Citations

The mechanical response of tissues to external forces has gained considerable interest in medical diagnosis. One approach of imaging the tissue elastic properties is to apply an external force on the surface of the body. Another approach is to generate a localized force inside the tissue with the radiation force of ultrasound. In this paper, a noninvasive method for imaging the vibration mode of an object is investigated. The radiation force of ultrasound is used to scan the object at a resonant frequency of the object. The vibration of the object is measured by laser and the resulting acoustic emission from the object is measured by a hydrophone. Experimental studies are carried out on a mechanical heart valve and arterial phantoms. The mode images on the valve are made by the hydrophone measurement and confirmed by FEM simulations. Compared with conventional B-scan imaging on arterial phantoms, the mode imaging can show not only the interface of the artery and the gelatin, but also the vibration modes of the artery. The images taken on the phantom surface suggest that an image of an interior artery can be made by vibration measurements on the surface of the body.

Vibro-acoustography is an imaging method based on the sound emitted due to the harmonic motion of tissue induced by the radiation force of focused ultrasound. This paper presents recent progress in vibro-acoustography of human breast. Matching vibro-acoustography and mammography images are acquired using a combined mammography/vibro-acoustography machine. Images of various types of breast masses are investigated and their characteristics features are compared. The study includes malignant masses, benign masses such as fibroadenomas, cysts, calcified lesions, and surgical scars. Image features, such as lesion borders, interior texture, and speculation pattern, are studied. Vibro-acoustography images demonstrate that calcified masses normally show sharp borders, benign noncalcified masses have smooth texture, and, in some cases, malignant masses are characterized with spiculation. These results, which are based on the data collected from an ongoing study, indicate that the above-mentioned image features may be used for differentiation of breast masses. It is noted that more data from a larger population are needed to establish the results in a statistical sense. [Work supported by NIH Grant EB-00535 and Grant BCTR0504550 from the Susan G. Komen Breast Cancer Foundation. Disclosure: Parts of the techniques used here are patented by MF and JFG.]

Dynamic ultrasound radiation force can induce local tissue vibration. To understand the tissue response, we developed a method to ultrasonically measure the induced motion. We present a model for motion detection in a vibrating scattering medium. We developed a harmonic pulsed excitation method using tonebursts repeated at frequency fr. This excitation method creates a multifrequency force with components at harmonics of fr. We use pulse-echo ultrasound and narrowband Kalman filtering to perform multifrequency motion detection. In an experiment, a 3.0 MHz air-backed transducer was used to create the radiation force, and with the same transducer performed pulse-echo motion detection at 9.0 MHz. Bovine muscle was used for radiation force excitation and motion detection. We present simulation results for a parameterized model of motion detection in a scattering medium. We show experimental measurements of vibration displacement and phase for motion with amplitude of 0.1–10 μm. Our simulation model provides a platform to optimize detection of small harmonic motion in a scattering medium. Multifrequency harmonic pulsed excitation and detection methods are presented that induce and measure very small amplitude harmonic motion.

BACKGROUNDBurnout is a syndrome affecting the entirety of work life and characterized by cynicism, detachment, and inefficacy. Despite longstanding concerns about burnout in hospital medicine, few data about burnout in hospitalists have been published.PURPOSEA systematic review of the literature on burnout in inpatient‐based and outpatient‐based physicians worldwide was undertaken to determine whether inpatient physicians experience more burnout than outpatient physicians.DATA SOURCESFive medical databases were searched for relevant terms with no language restrictions. Authors were contacted for unpublished data and clarification of the practice location of study subjects.STUDY SELECTIONTwo investigators independently reviewed each article. Included studies provided a measure of burnout in inpatient and/or outpatient nontrainee physicians.DATA EXTRACTIONFifty‐four studies met inclusion criteria, 15 of which provided direct comparisons of inpatient and outpatient physicians. Twenty‐eight studies used the same burnout measure and therefore were amenable to statistical analysis.DATA SYNTHESISOutpatient physicians reported more emotional exhaustion than inpatient physicians. No statistically significant differences in depersonalization or personal accomplishment were found. Further comparisons were limited by the heterogeneity of instruments used to measure burnout and the lack of available information about practice location in many studies.CONCLUSIONSThe existing literature does not support the widely held belief that burnout is more frequent in hospitalists than outpatient physicians. Better comparative studies of hospitalist burnout are needed. Journal of Hospital Medicine 2013;8:653–664. © 2013 Society of Hospital Medicine

The ultrasound radiation force can be used as a noncontact method for exciting vibrational modes. In one implementation, an audio-range modulation signal is combined with a 40-kHz carrier to form a double-sideband suppressed-carrier (DSSC) waveform that is emitted from an ultrasound transducer. The frequency of the ultrasound radiation force is the difference between the frequencies of the sidebands, therefore at twice the modulation frequency. We describe a method for selective mode excitation using a transducer pair, each emitting a DSSC signal. The modulation signal for one transducer was phase shifted relative to the other transducer. When the two modulation signals were in phase, the radiation stress field was uniform. For devices studied, this excited transverse modes while suppressing torsional modes. Similarly, if the modulation signals were out of phase, torsional modes were excited while suppressing transverse modes. This technique has been demonstrated for devices including an MEMS mirror, hard-drive suspension, and brass cantilever. For a cantilever, the ratio of the amplitudes of 180-Hz transverse over 1.3-kHz torsional modes was varied by over two orders of magnitude by shifting the phase difference between the two beams over a range of 90 deg. This noncontact technique allows control over the types of vibration excited.

Vibro-acoustography is a method that uses ultrasound radiation force to investigate the mechanical response of an object [Fatemi and Greenleaf, ‘‘Ultrasound-stimulated vibro-acoustic spectrography,’’ Science, 280, 82–85 (1998)]. The dynamic radiation force causes the object to vibrate within the focal region of the transducer, which creates a sound field called acoustic emission measured by a hydrophone. To gain a better understanding of the objects investigated with vibro-acoustography, a method is proposed that measures the displacement of the local vibration. A pulse-echo ultrasound method is implemented with the same transducer used to create the radiation force to measure the displacement of the object. A Kalman filter approach is used to process the pulse-echo data to obtain estimates of the local displacement [Zheng et al., ‘‘Kalman filter motion detection for vibroacoustography using pulse-echo ultrasond,’’ 2003 IEEE Ultrasonics Symposium, pp. 1812–1815 (2003)]. Results using this ultrasound method to measure vibration of a metal beam compare well with laser vibrometer results. This method can reliably measure displacements with amplitudes of 120–2000 nm with error less than 9%. A pulse-echo method is described that provides displacement detection with simultaneous radiation force generation for vibro-acoustography applications using a single transducer. [Work supported by Grants EB002640 and EB002167 from NIH.]

Increased stiffness of arteries has recently gained acceptance as an independent risk factor for cardiovascular and many other diseases. Pulse wave velocity (PWV) is widely used for estimating the stiffness of an artery. However, PWV is an average indicator of artery stiffness between the two measuring points, and therefore does not identify local stiffness. In addition, the thickness of the artery is needed to calculate artery elastic modulus. We have discovered that there is a series of ring resonant frequency in arteries that can be used for estimating local arterial elastic modulus. These resonances can be excited by remote palpation with selectively placed ultrasonic radiation force. Experiments on ring resonance were carried out on an excised artery as well as a rubber tube. This family of resonances occurs at relatively low frequency, around 356 Hz for the artery. Estimation of the Young’s modulus is 135 kPa by the ring frequency 356 Hz without the requirement of the arterial thickness. The estimated modulus is very consistent with the other ring resonant frequency measurements. The theory of ring resonance is developed and verified with the rubber tube experiments.

Low-intensity pulsed ultrasound is a commonly prescribed therapy for delayed unions and nonunions after fractures. Several prospective double-blind clinical trials have shown that a 1.5-MHz ultrasound signal at a 200-s tone burst repeating at 1 kHz shortens the time to clinically evaluated healing by 30%. We have shown in vitro that pulsed ultrasound increases aggrecan gene expression and aggrecan production in chondrocytes, a key cell in the fracture healing process. Because the square wave pulsed 1.5-MHz signal produces radiation force vibration at 1 kHz, we tested the hypothesis that dynamic radiation force, not ultrasound, is responsible for the biological effect of the signal. Experiments showed that a 1-kHz acoustic square wave, simulating the radiation force caused by the clinically used signal, induced chondrogenesis similarly to pulsed ultrasound treatment in ATDC5 cells. These results may have implications for remote ultrasound or acoustic stimulation of different types of strain-sensitive cells in addition to those responsible for fracture healing.