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Biomedical subjects

J P Hornak

Publications and source records attributed to J P Hornak.

16 recordsLinked to original sources
PubMed1998

Multivariate image analysis of magnetic resonance images with the direct exponential curve resolution algorithm (DECRA). Part 2: Application to human brain images.

Owing to the heterogeneity of living tissues, it is challenging to quantify tissue properties using magnetic resonance imaging. Within a single voxel, contributions to the signal may result from several types of 1H nuclei with varied chemical (e.g., -CH2-, -OH) and physical environments (e.g., tissue density, compartmentalization). Therefore, mixtures of 1H environments are prevalent. Furthermore, each unique type of 1H environment may possess a unique and characteristic spin-lattice relaxation time (T1) and spin-spin relaxation time (T2). A method for resolving these unique exponentials is introduced in a separate paper (Part 1. Algorithm and Model System) and uses the direct exponential curve resolution algorithm (DECRA). We present results from an analysis of images of the human head comprising brain tissues.

Adult↗
PubMed1996

A multipurpose MRI phantom based on a reverse micelle solution.

Many chemical solutions for use in magnetic resonance imaging phantoms have been reported in the literature. Each of these solutions has its application-specific advantages and disadvantages. We propose a single reverse micelle phantom solution, which, although not a universal phantom solution, may find applications in testing of the radio frequency transmit and receive fields of an imaging coil, the homogeneity of the static magnetic field, and the suppression in a fat or water saturation imaging sequence. The solution is thermodynamically stable and biologically inert, it possesses a smaller standing wave artifact than water, and its overall spin lattice relaxation times may be adjusted.

Alkanes↗
PubMed1993

Classification of trabecular structure in magnetic resonance images based on morphological granulometries.

A new method of detecting structured changes in trabecular bone, such as those associated with osteoporosis, was evaluated on magnetic resonance images of the wrist. The method was based on gray-scale morphological granulometries which classify image texture by iteratively filtering an image and measuring the rate of change of structural diminution in a filtered-image sequence. A classification scheme capable of distinguishing structural changes in trabecular bone starting from normal trabeculae through sclerotic, cystic, and grossly porotic bone is presented. Results of the application of this technique to the evaluation of high resolution magnetic resonance images of the wrist are presented.

Adolescent↗
PubMed1993

A multispectral analysis of brain tissues.

With the increasing use of three-dimensional MRI techniques it is becoming necessary to explore automated techniques for locating pathology in the volume images. The suitability of a specific technique to locate and identify healthy tissues of the brain was examined as a first step toward eventually identifying pathology in images. This technique, called multispectral image segmentation, is based on the classification of tissue types in an image according to their characteristics in various spectral regions. The spectral regions chosen for this study were the hydrogen spin-lattice relaxation time T1, spin-spin relaxation time T2, and spin density, rho. Single-echo, spin-echo magnetic resonance images of axial slices through the brain at the level of the lateral ventricles were recorded on a 1.5 Tesla imager from 20 volunteers ranging in age from 17 to 72 years. These images were used to calculate the T1, T2, and rho images used for the classification. Tissue classification was performed by locating clusters of pixels in a three-dimensional T1(-1)-T2(-1)-rho histogram. Gray matter, white matter, cerebrospinal fluid, meninges, muscle, and adipose tissues were readily classified in magnetic resonance images of the volunteers with a single set of T1, T2, and rho values. Cluster characteristics, such as size, shape, and location, provided information on the imaging procedure and tissue characteristics.

Adolescent↗
PubMed1993

Asymmetric single-turn solenoid for MRI of the wrist.

A single-turn solenoid, lacking the conventional cylindrical or rectangular symmetry, was constructed for magnetic imaging of the human wrist at 1.5 Tesla. As is characteristic of single-turn solenoids, this wedge-shaped geometry solenoid was found to have a high filling factor and quality factor, as well as produce images with a favorable signal-to-noise ratio. The radio frequency (RF) transmitter power required to produce a 90 degrees pulse in the solenoid was 130 mW. The asymmetric solenoid had a small variation in the intensity of the RF field across its volume, which is characteristic of symmetrical single-turn solenoids. The combination of these four properties of the coil resulted in the production of excellent 1.5-mm thick anatomical images of the wrist.

Adult↗
PubMed1992

A fast T1 algorithm.

Multispectral tissue classification using magnetic resonance T1, T2, and rho images may be useful in diagnosing and locating certain pathology. Techniques for generating the T1 images necessary for this classification scheme often require longer data collection and post processing times than are practical. As a consequence, further development of this classification scheme may be limited. This paper addresses an improvement in the post processing time required to generate T1 images. A nonlinear least-squares algorithm is described for rapidly generating spin-lattice relaxation time images from variable repetition time magnetic resonance images. The algorithm generates a 256 x 256 pixel T1 image from nine variable repetition time images in approximately 60 sec on a VAX-6510 computer.

Algorithms↗
PubMed1990

Spin-lattice relaxation time measurements using hybrid CSI--phantom study.

The spin-lattice relaxation time T1 of multicomponent tissues is often determined by fitting relaxation data to monoexponential functions. This process can lead to large errors in the relaxation time. We describe a procedure using chemical-shift imaging (CSI) which separates the NMR signal into water and lipid components thus allowing the two signals to be individually analyzed for relaxation times. This procedure yields more representative relaxation times than those obtained by both monoexponential and biexponential fitting schemes.

Body Water↗
PubMed1989

Magnetic resonance imaging using a ribbonator: hand and wrist.

A modified version of a single-turn solenoid with rectangular symmetry, which we call a ribbonator, provides excellent magnetic resonance images of the hand and wrist when used as both the transmitter and the receiver in a 1.5-T clinical imaging system. The very high RF efficiency provides excellent signal-to-noise and anatomical resolution. Design equations and RF properties of the resonator are discussed.

Copper↗
PubMed1988

Magnetic field mapping.

Homogeneous radiofrequency magnetic fields are necessary for production of high-quality magnetic images and for most forms of magnetic resonance spectroscopy. It is often convenient to map the radiofrequency homogeneity associated with a resonant device by measuring the magnetic image intensity of an aqueous phantom placed within the resonator. The rf field intensity is not related trivially to the magnetic image intensity, and the relationship is different for different image acquisition methods. In this report relationships between rf field intensity and magnetic image intensity are derived and radio-frequency field maps presented for comparison using (1) an rf probe moved about within the resonant volume, (2) spin-echo images, and (3) small tip angle gradient refocused echo images.

Magnetic Resonance Spectroscopy↗
PubMed1988

MRI of extremities using perforated single-turn solenoids.

A class of single-turn solenoids that permits magnetic imaging of the extremities in horizontal bore magnets with improved coupling between the imaged anatomy and the rf section of the imager is described and demonstrated. These devices differ from more conventional designs primarily by the placement of one or more access holes in the side of the generally cylindrical resonant structure to permit extremity insertion. The image quality is excellent, rf efficiency and homogeneity are good, and signal-to-noise is high, permitting rapid acquisition of magnetic images with small fields of view.

Extremities↗
PubMed1987

Noise reduction in wide-bore magnets using a patient cage.

Significant rf noise reduction is demonstrated by enclosing the patient in a conducting cage grounded to the magnet bore in a 22-cm-bore spectrometer system capable of examining magnetic images and spectroscopy of human limbs. This method of noise reduction was found to be reliable, simple, and efficient for dealing with ambient rf noise in an unshielded room.

Extremities↗
PubMed1987

Elementary single turn solenoids used as the transmitter and receiver in magnetic resonance imaging.

A single turn solenoid, also called a loop-gap resonator, is a device that is efficient for radio frequency spectroscopy on relatively large samples. Thus, the device provides an effective means for magnetic imaging where the single turn solenoid may serve both as the transmitter and receiver coil. The device is readily constructed and provides very efficient use of radio frequency (RF) power for imaging extremities such as breasts, arms, feet, and hands. The resulting magnetic images are acquired in short times with good anatomical resolution and considerable reduction of the RF power delivered to the patient.

Breast↗
PubMed1986

Breast MR imaging with loop-gap resonators.

Breast images obtained at 1.5 T using a loop-gap resonator pair as both the excitation and detection device are presented. The efficiency of this approach is high, as judged by the low level of radio frequency (RF) power required to obtain a 90 degree pulse and the uniformity of the RF field within the resonator pair. A modification of the pair geometry provides for reasonable observation of the tissues through the chest wall and laterally to the axillae.

Breast Diseases↗
PubMedDate not supplied

MR fat suppression technique in the evaluation of normal structures of the knee.

The chopper fat suppression (CFS) pulse sequence, which is a phase sensitive implementation of the Dixon fat suppression method and the spin echo (SE) pulse sequence, was used in the evaluation of anatomic structures of the normal knee using 48 sets of imaging sequences in six volunteers using a repetition time/echo time combination of 1,500/30, 60 ms. A demonstration of the CFS technique in 10 patients with suspected knee pathology is also presented. A semiquantitative grading scale was established to rate anatomic visualization and used to compare CFS and SE pulse sequence techniques. The results in normal subjects demonstrate that hyaline cartilage is significantly better visualized by fat suppression pulse sequence than by conventional SE pulse sequence in the coronal and sagittal planes of imaging (p less than 0.001). The preliminary results from patients studies suggest that CFS imaging may be useful in the evaluation of meniscal tears, in the differentiation of hyaline cartilage from joint fluid, and in the detection of both soft tissue and bone injuries.

Adipose Tissue↗