Purpose:
Transcranial magnetic resonance‐guided focused ultrasound (TcMRgFUS) brain treatment systems compensate for skull‐induced beam aberrations by adjusting the phase and amplitude of individual ...ultrasound transducer elements. These corrections are currently calculated based on a preacquired computed tomography (CT) scan of the patient's head. The purpose of the work presented here is to demonstrate the feasibility of using ultrashort echo‐time magnetic resonance imaging (UTE MRI) instead of CT to calculate and apply aberration corrections on a clinical TcMRgFUS system.
Methods:
Phantom experiments were performed in three ex‐vivo human skulls filled with tissue‐mimicking hydrogel. Each skull phantom was imaged with both CT and UTE MRI. The MR images were then segmented into “skull” and “not‐skull” pixels using a computationally efficient, threshold‐based algorithm, and the resulting 3D binary skull map was converted into a series of 2D virtual CT images. Each skull was mounted in the head transducer of a clinical TcMRgFUS system (ExAblate Neuro, Insightec, Israel), and transcranial sonications were performed using a power setting of approximately 750 acoustic watts at several different target locations within the electronic steering range of the transducer. Each target location was sonicated three times: once using aberration corrections calculated from the actual CT scan, once using corrections calculated from the MRI‐derived virtual CT scan, and once without applying any aberration correction. MR thermometry was performed in conjunction with each 10‐s sonication, and the highest single‐pixel temperature rise and surrounding‐pixel mean were recorded for each sonication.
Results:
The measured temperature rises were ∼45% larger for aberration‐corrected sonications than for noncorrected sonications. This improvement was highly significant (p < 10−4). The difference between the single‐pixel peak temperature rise and the surrounding‐pixel mean, which reflects the sharpness of the thermal focus, was also significantly larger for aberration‐corrected sonications. There was no significant difference between the sonication results achieved using CT‐based and MR‐based aberration correction.
Conclusions:
The authors have demonstrated that transcranial focal heating can be significantly improved in vitro by using UTE MRI to compute skull‐induced ultrasound aberration corrections. Their results suggest that UTE MRI could be used instead of CT to implement such corrections on current 0.7 MHz clinical TcMRgFUS devices. The MR image acquisition and segmentation procedure demonstrated here would add less than 15 min to a clinical MRgFUS treatment session.
Purpose:
An iterative reconstruction method has been previously reported by the authors of this paper. However, the iterative reconstruction method was demonstrated by solely using the numerical ...simulations. It is essential to apply the iterative reconstruction method to practice conditions. The objective of this work is to validate the capability of the iterative reconstruction method for reducing the effects of acoustic heterogeneity with the experimental data in microwave induced thermoacoustic tomography.
Methods:
Most existing reconstruction methods need to combine the ultrasonic measurement technology to quantitatively measure the velocity distribution of heterogeneity, which increases the system complexity. Different to existing reconstruction methods, the iterative reconstruction method combines time reversal mirror technique, fast marching method, and simultaneous algebraic reconstruction technique to iteratively estimate the velocity distribution of heterogeneous tissue by solely using the measured data. Then, the estimated velocity distribution is used subsequently to reconstruct the highly accurate image of microwave absorption distribution. Experiments that a target placed in an acoustic heterogeneous environment are performed to validate the iterative reconstruction method.
Results:
By using the estimated velocity distribution, the target in an acoustic heterogeneous environment can be reconstructed with better shape and higher image contrast than targets that are reconstructed with a homogeneous velocity distribution.
Conclusions:
The distortions caused by the acoustic heterogeneity can be efficiently corrected by utilizing the velocity distribution estimated by the iterative reconstruction method. The advantage of the iterative reconstruction method over the existing correction methods is that it is successful in improving the quality of the image of microwave absorption distribution without increasing the system complexity.
Ultrasound imaging is widely used in medicine because of its benign characteristics and real-time capabilities. Physics theory suggests that the application of tomographic techniques may allow ...ultrasound imaging to reach its full potential as a diagnostic tool allowing it to compete with other tomographic modalities such as x-ray computer tomography, and MRI. This paper describes the construction and use of a prototype tomographic scanner and reports on the feasibility of implementing tomographic theory in practice and the potential of ultrasound (US) tomography in diagnostic imaging. Data were collected with the prototype by scanning two types of phantoms and a cadaveric breast. A specialized suite of algorithms was developed and utilized to construct images of reflectivity and sound speed from the phantom data. The basic results can be summarized as follows. (i) A fast, clinically relevant US tomography scanner can be built using existing technology. (ii) The spatial resolution, deduced from images of reflectivity, is
0.4
mm
. The demonstrated
10
cm
depth-of-field is superior to that of conventional ultrasound and the image contrast is improved through the reduction of speckle noise and overall lowering of the noise floor. (iii) Images of acoustic properties such as sound speed suggest that it is possible to measure variations in the sound speed of
5
m
∕
s
. An apparent correlation with x-ray attenuation suggests that the sound speed can be used to discriminate between various types of soft tissue. (iv) Ultrasound tomography has the potential to improve diagnostic imaging in relation to breast cancer detection.
Sonoelastography has been developed as an ultrasound-based elasticity imaging technique. In this technique, external vibration is induced into the target tissue. In general, tissue stiffness is ...inversely proportional to the amplitude of tissue vibration. Imaging tissue vibration will provide the elasticity distribution in the target region. This study investigated the feasibility of using real-time sonoelastography to detect and estimate the volume of thermal lesions in porcine livers in vivo. A total of 32 thermal lesions with volumes ranging from
0.2
to
5.3
cm
3
were created using radiofrequency ablation (RFA) or high-intensity focused ultrasound (HIFU) technique. Lesions were imaged using sonoelastography and coregistered B-mode ultrasound. Volumes were reconstructed from a sequence of two-dimensional scans. The comparison of sonoelastographic measurements and pathology findings showed good correlation with respect to the area of the lesions (
r
2
=
0.8823
for RFA lesions,
r
2
=
0.9543
for HIFU lesions). In addition, good correspondence was found between three-dimensional sonoelastography and gross pathology (3.6% underestimate), demonstrating the feasibility of sonoelastography for volume estimation of thermal lesions. These results support that sonoelastography outperforms conventional B-mode ultrasound and could potentially be used for assessment of thermal therapies.
Women with high mammographic breast density have a four- to fivefold increased risk of developing breast cancer compared to women with fatty breasts. Many preventative strategies have attempted to ...correlate changes in breast density with response to interventions including drugs and diet. The purpose of this work is to investigate the feasibility of assessing breast density with acoustic velocity measurements with ultrasound tomography, and to compare the results with existing measures of mammographic breast density. An anthropomorphic breast tissue phantom was first imaged with our computed ultrasound tomography clinical prototype. Strong positive correlations were observed between sound speed and material density, and sound speed and computed tomography number (Pearson correlation
coefficients
=
0.87
and 0.91, respectively). A cohort of 48 women was then imaged. Whole breast acoustic velocity was determined by creating image stacks and evaluating the sound speed frequency distribution. The acoustic measures of breast density were evaluated by comparing these results to two mammographic density measures: (1) qualitative estimates determined by a certified radiologist using the BI-RADS Categorical Assessment based on a 1 (fatty) to 4 (dense) scale, and (2) quantitative measurements via digitization and computerized analysis of archival mammograms. A one-way analysis of variance showed that a significant difference existed between the mean values of sound speed according to BI-RADS category, while post hoc analyses using the Scheffé criterion for significance indicated that BI-RADS 4 (dense) patients had a significantly higher sound speed than BI-RADS 1, 2, and 3 at an alpha level of 0.05. Using quantitative measures of breast density, a direct correlation between the mean acoustic velocity and calculated mammographic percent breast density was demonstrated with correlation coefficients ranging from 0.75 to 0.89. The results presented here support the hypothesis that sound speed can be used as an indicator of breast tissue density. Noninvasive, nonionizing monitoring of dietary and chemoprevention interventions that affect breast density are now possible.
A modified back-projection approach deduced from an exact reconstruction solution was applied to our photoacoustic tomography of the optical absorption in biological tissues. Pulses from a ...Ti:sapphire laser (4.7 ns FWHM at 789.2 nm) were employed to generate a distribution of photoacoustic sources in a sample. The sources were detected by a wide-band nonfocused ultrasonic transducer at different positions around the imaging cross section perpendicular to the axis of the laser irradiation. Reconstructed images of phantoms made from chicken breast tissue agreed well with the structures of the samples. The resolution in the imaging cross section was experimentally demonstrated to be better than 60 μm when a 10 MHz transducer (140% bandwidth at −60 dB) was employed, which was nearly diffraction limited by the detectable photoacoustic waves of the highest frequency.
Purpose:
In this work, the authors propose a modeling approach to compute the nonlinear acoustic field generated by a flat piston transmitter with an attached aluminum lens.
Methods:
In this ...approach, the geometrical parameters (radius and focal length) of a virtual source are initially determined by Snell’s refraction law and then adjusted based on the Rayleigh integral result in the linear case. Then, this virtual source is used with the nonlinear spheroidal beam equation (SBE) model to predict the nonlinear acoustic field in the focal region.
Results:
To examine the validity of this approach, the calculated nonlinear result is compared with those from the Westervelt and (Khokhlov-Zabolotskaya-Kuznetsov) KZK equations for a focal intensity of 7 kW/cm2. Results indicate that this approach could accurately describe the nonlinear acoustic field in the focal region with less computation time.
Conclusions:
The proposed modeling approach is shown to accurately describe the nonlinear acoustic field in the focal region. Compared with the Westervelt equation, the computation time of this approach is significantly reduced. It might also be applicable for the widely used concave focused transmitter with a large aperture angle.
High mechanical index (MI) echocardiography with contrast agent has been shown to induce Evans blue staining of cardiomyocytes, seen 1 d after exposure, in addition to contraction band necrosis, seen ...immediately after exposure. This research examined the roles of necrosis vs. apoptosis in these bioeffects. Myocardial contrast echocardiography at high MI with 1:4 electrocardiogram triggering was performed in anesthetized rats at 1.5 MHz. Histologically observable cell injury was accumulated by infusing a high dose of 50 microL/kg ultrasound contrast media via tail vein for 5 min at the start of 10 min of scanning. Evans blue dye or propidium iodide was injected as an indicator of cardiomyocyte plasma membrane integrity. Histologic sections were stained using the terminal dUTP nick-end labeling (TUNEL) method for labeling nuclei with DNA degradation (e.g., apoptosis). Evans blue fluorescent cells were counted on frozen sections or on hematoxylin-stained and TUNEL-labeled paraffin sections. In addition, transmission electron microscopy was used to assess potential apoptotic nuclei. Hypercontraction and propidium iodide staining were observed immediately after imaging exposure. Although TUNEL-positive cells were evident after 4 h, these also had indications of contraction band necrosis, and features of apoptosis were not confirmed by electron microscopy. Inflammatory cell infiltration was evident after 24 h. A second, more subtle injury was recognized by Evans blue staining, with minimal inflammatory cell infiltration at the morphologically intact stained cells after 24 h. Apoptosis was not detected by the TUNEL method in the cardiomyocytes stained with Evans blue at 24 h. However, Evans blue-stained cell numbers declined after 48 h, with continued inflammatory cell infiltration. The initial insult for Evans blue-stained cardiomyocytes apparently induced partial permeability of the plasma membrane, which led to gradual degeneration (but not apoptosis) and necrosis after 24 to 48 h.
A new ultrasound applicator with three-dimensional power distribution control was developed for simultaneous thermoradiotherapy. The system was named SURLAS for Scanning Ultrasound Reflector Linear ...Arrays System. In this paper, the hardware of the first clinical grade SURLAS applicator is described with emphasis on clinically important static acoustic characteristics and on construction aspects not reported before. Functionally, the SURLAS applicator consists of two parallel opposed ultrasound linear arrays aiming their acoustic beams to a
V
-shape scanning ultrasound reflector, which deflects the beams coming from opposite directions toward the treatment area. The reciprocating motion of the reflector in-between the arrays spreads the ultrasonic energy over the target area scanned. Control of power deposition over the 16 cm by 16 cm treatment window area is achieved by adjusting the power input into the transducer elements of the arrays as a function of the position of the scanning reflector. Furthermore, the arrays operate at significantly different frequencies (1.9 and 4.9 MHz) so that intensity modulation of beams of different frequencies can be exploited to adjust the depth of energy penetration. With this design, external electron or photon beams can be concurrently delivered with hyperthermia by irradiating through the applicator’s body. Safety features were implemented into the applicator’s design to monitor its performance during operation. A detailed description of the applicator including impedance matching circuits/filters, radiation force balance power measurements, hydrophone pressure field distribution measurements, as well as safety test results are reported.
An imaging technology, thermoacoustic tomograpy (TAT), was applied to the visualization of high-intensity focused ultrasound (HIFU)-induced lesions. A single, spherically focused ultrasonic ...transducer, operating at a central frequency of approximately 4 MHz, was used to generate a HIFU field in fresh porcine muscle. Microwave pulses from a 3-GHz microwave generator were then employed to generate thermoacoustic sources in this tissue sample. The thermoacoustic signals were detected by an unfocused ultrasonic transducer that was scanned around the sample. To emphasize the boundaries between the lesion and its surrounding tissue, a local-tomography-type reconstruction method was applied to reconstruct the TAT images of the lesions. Good contrast was obtained between the lesion and the tissue surrounding it. Gross pathologic photographs of the tissue samples confirmed the TAT images.This work demonstrates that TAT may potentially be used to image HIFU-induced lesions in biological tissues.
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