Ultrasonic heating of the skull

Comparatively simple analysis shows that diagnostic ultrasound devices, in some cases, may approach output levels that can produce significant heating of tissues, particularly if the propagation path includes bone. Experimental tests of these predictions using rodents show that temperature increments of the order of 3 degrees C/W/cm2 are produced in skull bone with sharply focused fields at medically relevant frequencies.     

Experimental analysis of the vibrational characteristics of the human skull

Analytical models of the human skull structure have generally been constructed so as to characterize the gross geometric features and material properties; however, a model should also have accurate frequency response characteristics since these are essential for collision and head injury analyses. An experimental investigation was conducted to identify the dynamic characteristics of freely vibrating human skulls. Resonant frequencies and associated mode shapes in the frequency band from 20 Hz to 5000 Hz were delineated for two dry human skulls. Osteometrically, one skull corresponds to a 50th percentile male (skull 1) and the second is representative of a 5th percentile female skull (skull 2). Digital Fourier analysis techniques were used to identify the resonant frequencies and corresponding mode shapes of each skull. Eleven resonant frequencies were identified for skull 1, with the lowest being 1385 Hz. In contrast, skull 2 exhibited only 6 resonant frequencies with the first being 1641 Hz. Nine mode shapes were identified for skull 1, but only 5 modes were recognized for skull 2. The vibrational pattern of the human skull, as indicated by its mode shapes in this limited study, seems to be a unique property of a particular skull. Skull satures did not appear to influence the modal pattern.     

MRI of the normal hippocampus.

Before it is possible to use MR imaging to investigate changes in the hippocampus in disease processes such as epilepsy and memory disorders, it is imperative that normal variations are defined. Using specific anatomic locations, we evaluated the hippocampi of 29 normal volunteers with coronal MR studies. Mild variations occur with regard to hippocampal size and shape, and hippocampal fissure visualization. Hippocampal signal intensity is isointense to cortical gray matter. Recognition of normally occurring variations should help prevent over-interpretation of hippocampal changes in pathologic disorders.     

MRI angiography of the carotid artery

Spin echo images of the carotid arteries in longitudinal view have been obtained by selection of oblique imaging planes. Blood flow within the lumen in the region of the carotid bifurcation has been visualized through the use of cardiac gating during end diastole. Using a surface coil placed about the mandible, high resolution images [(0.75 mm)² per pixel) were obtained with scan times typically equal to 9 min and image data matrix equal to 256 × 256. Images obtained with this technique of MRI carotid angiography demonstrate blood flow phenomenon as well as vascular anatomy.     

Enhanced ultrasound transmission through the human skull using shear mode conversion

A new transskull propagation technique, which deliberately induces a shear mode in the skull bone, is investigated. Incident waves beyond Snell's critical angle experience a mode conversion from an incident longitudinal wave into a shear wave in the bone layers and then back to a longitudinal wave in the brain. The skull's shear speed provides a better impedance match, less refraction, and less phase alteration than its longitudinal counterpart. Therefore, the idea of utilizing a shear wave for focusing ultrasound in the brain is examined. Demonstrations of the phenomena, and numerical predictions are first studied with plastic phantoms and then using an ex vivo human skull. It is shown that at a frequency of 0.74 MHz the transskull shear method produces an amplitude on the order of--and sometimes higher than--longitudinal propagation. Furthermore, since the shear wave experiences a reduced overall phase shift, this indicates that it is plausible for an existing noninvasive transskull focusing method [Clement, Phys. Med. Biol. 47(8), 1219-1236 (2002)] to be simplified and extended to a larger region in the brain.     

Evaluation of the vibrational modes of the human skull as it relates to bone-conducted sound

Analytic and numerical models are used to study bone-conducted sound and how it relates to the vibrational modes of the human skull. The analytic model is based on the solution to the acoustic and elastic wave equations and the constraining boundary conditions for a fluid-filled elastic sphere. Both models predict that most of the acoustic energy of bone-conducted sound exists in the form of surface wave vibrations at the interface between two acoustic media rather than in the bone or cranial chamber. These surface waves have phase speeds much slower than the bulk sound speed for bone. The analytic model, based on spherical elastic shells, predicts a phase speed of 775 m/s and the first resonance frequency at 1500 Hz while the numerical solution yields approximate phase speeds of 450 m/s and provides a visual display of the surface waves and diffraction effects.     

Three-dimensional laser-induced photoacoustic tomography of mouse brain with the skin and skull intact

Three-dimensional laser-induced photoacoustic tomography, also referred to as optoacoustic tomography, is developed to image animal brain structures noninvasively with the skin and skull intact. This imaging modality combines the advantages of optical contrast and ultrasonic resolution. The distribution of optical absorption in a mouse brain is imaged successfully. The intrinsic optical contrast reveals not only blood vessels but also other detailed brain structures, such as the cerebellum, hippocampus, and ventriculi lateralis. The spatial resolution is primarily diffraction limited by the received photoacoustic waves. Imaged structures of the brain at different depths match the corresponding histological pictures well.     

MRI of polysplenia syndrome

The polysplenia syndrome is the association of multiple spleens, situs inversus, congenital heart disease, and azygous continuation of the inferior vena cava. Magnetic resonance (MR) is a noninvasive imaging modality which can easily confirm the multiplicity of spleens, situs inversus, and identify complex congenital cardiovascular malformations. The anomalies of the polysplenia syndrome as imaged by MR are presented.     

SQUID-detected microtesla MRI in the presence of metal

In magnetic resonance imaging performed at fields of 1 T and above, the presence of a metal insert can distort the image because of susceptibility differences within the sample and modification of the radiofrequency fields by screening currents. Furthermore, it is not feasible to perform nuclear magnetic resonance (NMR) spectroscopy or acquire a magnetic resonance image if the sample is enclosed in a metal container. Both problems can be overcome by substantially lowering the NMR frequency. Using a microtesla imaging system operating at 2.8 kHz, with a superconducting quantum interference device as the signal detector, we have obtained distortion-free images of a phantom containing a titanium bar and three-dimensional images of an object enclosed in an aluminum can; in both cases high-field images are inaccessible.     

高温超导MRI磁体试验样机的设计

对MRI用高温超导磁体试验样机的设计进行了介绍,包括项目的市场需求、磁体的基本参数和系统热负荷的预设计结果等。     

Claustrophobia in MRI: the role of cognitions

Purpose

This study aimed to investigate the role of cognitive and behavioural factors in the experience of claustrophobia in the context of magnetic resonance imaging (MRI) scanners.

Materials and Methods

One hundred and thirty outpatients attending an MRI unit completed questionnaires before and after their scans. Specific measures of experience in the scanner included subjective anxiety, panic symptoms, strategies used to stay calm and negative cognitions (such as ‘I will suffocate’ and ‘I am going to faint in here’). Other general measures used included anxiety, depression, health anxiety and fears of restriction and suffocation.

Results

The amount of anxiety experienced during the scan was related to the perceived amount of time spent having physical symptoms of panic. Cognitions reported concerned the following: suffocation, harm caused by the machine and lack of perceived control. The number of strategies patients used to cope in the machine was also a related factor. Neither position in the scanner, nor head coil use nor previous experience of being in the scanner was related to levels of anxiety.

Conclusion

The cognitions identified here may be used to construct a measure to identify those unable to enter the scanner or those most likely to become claustrophobic whilst undergoing the procedure and to further inform future brief, effective interventions.     

Improvements to the quality of MRI cluster analysis

Cluster analysis techniques are gaining widespread use for segmentation of MRI data, especially for volume measurement and 3-D display purposes. This paper describes four improvements to such techniques: (1) The use of intensity simulations to model cluster plots; (2) Correction of image nonuniformity; (3) Anisotropic smoothing of data; and (4) Automatic isolation of tissues of interest. Simulation of cluster plots allows an informed choice of pulse sequence(s) and acquisition parameters to be made. Correction of image nonuniformity and anisotropic smoothing reduce the spread of signal intensity from a single tissue thus producing significantly more compact clusters, whilst the isolation of tissues of interest prevents overlap of clusters from the tissues of interest with those not under consideration. These techniques may be used to improve the results of cluster analysis or traded off, for example to allow lower signal-to-noise images, shorter repetition time images, or fewer images to be used for segmentation.     

Making MRI quieter

We have mitigated acoustic noise in a 1.5 T cylindrical MRI scanner equipped with epoxy-potted, shielded gradients. It has been widely assumed that MRI acoustic noise comes overwhelmingly from vibrations of the gradient assembly. However, with vibration-isolated gradients contained in an airtight enclosure, we found the primary sources of acoustic noise to be eddy-current-induced vibrations of metal structures such as the cryostat inner bore and the rf body coil. We have elucidated the relative strengths of source-pathways of acoustic noise and assembled a reduced-acoustic-noise demonstration MRI system. This scanner employed a number of acoustic noise reduction measures including a vacuum enclosure of a vibrationally isolated gradient assembly, a low-eddy-current rf coil and a non-conducting inner bore cryostat. The demonstration scanner reduced, by about 20 dBA, the acoustic noise levels in the patient bore to 85 dBA and below for several typical noisy pulse sequences. The noise level standing near the patient bore is 71 dBA and below. We have applied Statistical Energy Analysis to develop a vibroacoustic model of the MR system. Our model includes vibrational sources and acoustic pathways to predict acoustic noise and provides a good spectral match above 400 Hz to experimentally measured sound levels. This tool enables us to factor acoustics into the design parameters of new MRI systems.     

Computing the B1 field of the toroidal MRI coil

We present an analytic solution for the B1 field produced in a gapped toroidal cavity resonator designed as a probe for high field MRI. This resonator supports standing TEM waves, so its electric and magnetic fields are identical to those produced by a stationary planar current source with the same (constant) cross-section multiplied by a complex exponential propagation factor. An explicit expression for the field may therefore be found by solving Laplace's equation for the static potential, which is accomplished with a two-dimensional logarithmic conformal transformation algorithm. The equipotential curves are also the contours of the field strength B, and the B (vector) field at any point is directed along the contour passing through that point. With this information, we construct the solution by computing the angle made by the equipotential curve with the horizontal axis at each point, using this angle to analyze the B field into its x and y components, and adding the contributions from the current sources to obtain the magnitude and direction of B at each point in the region of interest. Some proposed extensions of this algorithm are also discussed.