Tissue-mimicking oil-in-gelatin dispersions for use in heterogeneous elastography phantoms.

A ten-month study is presented of materials for use in heterogeneous elastography phantoms. The materials consist of gelatin with or without a suspension of microscopic safflower oil droplets. The highest volume percent of oil in the materials is 50%. Thimerosal acts as a preservative. The greater the safflower oil concentration, the lower the Young's modulus. Elastographic data for heterogeneous phantoms, in which the only variable is safflower oil concentration, demonstrate stability of inclusion geometry and elastic strain contrast. Young's modulus ratios (elastic contrasts) producible in a heterogeneous phantom are as high as 2.7. The phantoms are particularly useful for ultrasound elastography. They can also be employed in MR elastography, although the highest achievable ratio of longitudinal to transverse relaxation times is considerably less than is the case for soft tissues.

Poroelastography: imaging the poroelastic properties of tissues.

In the field of elastography, biological tissues are conveniently assumed to be purely elastic solids. However, several tissues, including brain, cartilage and edematous soft tissues, have long been known to be poroelastic. The objective of this study is to show the feasibility of imaging the poroelastic properties of tissue-like materials. A poroelastic material is a material saturated with fluid that flows relative to a deforming solid matrix. In this paper, we describe a method for estimating the poroelastic attributes of tissues. It has been analytically shown that during stress relaxation of a poroelastic material (i.e., sustained application of a constant applied strain over time), the lateral-to-axial strain ratio decreases exponentially with time toward the Poisson's ratio of the solid matrix. The time constant of this variation depends on the elastic modulus of the solid matrix, its permeability and its dimension along the direction of fluid flow. Recently, we described an elastographic method that can be used to map axial and lateral tissue strains. In this study, we use the same method in a stress relaxation case to measure the time-dependent lateral-to-axial strain ratio in poroelastic materials. The resulting time-sequenced images (poroelastograms) depict the spatial distribution of the fluid within the solid at each time instant, and help to differentiate poroelastic materials of distinct Poisson's ratios and permeabilities of the solid matrix. Results are shown from finite-element simulations.

Equipment specifications for supported treadmill ambulation training.

Supported Treadmill Ambulation Training (STAT) is a mode of therapy for gait retraining for patients with spinal cord injuries or other upper motor neuron dysfunction. The STAT program involves simultaneously supporting a portion of the patient's weight while gait training on a treadmill. STAT has been successful in improving the gait of many research subjects, but has not been widely applied in clinical practice. The goal of this study was to acquire practical, clinically useful information regarding this therapeutic intervention in order to remove barriers to its use. This manuscript enumerates equipment specifications for the treadmill, body weight support (BWS) system, and harness. The ergonomics of the work space are also considered, since the therapist(s) will need access to the patient's legs during therapy. The specific recommendations were determined through prior clinical experience, consultation of anthropometric tables, and application of engineering principles. The guidelines listed are intended to facilitate safe and effective application of the therapy at minimum hardware cost.

Nonlinear stress-strain relationships in tissue and their effect on the contrast-to-noise ratio in elastograms.

The practice of elastography is generally limited to small applied compressions (typically 1%), under the assumption of a linear stress-strain relationship in biological tissue. However, the recent reports of larger applied compressions and precompression levels to increase the strain contrast violate the above assumption. The nonlinear stress-strain relationships in different breast tissue types significantly alter the contrast in elastography, especially for large applied compression. The moduli of normal fibrous and glandular breast tissue (along with cancerous lesions) are strain-dependent, with tissue stiffness increasing with applied compression. In this paper, we illustrate that the strain-dependence of the modulus has a significant impact on the elastographic contrast and on the contrast-to-noise ratio, and may even cause a reversal of the contrast in certain situations. This paper also emphasizes the effect of the precompression strain level on the strain contrast.

Elastographic characterization of HIFU-induced lesions in canine livers.

The elastographic visualization and evaluation of high-intensity focused ultrasound (HIFU)-induced lesions were investigated. The lesions were induced in vitro in freshly excised canine livers. The use of different treatment intensity levels and exposure times resulted in lesions of different sizes. Each lesion was clearly depicted by the corresponding elastogram as being an area harder than the background. The strain contrast of the lesion/background was found to be dependent on the level of energy deposition. A lesion/background strain contrast between -2.5 dB and -3.5 dB was found to completely define the entire zone of tissue damage. The area of tissue damage was automatically estimated from the elastograms by evaluating the number of pixels enclosed inside the isointensity contour lines corresponding to a strain contrast of -2.5, -3 and -3.5 dB. The area of the lesion was measured from a tissue photograph obtained at approximately the same plane where elastographic data were collected. The estimated lesion areas ranged between approximately 10 mm2 and 110 mm2. A high correlation between the damaged areas as depicted by the elastograms and the corresponding areas as measured from the gross pathology photographs was found (r2 = 0.93, p value < 0.0004, n = 16). This statistically significant high correlation demonstrates that elastography has the potential to become a reliable and accurate modality for HIFU therapy monitoring.

Comparison of upper limb muscle activity in four walking canes: a preliminary study.

It is hypothesized that the cane and forearm can be aligned to reduce muscular activity needed during ambulation. In this prospective study, 10 nonimpaired control subjects were tested both in stationary and dynamic weight-bearing states while using different cane prototypes and the platform cane (PC) as compared to the standard cane (SC). The outcome measures were: 1) root mean square (RMS) voltage (µV) of electromyographic (EMG) signal as a measure of muscle power and 2) distance of ambulation. Results of stationary cane use showed that Prototype 1 decreased RMS output by 19 percent (p=0.01), Prototype 2 with wrist splint decreased it by 23 percent (p=0.003), and the PC decreased it by 68 percent (p<0.0001) as compared to the SC (ANOVA, posthoc LSM). In conclusion, the two prototypes and the PC significantly decrease RMS voltage muscle output in the upper limb, compared to the SC.

Electromyographic localization of the palmaris brevis muscle.

The palmaris brevis (PB) is the only muscle frequently innervated by the superficial branch of the ulnar nerve. It is clinically involved in patients with sensory impairment of the ulnar nerve sparing the deep branch as seen in mild cases of cyclist or crutch palsy. It is also involved in palmaris brevis spasm syndrome. A technique is described that makes it possible to localize the PB distinctly from its neighbors by electromyographic (EMG) pin examination. Methods include prospective (1) anatomic measurement in 10 cadavers and (2) EMG pin examination in 12 normal subjects measuring rise time (ms) and RMS voltage output (microV). Results showed (1) the area of the PB in elderly cadavers was 2.1+/-0.7 x 2.1+/-0.3 cm with a thickness of 0.2+/-.01 cm and (2) in the live subjects, the muscle could be located with identification of motor unit potentials having a rise time <1 ms 100% of the time. In conclusion, the PB can be localized for EMG analysis by using an approach parallel to surface of the palm. Clinically, it could be a useful complement to diagnose neuropathy involving the superficial fibers of the ulnar nerve.

Elastic moduli of breast and prostate tissues under compression.

To evaluate the dynamic range of tissue imaged by elastography, the mechanical behavior of breast and prostate tissue samples subject to compression loading has been investigated. A model for the loading was validated and used to guide the experimental design for data collection. The model allowed the use of small samples that could be considered homogeneous; this assumption was confirmed by histological analysis. The samples were tested at three strain rates to evaluate the viscoelastic nature of the material and determine the validity of modeling the tissue as an elastic material for the strain rates of interest. For loading frequencies above 1 Hz, the storage modulus accounted for over 93 percent of the complex modulus. The data show that breast fat tissue has a constant modulus over the strain range tested while the other tissues have a modulus that is dependent on the strain level. The fibrous tissue samples from the breast were found to be 1 to 2 orders of magnitude stiffer than fat tissue. Normal glandular breast tissue was found to have an elastic modulus similar to that of fat at low strain levels, but the modulus of the glandular tissue increased by an order of magnitude above fat at high strain levels. Carcinomas from the breast were stiffer than the other tissues at the higher strain level; intraductal in situ carcinomas were like fat at the low strain level and much stiffer than glandular tissue at the high strain level. Infiltrating ductal carcinomas were much stiffer than any of the other breast tissues. Normal prostate tissue has a modulus that is lower than the modulus of the prostate cancers tested. Tissue from prostate with benign prostatic hyperplasia (BPH) had modulus values significantly lower than normal tissue. There was a constant but not significant difference in the modulus of tissues taken from the anterior and posterior portions of the gland.

Elastographic imaging of thermal lesions in soft tissue: a preliminary study in vitro.

The use of elastography for the visualization of thermal lesions in biological soft tissue in vitro was investigated. Thermal lesions were created in samples of postmortem ovine kidney using a surgical neodymium: YAG (Nd:YAG) laser. The kidney samples were cast in gel, and elastographic images of the lesions were constructed using sonographic information and external markers to locate the region of interest. Gross pathology of the kidney samples confirmed the dimensions of the lesions. Good correlation between the lesion length along the laser fiber axis and maximum diameter measured off of the fiber axis determined from elastographic images and gross pathology photographs was found.

Augmented sensory nerve action potentials during distant muscle contraction.

We previously reported that the median sensory nerve action potentials (SNAP) increased in amplitude during ipsilateral abductor pollicis brevis contraction. The objectives of the present project were to study the timing and origin of this phenomenon and to eliminate the possibility of local artifact. Ten normal subjects were recruited. The baseline was established using ten threshold stimuli, which were delivered to the median nerve at the wrist at 0.2 Hz. Using the same stimulus strength, the SNAP was recorded while the tibialis anterior was contracted at 25, 50, 75, and 100% of maximum force. Responses were signal-averaged. Results showed an increase in ipsilateral SNAP amplitude between baseline and maximum contraction of 6 +/- 2 microV (standard error, P = 0.004) and contralateral amplitude of 8 +/- 2 microV (standard error, P = 0.01). Statistical analysis was performed with analysis of variance for repeated measures and paired t test. The effect peaked between 0 and 10 min after contraction and lasted from 1.5 to more than 20 min after muscle relaxation. In conclusion, SNAP appear to be enhanced during and after muscle contraction. Theories concerning underlying causes for this event are discussed.

Demonstrating a model approach to independent living center-based assistive technology services.

This project examined the effectiveness of an assistive technology preventive maintenance service for consumers. Sixteen independent living centers that offer mobility equipment repair services for consumers were surveyed. Thirty-seven consumers and 22 vendors supplying mobility equipment and repair services within the Houston area were interviewed. Problems identified included lack of available services to maintain mobility equipment in good working order, no centralized information and evaluation system, denials of needed equipment by third party payers, and high equipment costs. A collaborative effort between the Independent Living Research Utilization Research and Training Center on Independent Living at The Institute for Rehabilitation and Research in the Texas Medical Center, and the Houston Center for Independent Living designed and implemented a model preventive maintenance service--The Wheelchair Maintenance Clinic. Evaluation after the first year of operating the clinic showed 111 service visits were delivered to 50 consumers. Consumers reported high levels of satisfaction with the service.

Evaluating the long-term performance of a foam-core hospital replacement mattress.

Many hospitals have begun replacing their standard mattresses with pressure-reducing foam mattresses to eliminate the costly overlays that have been used for comfort and to protect patients at risk for development of pressure ulcers. Although researchers have tested the performance of new mattresses and have shown that many may be of use with patients at high risk, no information is available on the pressure-reducing capabilities of used mattresses. This study was designed to evaluate changes with use in the support characteristics of foam-core hospital replacement mattresses. In the first part of the study, interface pressure measurements were made at regular intervals over an extended period of use. Sixty mattresses were randomly selected from 750 mattresses currently in use at St. Luke's Episcopal Hospital, Texas Medical Center, Houston. The mattresses were labeled and marked at three selected points, and interface pressures under a standard load were measured at approximately 3-month intervals for 12 months. In the second part of the study, the stiffness of the foam, a measure of the foam's ability to support a weight, was measured at selected points on the mattresses. Patient reports of discomfort in the sacral area and of "sinking" into the middle of the mattress prompted this phase of the study. The stiffness profiles of 25 new mattresses were compared with those of 25 randomly selected mattresses that had been in regular use for at least 24 months. Data analyzed from part one showed trends that indicated that the interface pressures increased at each site during the course of the study; however, an analysis of variance indicated that the changes were not statistically significant during the time frame of the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Custom selection of support surfaces for wheelchairs and beds: one size does not fit all.

Support surfaces aid in preventing pressure sores and trauma, correct posture, and provide patient comfort. Knowing how to custom select a support surface is vital for meeting the patient's protective, postural, and pain-prevention needs.

Pressure relief characteristics of a new foam overlay: a preliminary performance evaluation.

To evaluate a newly designed foam mattress to be used by patients who are susceptible to pressure ulcers, interface pressures that are produced between body and mattress were measured with the Mini-Texas interface pressure evaluator (Tee-Kay Applied Technology, Inc., Stafford, Texas). Fifteen subjects with builds that ranged from thin to large were measured at four body locations--the scapular area, sacral area, trochanter, and heel. The interface pressure measurements were found to increase with body size in the sacral and heel areas. The pressures under the sacral, scapular, and heel areas remained within a narrow range for a given body type.

Toward a practical mobile robotic aid system for people with severe physical disabilities.

A simple, relatively inexpensive robotic system that can aid severely disabled persons by providing pick-and-place manipulative abilities to augment the functions of human or trained animal assistants is under development at Rice University and the Baylor College of Medicine. A stand-alone software application program runs on a Macintosh personal computer and provides the user with a selection of interactive windows for commanding the mobile robot via cursor action. A HERO 2000 robot has been modified such that its workspace extends from the floor to tabletop heights, and the robot is interfaced to a Macintosh SE via a wireless communications link for untethered operation. Integrated into the system are hardware and software which allow the user to control household appliances in addition to the robot. A separate Machine Control Interface device converts breath action and head or other three-dimensional motion inputs into cursor signals. Preliminary in-home and laboratory testing has demonstrated the utility of the system to perform useful navigational and manipulative tasks.

Kinetics of colonization of a porous vitreous carbon percutaneous implant.

Implants of porous vitreous carbon with pore diameters 200-500 microns were surgically placed in rabbits and pigs. Skin colonization experiments were carried out by topically inoculating concentrations of Staphylococcus aureus and Escherichia coli in a test area adjacent to the implant and to a remote control area. Subsequent swab cultures were taken at 6, 24, 48 and 72 h and one or more weeks. In vitro attachment studies were also performed using bacteria stained with FITC on 1 mm slices of the porous carbon. Proplast was used as a control. Results showed that despite a temporary high rate of colonization and obvious binding of the bacteria to the carbon, the skin-implant interface resists infection by both normal and pathogenic flora.

Noninvasive measurement of the stiffness of tissue in the above-knee amputation limb.

In order to characterize the mechanical properties of the soft tissue in above-knee amputations, nine subjects were measured with a Doppler ultrasound system. Measurements were made at four locations: anterior, posterior, lateral, and medial. The characterizations included tissues up to 2.5 cm deep. The average posterior moduli are significantly greater than the anterior and lateral parameters. No significant difference was found among moduli from the medial zone compared to other areas. Superficial tissue had a significantly higher modulus than the tissue beneath. A simple method for transducer placement produced repeatable results. The present technique proved to be useful with patients in good health, and with no severe residual limb complications. The information generated with the ultrasonic device may aid in prosthesis fitting and will be used in a Computer-Aided Design (CAD) system as well as in other clinical applications.