on T1w contrast-enhanced images immediately after
treatment. These changes were also evident on T2w FSE images taken 2
hours after the 1st treatment and correlated well with the temperature
image. On average, the targeted volume was 4.7 +/- 2.3 cm3 and the
55°C treated volume was 6.7 +/- 4.4 cm3. The volumetric
undertreatment and overtreatment were respectively 0.1 +/- 0.1 cm3 and
0.7 +/- 0.6 cm3. The radial targeting accuracy was on average 1 +/- 3
mm. Treatments were completed within 7 +/- 3 min, that is an treatment
rate of 0.9 +/- 0.7 cm3/min. MRI-controlled interstitial ultrasound
therapy of brain tissue is feasible. This minimally-invasive approach
avoids the need to propagate ultrasound through the skull and allows
spatially controlled heating which could be used for tissue ablation or
[Show abstract][Hide abstract] ABSTRACT: MRI-guided transurethral ultrasound therapy has previously been
evaluated in human volunteers by treating a prostate subvolume, prior to
radical prostatectomy. Extension of this technique to full clinical use
requires validation of the technique for coagulating a volume of tissue
equivalent to the entire gland. This in-vitro study evaluates approaches
for treating the whole prostate gland by comparing various treatment
strategies. An 8-element ultrasound therapy system was evaluated in gel
phantoms using 6 human prostate profiles segmented from MR clinical
images (average volume: 36 c\mcirc 3). Real-time MR-thermometry feedback
was performed over nine slices covering the entire prostate. Decisions
on acoustic power, frequency, and device rotation rate were made based
on prostate target radii and temperature maps updated every 7s. Low and
high power treatment approaches (10, 20 W/c\mcirc 2) were tested as well
as single-and dual-frequency strategies (8.1, 4.6/14.4 MHz). Decreasing
frequency to 4.6 MHz enabled treating 97% of the gland on average. A
20/10 W/c\mcirc 2 dual-frequency 4.6/14.4 MHz treatment was the most
efficient configuration in achieving full human prostate treatments.
Treatments were performed more quickly (<30min) with enhanced
treatment safety close to the base and apex. Transurethral thermal
therapy has been validated in vitro for full gland treatment in human
[Show abstract][Hide abstract] ABSTRACT: High-intensity contact ultrasound therapy can generate precise volumes
of thermal damage in deep-seated tissue using interstitial or
intracavitary devices. Multi-element, dual-frequency transducers offer
increased spatial control of the heating pattern by enabling modulation
of ultrasound power and frequency along the device. The performance and
acoustic coupling between elements of simple, multi-element,
dual-frequency transducers was measured. Transducer arrays were
fabricated by cutting halfway through a rectangular plate of PZT,
creating individual 4 × 5 mm segments with fundamental frequency
(4.1 MHz) and third harmonic (13.3 MHz). Coupling between elements was
investigated using a scanning laser vibrometer to measure transducer
surface displacements at each frequency and different acoustic powers
(0, 10, 20 W/cm2). The measured acoustic power was proportional to the
input electrical power with no hysteresis and efficiencies >50% at
both frequencies. Maximum transducer surface displacements were observed
near element centers, reducing to ~1/3-maximum near edges. The power and
frequency of neighboring transducer segments had little impact on an
element's output. In the worst case, an element operating at 4.1 MHz and
20 W/cm2 coupled only 1.5 W/cm2 to its immediate neighboring element.
Multi-element, dual-frequency transducers were successfully constructed
using a simple dicing method. Coupling between elements was minor,
therefore the power and frequency of each transducer element could be
[Show abstract][Hide abstract] ABSTRACT: Purpose:
To evaluate the feasibility and safety of magnetic resonance (MR) imaging-controlled transurethral ultrasound therapy for prostate cancer in humans.
Materials and methods:
This pilot study was approved by the institutional review board and was performed in eight men (mean age, 60 years; range, 49-70 years) with localized prostate cancer (Gleason score≤7, prostate-specific antigen level #15 μg/L) immediately before radical prostatectomy. All patients provided written informed consent. This phase 0 feasibility and safety study is the first evaluation in humans. Transurethral ultrasound therapy was performed with the patient under spinal anesthesia by using a clinical 1.5-T MR unit. Patients then underwent radical prostatectomy, and the resected gland was sliced in the plane of treatment to compare the MR imaging measurements with the pattern of thermal damage. The overall procedure time and coagulation rate were measured. In addition, the spatial targeting accuracy was evaluated, as was the thermal history along the thermal damage boundaries in the gland.
The average procedure time was 3 hours, with 2 or fewer hours spent in the MR unit. The treatment was well tolerated by all patients, and a temperature uncertainty of less than 2°C was observed in the treatments. The mean temperature and thermal dose measured along the boundary of thermal coagulation were 52.3°C±2.1 and 3457 (cumulative equivalent minutes at 43°C)±5580, respectively. The mean treatment rate was 0.5 mL/min, and a spatial targeting accuracy of -1.0 mm±2.6 was achieved.
MR imaging-controlled transurethral ultrasound therapy is feasible, safe, and well tolerated. This technology could be an attractive approach for whole-gland or focal therapy.
[Show abstract][Hide abstract] ABSTRACT: The feasibility and safety of magnetic resonance imaging (MRI)-controlled transurethral ultrasound therapy were demonstrated recently in a preliminary human study in which a small subvolume of prostate tissue was treated prior to radical prostatectomy. Translation of this technology to full clinical use, however, requires the capability to generate thermal coagulation in a volume up to that of the prostate gland itself. The aim of this study was to investigate the parameters required to treat a full 3D human prostate accurately with a multi-element transurethral applicator and multiplanar MR temperature control.
The approach was a combination of simulations (to select appropriate parameters) followed by experimental confirmation in tissue-mimicking phantoms. A ten-channel, MRI-compatible transurethral ultrasound therapy system was evaluated using six human prostate models (average volume: 36 cm(3)) obtained from the preliminary human feasibility study. Real-time multiplanar MR thermometry at 3 T was used to control the spatial heating pattern in up to nine planes simultaneously. Treatment strategies incorporated both single (4.6 or 8.1 MHz) and dual (4.6 and 14.4 MHz) frequencies, as well as maximum acoustic surface powers of 10 or 20 W cm(-2).
Treatments at 4.6 MHz were capable of coagulating a volume equivalent to 97% of the prostate. Increasing power from 10 to 20 W cm(-2) reduced treatment times by approximately 50% with full treatments taking 26 ± 3 min at a coagulation rate of 1.8 ± 0.4 cm(3) min(-1). A dual-frequency 4.6∕14.4 MHz treatment strategy was shown to be the most effective configuration for achieving full human prostate treatment while maintaining good treatment accuracy for small treatment radii. The dual-frequency approach reduced overtreatment close to the prostate base and apex, confirming the simulations.
This study reinforces the capability of MRI-controlled transurethral ultrasound therapy to treat full prostate volumes in a short treatment time with good spatial targeting accuracy and provides key parameters necessary for the next clinical trial.
[Show abstract][Hide abstract] ABSTRACT: Transurethral ultrasound therapy uses real-time magnetic resonance (MR) temperature feedback to enable the 3D control of thermal therapy accurately in a region within the prostate. Previous canine studies showed the feasibility of this method in vivo. The aim of this study was to reduce the procedure time, while maintaining targeting accuracy, by investigating new combinations of treatment parameters. Simulations and validation experiments in gel phantoms were used, with a collection of nine 3D realistic target prostate boundaries obtained from previous preclinical studies, where multi-slice MR images were acquired with the transurethral device in place. Acoustic power and rotation rate were varied based on temperature feedback at the prostate boundary. Maximum acoustic power and rotation rate were optimised interdependently, as a function of prostate radius and transducer operating frequency. The concept of dual frequency transducers was studied, using the fundamental frequency or the third harmonic component depending on the prostate radius. Numerical modelling enabled assessment of the effects of several acoustic parameters on treatment outcomes. The range of treatable prostate radii extended with increasing power, and tended to narrow with decreasing frequency. Reducing the frequency from 8 MHz to 4 MHz or increasing the surface acoustic power from 10 to 20 W/cm(2) led to treatment times shorter by up to 50% under appropriate conditions. A dual frequency configuration of 4/12 MHz with 20 W/cm(2) ultrasound intensity exposure can treat entire prostates up to 40 cm(3) in volume within 30 min. The interdependence between power and frequency may, however, require integrating multi-parametric functions in the controller for future optimisations.
No preview · Article · Feb 2012 · International Journal of Hyperthermia
[Show abstract][Hide abstract] ABSTRACT: MRI‐guided transurethral ultrasound therapy shows promise for minimally invasive treatment of localized prostate cancer. Previous in‐vivo studies demonstrated the feasibility of performing conservative treatments using real‐time temperature feedback to control accurately the establishment of coagulative lesions within circumscribed prostate regions. This in‐vitro study tested device configuration and control options for achieving full prostate treatments. A multi‐channel MRI compatible ultrasound therapy system was evaluated in gel phantoms using 3 canine prostate models. Prostate profiles were 5 mm‐step‐segmented from T2‐weighted MR images performed during previous in‐vivo experiments. During ultrasound exposures, each ultrasound element was controlled independently by the 3D controller. Decisions on acoustic power, frequency, and device rotation rate were made in real time based on MR thermometry feedback and prostate radii. Low and high power treatment approaches using maximum acoustic powers of 10 or 20 W.cm−2 were tested as well as single and dual‐frequency strategies (4.05∕13.10 MHz). The dual‐frequency strategy used either the fundamental frequency or the 3rd harmonic component, depending on the prostate radius. The 20 W.cm−2 dual frequency approach was the most efficient configuration in achieving full prostate treatments. Treatment times were about half the duration of those performed with 10 W.cm−2 configurations. Full prostate coagulations were performed in 16.3±6.1 min at a rate of 1.8±0.2 cm3.min−1, and resulted in very little undertreated tissue (<3%). Surrounding organs positioned beyond a safety distance of 1.4±1.0 mm from prostate boundaries were not damaged, particularly rectal wall tissues. In this study, a 3D, MR‐thermometry‐guided transurethral ultrasound therapy was validated in vitro in a tissue‐mimicking phantom for performing full prostate treatment. A dual‐frequency configuration with 20 W.cm−2 ultrasound intensity exposure showed good results with direct application to full human prostate treatments.
[Show abstract][Hide abstract] ABSTRACT: The feasibility of performing MRI-guided transurethral ultrasound therapy in humans has been shown by our group for the treatment of small subvolumes prior to radical prostatectomy. One challenge is to use this technology to treat a volume of tissue equivalent to the entire prostate gland. This simulation study evaluates the feasibility of treating the whole prostate gland and characterizes the nature of the treatment with respect to treatment time, accuracy, and safety. A numerical model was used to simulate a multi-element heating applicator rotating inside the urethra in five human prostates. 3D prostate profiles were segmented from clinical MR images obtained from subjects after insertion of a transurethral heating applicator into the prostate gland. Clinical treatment planning conditions were simulated including device orientation, prostate & rectum geometry, temperature uncertainty, imaging time, and spatial resolution. During ultrasound exposures, acoustic power, frequency and rotation rate were varied based on the prostate radius and on temperature feedback every 5 seconds using MR thermometry. Two treatment approaches (10 or 20 W.cm-2 acoustic power) were tested as well as single and dual-frequency strategies (4.05/13.10 MHz). A 20 W.cm-2 dual-frequency treatment was shown to be the most efficient configuration in achieving full human prostate treatments. Increasing the power from 10 to 20 W.cm-2 led, on average, to treatment times shorter by 50%. Full prostate coagulations were performed in 20.6+/-1.6 min at a rate of 1.6+/-0.2 cm3.min-1, and resulted in
[Show abstract][Hide abstract] ABSTRACT: MRI-guided transurethral ultrasound therapy uses a linear array of transducer elements and active temperature feedback to create volumes of thermal coagulation shaped to predefined prostate geometries in 3-D. Numerical simulations have been used to determine robust feedback control algorithms, optimal transducer designs, effects of various tissue and imaging parameters, as well as to evaluate potential treatment accuracy and safety in patient-specific anatomical models. The goal of this work is to evaluate quantitatively the accuracy with which these numerical simulations predict the extent, shape and temperature pattern of 3-D heating produced in tissue-mimicking Zerdine* gel phantoms. Methods. Eleven experiments were performed in a 1.5T MRI scanner. Temperature feedback was used to control the rotation rate and ultrasound power of a transurethral device with five 3.5×5 mm transducer elements. Heating patterns shaped to 23 and 11 cc human prostate geometries were generated using devices operating at 4.7 and 8.0 MHz, respectively, and 10 W/cm2 surface acoustic intensity. Transducer surface velocity measurements were acquired using a vibrometer and used to calculate the resulting acoustic pressure distribution in gel. Temperature dynamics were determined according to a FDTD solution to Pennes' BHTE. Results. The numerical simulations predicted the extent and shape of the coagulation boundary produced in gel to within (mean+/-stdev [min, max]): 0.1+/-0.4 [-1.4, 1.7] and 0.0+/-0.3 [-1.0, 1.5] mm for the treatments at 4.7 and 8.0 MHz, respectively. The temperatures across all MRI thermometry images were predicted to within 10%, and the treatment time (~20 min) to within 20%. The simulations showed excellent agreement in regions of sharp temperature gradients, near the transurethral and endorectal devices. Conclusion. Heating patterns predicted by the numerical simulations correspond closely to those produced experimentally in gel. This work quantifies the accuracy and demonstrates the validity of using numerical simulations to model MRI-guided transurethral ultrasound prostate therapy.
[Show abstract][Hide abstract] ABSTRACT: Minimally invasive treatments for localised prostate cancer are being developed with the aim of achieving effective disease control with low morbidity. High-temperature thermal therapy aimed at producing irreversible thermal coagulation of the prostate gland is attractive because of the rapid onset of thermal injury, and the immediate visualisation of tissue response using medical imaging. High-intensity ultrasound therapy has been shown to be an effective means of achieving thermal coagulation of prostate tissue using minimally invasive devices inserted into the rectum, urethra, or directly into the gland itself. The focus of this review is to describe the work done in our group on the development of MRI-controlled transurethral ultrasound therapy. This technology utilises high intensity ultrasound energy delivered from a transurethral device to achieve thermal coagulation of prostate tissue. Control over the spatial pattern of thermal damage is achieved through closed-loop temperature feedback using quantitative MR thermometry during treatment. The technology, temperature feedback algorithms, and results from numerical modelling, along with experimental results obtained in animal and human studies are described. Our experience suggests that this form of treatment is technically feasible, and compatible with existing MR imaging systems. Temperature feedback control algorithms using MR thermometry can achieve spatial treatment accuracy of a few millimetres in vivo. Patient-specific simulations predict that surrounding tissues can be spared from thermal damage if appropriate measures are taken into account during treatment planning. Recent human experience has been encouraging and motivates further evaluation of this technology as a potential treatment for localised prostate cancer.
No preview · Article · Dec 2010 · International Journal of Hyperthermia
[Show abstract][Hide abstract] ABSTRACT: MRI-controlled transurethral ultrasound therapy uses a linear array of transducer elements and active temperature feedback to create volumes of thermal coagulation shaped to predefined prostate geometries in 3D. The specific aims of this work were to demonstrate the accuracy and repeatability of producing large volumes of thermal coagulation (>10 cc) that conform to 3D human prostate shapes in a tissue-mimicking gel phantom, and to evaluate quantitatively the accuracy with which numerical simulations predict these 3D heating volumes under carefully controlled conditions. Eleven conformal 3D experiments were performed in a tissue-mimicking phantom within a 1.5T MR imager to obtain non-invasive temperature measurements during heating. Temperature feedback was used to control the rotation rate and ultrasound power of transurethral devices with up to five 3.5 × 5 mm active transducer elements. Heating patterns shaped to human prostate geometries were generated using devices operating at 4.7 or 8.0 MHz with surface acoustic intensities of up to 10 W cm(-2). Simulations were informed by transducer surface velocity measurements acquired with a scanning laser vibrometer enabling improved calculations of the acoustic pressure distribution in a gel phantom. Temperature dynamics were determined according to a FDTD solution to Pennes' BHTE. The 3D heating patterns produced in vitro were shaped very accurately to the prostate target volumes, within the spatial resolution of the MRI thermometry images. The volume of the treatment difference falling outside ± 1 mm of the target boundary was, on average, 0.21 cc or 1.5% of the prostate volume. The numerical simulations predicted the extent and shape of the coagulation boundary produced in gel to within (mean ± stdev [min, max]): 0.5 ± 0.4 [-1.0, 2.1] and -0.05 ± 0.4 [-1.2, 1.4] mm for the treatments at 4.7 and 8.0 MHz, respectively. The temperatures across all MRI thermometry images were predicted within -0.3 ± 1.6 °C and 0.1 ± 0.6 °C, inside and outside the prostate respectively, and the treatment time to within 6.8 min. The simulations also showed excellent agreement in regions of sharp temperature gradients near the transurethral and endorectal cooling devices. Conformal 3D volumes of thermal coagulation can be precisely matched to prostate shapes with transurethral ultrasound devices and active MRI temperature feedback. The accuracy of numerical simulations for MRI-controlled transurethral ultrasound prostate therapy was validated experimentally, reinforcing their utility as an effective treatment planning tool.
No preview · Article · Oct 2010 · Physics in Medicine and Biology
[Show abstract][Hide abstract] ABSTRACT: MRI-guided transurethral ultrasound therapy can generate highly accurate volumes of thermal coagulation conforming to 3D human prostate geometries. This work simulated, quantified, and evaluated the thermal impact of these treatments on the rectum, pelvic bone, neurovascular bundles (NVBs), and urinary sphincters because damage to these structures can lead to complications.
Twenty 3D anatomical models of prostate cancer patients were used with detailed bioacoustic simulations incorporating an active feedback algorithm which controlled a rotating, planar ultrasound transducer (17, 4 x 3 mm2 elements, 10 W(acoustic)/cm2). Heating of the adjacent surrounding anatomy was evaluated at 4.7, 9.7, and 14.2 MHz using thermal tolerances reported in literature.
Heating of the rectum posed the most important safety concern, influenced largely by the water temperature of an endorectal cooling device (ECD); depending on anatomy, temperatures of 7-37 degrees C were required to limit potential damage to less than 10 mm3 on the outer 1 mm layer of the rectal wall. Heating of the pelvic bone could be important at 4.7 MHz. A smaller sized ECD or a higher ultrasound frequency in sectors where the bone was less than 10 mm from the prostate reduced heating in all cases below the threshold for irreversible damage. Heating of the NVB was significant in 75% of the patient models in the absence of treatment planning; this proportion was reduced to 5% by increasing treatment margins up to 4 mm. To avoid damaging the urinary sphincters, the transducer should be positioned at least 2-4 mm from the sphincters, depending on the transurethral cooling temperature.
Simulations show that MRI-guided transurethral therapy can treat the prostate accurately, but in the absence of treatment planning, some thermal impact can be predicted on the surrounding anatomy. Treatment planning strategies have been developed, which reduce thermal injury to the surrounding anatomy.
[Show abstract][Hide abstract] ABSTRACT: Development of non-invasive techniques for prostate cancer treatment requires implementation of quantitative measures for evaluation of the treatment results. In this paper. we introduce measures that estimate spatial targeting accuracy and potential thermal damage to the structures surrounding the prostate. The measures were developed for the technique of treating prostate cancer with a transurethral ultrasound heating applicators guided by active MR temperature feedback. Variations of ultrasound element length and related MR imaging parameters such as MR slice thickness and update time were investigated by performing numerical simulations of the treatment on a database of ten patient prostate geometries segmented from clinical MR images. Susceptibility of each parameter configuration to uncertainty in MR temperature measurements was studied by adding noise to the temperature measurements. Gaussian noise with zero mean and standard deviation of 0, 1, 3 and 5° C was used to model different levels of uncertainty in MR temperature measurements. Results of simulations for each parameter configuration were averaged over the database of the ten prostate patient geometries studied. Results have shown that for update time of 5 seconds both 3- and 5-mm elements achieve appropriate performance for temperature uncertainty up to 3° C, while temperature uncertainty of 5° C leads to noticeable reduction in spatial accuracy and increased risk of damaging rectal wall. Ten-mm elements lacked spatial accuracy and had higher risk of damaging rectal wall compared to 3- and 5-mm elements, but were less sensitive to the level of temperature uncertainty. The effect of changing update time was studied for 5-mm elements. Simulations showed that update time had minor effects on all aspects of treatment for temperature uncertainty of 0° C and 1° C, while temperature uncertainties of 3° C and 5° C led to reduced spatial accuracy, increased potential damage to the rectal wall, and longer treatment times for update time above 5 seconds. Overall evaluation of results suggested that 5-mm elements showed best performance under physically reachable MR imaging parameters.
[Show abstract][Hide abstract] ABSTRACT: MRI-guided transurethral ultrasound therapy shows promise for minimally invasive treatment of localized prostate cancer. Real-time MR temperature feedback enables the 3D control of thermal therapy to define an accurate region within the prostate. Previous in-vivo canine studies showed the feasibility of this method using transurethral planar transducers. The aim of this simulation study was to reduce the procedure time, while maintaining treatment accuracy by investigating new combinations of treatment parameters. A numerical model was used to simulate a multi-element heating applicator rotating inside the urethra in 10 human prostates. Acoustic power and rotation rate were varied based on the feedback of the temperature in the prostate. Several parameters were investigated for improving the treatment time. Maximum acoustic power and rotation rate were optimized interdependently as a function of prostate radius and transducer operating frequency, while avoiding temperatures >90 degrees C in the prostate. Other trials were performed on each parameter separately, with the other parameter fixed. The concept of using dual-frequency transducers was studied, using the fundamental frequency or the 3rd harmonic component depending on the prostate radius. The maximum acoustic power which could be used decreased as a function of the prostate radius and the frequency. Decreasing the frequency (9.7-3.0 MHz) or increasing the power (10-20 W.cm-2) led to treatment times shorter by up to 50% under appropriate conditions. Dual-frequency configurations, while helpful, tended to have less impact on treatment times. Treatment accuracy was maintained and critical adjacent tissues like the rectal wall remained protected. The interdependence between power and frequency may require integrating multi-parametric functions inside the controller for future optimizations. As a first approach, however, even slight modifications of key parameters can be sufficient to reduce treatment time.
[Show abstract][Hide abstract] ABSTRACT: Previous numerical simulations have shown that MRI-guided transurethral ultrasound therapy can generate highly accurate volumes of thermal coagulation conforming to 3-D human prostate geometries. The goal of this work is to simulate, quantify and evaluate the thermal impact of these treatments on the rectum, pelvic bone, neurovascular bundles (NVB) and urinary sphincters. This study used twenty 3-D anatomical models of prostate cancer patients and detailed bio-acoustic simulations incorporating an active feedback algorithm which controlled a rotating, planar ultrasound transducer (17-4×3 mm elements, 4.7/9.7 MHz, 10 Wac/cm2). Heating of the adjacent surrounding anatomy was evaluated using thermal tolerances reported in the literature. Heating of the rectum poses the most important safety concern and is influenced largely by the water temperature flowing through an endorectal cooling device; temperatures of 7-37° C are required to limit potential damage to less than 10 mm3 on the outer 1 mm layer of rectum. Significant heating of the pelvic bone was predicted in 30% of the patient models with an ultrasound frequency of 4.7 MHz; setting the frequency to 9.7 MHz when the bone is less than 10 mm away from the prostate reduced heating in all cases below the threshold for irreversible damage. Heating of the NVB was significant in 75% of the patient models in the absence of treatment planning; this proportion was reduced to 5% by using treatment margins of up to 4 mm. To avoid damaging the urinary sphincters, margins from the transducer of 2-4 mm should be used, depending on the transurethral cooling temperature. Simulations show that MRI-guided transurethral therapy can treat the entire prostate accurately. Strategies have been developed which, along with careful treatment planning, can be used to avoid causing thermal injury to the rectum, pelvic bone, NVB and urinary sphincters.
[Show abstract][Hide abstract] ABSTRACT: High intensity ultrasound delivered transurethrally is a promising approach for the treatment of localized prostate cancer. The use of multiple planar ultrasound transducers mounted on an MR‐compatible applicator with rotational capability can provide precise control over the spatial deposition of energy within the gland. Using MR thermometry for adaptive temperature feedback, accurately shaped three‐dimensional heating patterns can potentially be achieved. The goal of this study was to evaluate the feasibility of simultaneously controlling multiple elements with real‐time MR temperature feedback in gel phantoms in a 1.5T MR imager. Numerical simulations were used initially to determine treatment delivery strategies and appropriate tuning of the temperature feedback control algorithm. Two typical prostate shapes were then treated in tissue‐mimicking polyacrylamide gel phantoms using a prototype system to demonstrate conformity of the thermal damage patterns to the target boundaries. Five planar gradient‐echo MRI slices with a spatial resolution of 1.7×3.4×5 mm and a temporal resolution of 5s were obtained. Each slice was centered on a transducer element which had a length of 9mm operating at 7.7MHz. Results showed high correlation between the desired target boundary and the 55° C isotherm with an average error of 1.0±1.5 mm (n = 5) for shape # 1 and 1.0±1.2 mm (n = 5) for shape # 2 across five slices with target volumes of approximately 53 cm3 and 58 cm3 respectively. The feasibility of MRI‐guided active feedback for accurate, 3D, multi‐planar treatments has been demonstrated and further investigation in vivo will be done.