Variability of CubeQuant T1ρ, quantitative DESS T2, and cones sodium MRI in knee cartilage.

OBJECTIVE: To measure the variability of T1ρ relaxation times using CubeQuant, T2 relaxation times using quantitative double echo in steady state (DESS), and normalized sodium signals using 3D cones sodium magnetic resonance imaging (MRI) of knee cartilage in vivo at 3 T. DESIGN: Eight healthy subjects were scanned at 3 T at baseline, 1 day, 5 months, and 1 year. Ten regions of interest (ROIs) of knee cartilage were segmented in the medial and lateral compartments of each subject's knee. T1ρ and T2 relaxation times and normalized sodium signals were measured and the root-mean-square coefficient of variation (CVRMS) was calculated. Intra-subject variability was measured over short, moderate and long-term, as well as intra-observer and inter-observer variability. RESULTS: The average intra-subject CVRMS measurements over short, moderate, and long-term time periods were 4.6%, 6.1%, and 6.0% for the T1ρ measurements, 6.4%, 9.3%, and 10.7% for the T2 measurements and 11.3%, 11.6%, and 12.9% for the sodium measurements, respectively. The average CVRMS measurements for intra-observer and inter-observer segmentation were 3.8% and 5.7% for the T1ρ measurements, 4.7% and 6.7% for the T2 measurements, and 8.1% and 11.4% for the sodium measurements, respectively. CONCLUSIONS: These CVRMS measurements are substantially lower than previously measured changes expected in patients with advanced osteoarthritis compared to healthy volunteers, suggesting that CubeQuant T1ρ, quantitative DESS T2 and 3D cones sodium measurements are sufficiently sensitive for in vivo cartilage studies.

Simultaneous magnetic resonance phase and magnitude temperature maps in muscle.

Noninvasive magnetic resonance temperature maps that are used to monitor thermal ablation of tissue are described. In magnetic resonance images, thermally induced proton nuclear magnetic resonance frequency shifts, and changes in the longitudinal relaxation time produce both phase and magnitude changes in the MR signal. Temperature maps with improved sensitivity are derived from the complex-difference nuclear magnetic resonance signal. Bovine muscle specimens were heated with focused ultrasound to model thermal surgery and create a known thermal distribution to test the method. Resulting MR images acquired in 2 s produce temperature maps with 1 min resolution and 2 degrees C temperature sensitivity. The temperature sensitivity was increased by extending the acquisition to 5 s, by decreasing the receiver bandwidth, and increasing the echo time.

A clinical, noninvasive, MR imaging-monitored ultrasound surgery method.

A noninvasive method of tissue ablation that is guided and monitored with magnetic resonance (MR) imaging has been developed. The method uses sharply focused ultrasound transducers of different focal lengths to induce a localized temperature elevation during a short exposure (1-20 seconds). A hydraulic, computer-controlled positioning device moves the transducer in an MR imager. The positioner is built into a standard cradle in the imager. The system includes cavitation detection and power monitoring circuitry for patient safety. The target volume is outlined with cross-sectional MR images obtained immediately before sonication. By means of the software, the focus is moved to ablate the volume defined with the images. The temperature elevation during the exposure is monitored by means of the proton resonance frequency shift with fast gradient-echo sequences, and the necrosed volume is demonstrated with T2-weighted fast spin-echo images. This method has been extensively tested in in vivo animal experiments and is now undergoing clinical trial.

Focused US system for MR imaging-guided tumor ablation.

PURPOSE: To measure the performance characteristics of a focused ultrasound (US) system for magnetic resonance (MR) imaging-guided tumor ablation. MATERIALS AND METHODS: The authors constructed a focused US system for MR imaging-guided tumor ablation. The location of the heated region and thermal dose were monitored with temperature-sensitive MR images obtained in phantoms and rabbit skeletal muscle after application of each sonic pulse. RESULTS: The region heated by the focused ultrasound beam was within 1 mm of that observed on temperature-sensitive fast gradient-echo MR images of in vivo rabbit skeletal muscle. Analysis of heat flow and the rate of coagulation necrosis provided an estimate of the size of the ablated region that was in agreement with experimental findings. CONCLUSION: MR imaging provides target definition and control for thermal therapy in regions of variable perfusion or in tissues that are not well characterized.

MR temperature mapping of focused ultrasound surgery.

Deep lying soft tissue tumors may be treated by a nonincisional surgical procedure executed inside an MR imaging system using a thermal effect delivered by a focused ultrasound transducer. A prototype system is constructed to assess MRI thermal monitoring and the localization of the heat zone in muscle. The temperature distribution of the focal spot is imaged with MRI while mechanically moving the transducer with an hydraulic 3-axis positioner. Acoustic power is applied with a spherical shell transducer using 1- to 10-s duration pulses at frequencies of 1.5 MHz to selectively coagulate tissue at 60-70 degrees C. The procedure is monitored with a series of fast second gradient echo, T1-weighted, temperature sensitive MR sequences. Acquisitions are optimized for high temperature sensitive images that yield the thermal diffusivity, heat flow time constant and the focal spot size in muscle. MR temperature maps of muscle provide localization and dosimetry both in the focal region and near field.

One-dimensional NMR thermal mapping of focused ultrasound surgery.

OBJECTIVE: The aim of this study was to evaluate a technique for real-time monitoring of tissue temperature and tracking of the heat source during minimally invasive thermal interventions such as focused ultrasound surgery. MATERIALS AND METHODS: A temperature-sensitive NMR line scan pulse sequence was directed interactively from a workstation during the application of focused ultrasound to samples of excised bovine skeletal muscle. The NMR signal along a sensitive line was monitored during and after heating by means of a scrolling display on the workstation. RESULTS: The temperature sensitivity was found to be approximately 2 degrees C with a time resolution of 300 ms along a line intersecting the ultrasonic focal point. Experimental temperature rises determined from the NMR signal showed close agreement with theoretical temperature behavior derived from the heat equation. Temperature quantitation capabilities were lost upon onset of thermal denaturation and coagulation. CONCLUSION: This technique could serve as a noninvasive guide in tracking the heat source and in monitoring thermal dose during focused ultrasound surgery and other minimally invasive thermal interventions.

Magnetic resonance-guided thermal surgery.

A demonstration of MR guided thermal surgery involved experiments with imaging of focused ultrasound in an MRI system, measurements of the thermal transients and a thermal analysis of the resulting images. Both the heat distribution and the creation of focused ultrasound lesions in gel phantoms, in vitro bovine muscle and in vivo rabbit muscle were monitored with magnetic resonance imaging. Thermal surgical procedures were modeled by an elongated gaussian heat source where heat flow is controlled by tissue thermal properties and tissue perfusion. Temperature profiles were measured with thermocouples or calculated from magnetic resonance imaging in agreement with the model. A 2-s T1-weighted gradient-refocused acquisition provided thermal profiles needed to localize the heat distribution produced by a 4-s focused ultrasound pulse. Thermal analysis of the images give an effective thermal diffusion coefficient of 0.0015 cm2/s in gel and 0.0033 cm2/s in muscle. The lesions were detected using a T2-weighted spin-echo or fast spin-echo pulse sequence in agreement with muscle tissue sections. Potential thermal surgery applications are in the prostate, liver, kidney, bladder, breast, eye and brain.

Real-time acquisition, display, and interactive graphic control of NMR cardiac profiles and images.

A highly interactive MRI scanner interface has been developed that allows, for the first time, real-time graphic control of one-dimensional (1D) and two-dimensional (2D) cardiac MRI exams. The system comprises a Mercury array processor (AP) in a Sun SPARCserver with two connections to the MRI scanner, a data link that passes the NMR data directly to the AP as they are collected, and a control link that passes commands from the Sun to the scanner to redirect the imaging pulse sequence in real time. In the 1D techniques, a cylinder or "pencil" of magnetization is repeatedly excited using gradient-echo or spin-echo line-scan sequences, with the magnetization read out each time along the length of the cylinder, and a scrolling display generated on the Sun monitor. Rubber-band lines drawn on the scout image redirect the pencil or imaging slice to different locations, with the changes immediately visible in the display. M-mode imaging, 1D flow imaging, and 2D fast cardiac imaging have been demonstrated on normal volunteers using this system. This platform represents an operator-"friendly" way of directing real-time imaging of the heart.

MR-guided focused ultrasound surgery.

Magnetic resonance guided focused ultrasound surgery provides a minimally invasive controlled method for selectively destroying deep-lying tissue. A thermal analysis of focused ultrasound provides an estimate of the time-dependent temperature distribution and thermal dose required for ultrasound surgery. The temperature distribution is estimated by accumulating heat sources, considering the effects of thermal conductivity, heat content, and perfusion. In this study, both gel phantoms and excised in vitro bovine muscle specimens were imaged in a 1.5 T MR system while heated with a 5 cm diameter, 10 cm focal length, 1.1 MHz transducer. During sonication, the thermal effects were observed with T1-weighted pulse sequences. Below a critical temperature, the heat zone appeared as a dark spot that moved with the focal spot. Above a critical thermal dose, the in vitro tissue was irreversibly altered and the focal lesion was observed on both the MR image and the specimen slice.

Definitions for hydration changes of hydrogel lenses.

Authors of studies on the hydration characteristics of hydrogel lenses have used a variety of definitions to describe the changes observed with exposure of the lenses to different environments. This has resulted in difficulties in the interpretation of published data. In particular, the single term "percentage dehydration" has been used to denote different mathematical expressions for alterations to the lens composition. A detailed theoretical analysis of hydrogel hydration levels is presented to examine definitions for lens hydration changes, compare results obtained under different definitions and propose appropriate usage for the alternative definitions according to the aspect of lens performance under consideration. The interrelationships between the commonly used definitions of dehydration are shown to be independent of initial lens mass but dependent on initial water content. Typically, higher water content lenses undergo considerably larger mass changes than lower water content lenses, an effect that may be masked if these changes are presented as changes of water content. There are a number of clinical consequences of lens dehydration, the importance of which will vary depending on the initial lens water content. It is therefore essential when comparing different water content lenses on the basis of the extent of dehydration to do so with respect to a specific clinical consequence rather than in general terms.

Mode conversion of Rayleigh and Lamb waves to compression waves at a metal-liquid interface.

A theoretical treatment is given of a potentially useful ultrasonic mode-conversion process, from Rayleigh or Lamb waves (SAW's) to angled compression waves in an adjacent liquid. Since the compression waves can form a well-defined beam, with typical angular width of only 1 degree or 2 degrees, it is possible to use this technique in under-liquid viewing applications. Another use, in liquid level measurement, has advantages over conventional ultrasonic methods. Attractive features are the high power efficiency of the mode-conversion, in both directions, and its inherent separation of electromechanical transducer from the liquid, which may be vital in certain hazardous environments. With such applications in mind, the emphasis here is on ultrasonic processes in the liquid. After a descriptive account of pressure wave generation in the liquid, the main part of the paper is a general treatment of far-field beam profiles and includes transverse beam-width and discussion of pulsed operation. Finally we examine inverse mode conversion (compression wave to SAW), to identify the main physical processes occurring and arrive at a figure for power efficiency. This turns out to be 80% (max) against nearly 100% for transmission.