Tissue properties such as elasticity and viscosity have been shown to be related to such tissue conditions as contraction, edema, fibrosis, and fat content among others. Magnetic Resonance Elastography has shown outstanding ability to measure the elasticity and in some cases the viscosity of tissues, especially in the liver, providing the ability to stage fibrotic liver disease similarly to biopsy. We discuss ultrasound methods of measuring elasticity and viscosity in tissues. Many of these methods are becoming widely available in the extant ultrasound machines distributed throughout the world. Some of the methods to be discussed are in the developmental stage. The advantages of the ultrasound methods are that the imaging instruments are widely available and that many of the viscoelastic measurements can be made during a short addition to the normal ultrasound examination time. In addition, the measurements can be made by ultrasound repetitively and quickly allowing evaluation of dynamic physiologic function in circumstances such as muscle contraction or artery relaxation. Measurement of viscoelastic tissue mechanical properties will become a consistent part of clinical ultrasound examinations in our opinion.
Effective brachytherapy procedures require precise placement of radioactive seeds in the prostate. Currently, transrectal ultrasound (TRUS) imaging is one of the main intraoperative imaging modalities to assist physicians in placement of brachytherapy seeds. However, the seed detection rate with TRUS is poor mainly because ultrasound imaging is highly sensitive to variations in seed orientation. The purpose of this study is to investigate the abilities of a new acoustic radiation force imaging modality, vibro-acoustography (VA), equipped with a 1.75D array transducer and implemented on a customized clinical ultrasound scanner, to image and localize brachytherapy seeds in prostatic tissue. To perform experiments, excised cadaver prostate specimens were implanted with dummy brachytherapy seeds, and embedded in tissue mimicking gel to simulate the properties of the surrounding soft tissues. The samples were scanned using the VA system and the resulting VA signals were used to reconstruct VA images at several depths inside the tissue. To further evaluate the performance of VA in detecting seeds, X-ray computed tomography (CT) images of the same tissue sample, were obtained and used as a gold-standard to compare the number of seeds detected by the two methods. Our results indicate that VA is capable of imaging of brachytherapy seeds with accuracy and high contrast, and can detect a large percentage of the seeds implanted within the tissue samples.
Vibro-acoustography is a speckle-free ultrasound based imaging modality that can visualize normal and abnormal soft
tissue through mapping stimulated acoustic emission. The acoustic emission is generated by focusing two ultrasound
beams of slightly different frequencies (Δf = f1-f2) to the same spatial location and vibrating the tissue as a result of
ultrasound radiation force. Reverberation of the acoustic emission can create dark and bright areas in the image that
affect overall image contrast and detectability of abnormal tissue. Using finite length tonebursts yields acoustic emission
at Δf and at sidebands centered about Δf that originate from the temporal toneburst gating. Separate images are formed by
bandpass filtering the acoustic emission at Δf and the associated sidebands. The data at these multiple frequencies are
compounded through coherent or incoherent processes to reduce the artifacts associated with reverberation of the acoustic emission. Experimental results from a urethane breast phantom and in vivo human breast scans are shown. The reduction in reverberation artifacts are analyzed using a smoothness metric which uses the variances of the gray levels of the original images and those formed through coherent and incoherent compounding of image data. This smoothness metric is minimized when the overall image background is smooth while image features are still preserved. The smoothness metric indicates that the images improved by factors from 1.23-4.33 and 1.09-2.68 in phantom and in vivo studies, respectively. The coherent and incoherent compounding of multifrequency data demonstrate, both qualitatively and quantitatively, the efficacy of this method for reduction of reverberation artifacts.
Vibro-acoustography is an imaging method based on the vibro-acoustic response of the object to a low-frequency radiation force of ultrasound. Here, we present the results of a study on detection mass lesions by vibro-acoustography. Experiments were conducted on excised human liver tissues that included focal mass lesions which were a few mm to a few cm in diameter. The 3 MHz transducer used for this purpose provided a 0.7 mm lateral resolution. The focal length of the transducer was long enough to cover the entire 5 mm thickness of the specimen. Several scans of each sample were obtained. Resulting images distinctively showed the normal liver tissue and the mass lesions. Masses appeared with enhanced boundary and rough textures in VA which allowed us to delineate the masses from the surrounded tissue. These results suggest that vibro-acoustography may be a clinically useful imaging modality for detection of mass lesions in soft tissue.