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Biomedical subjects

R M Henkelman

Publications and source records attributed to R M Henkelman.

At least 19 recordsLinked to original sources
PubMed1992

Quantitative magnetic resonance imaging parameters and their relationship to mammographic pattern.

BACKGROUND: Breast cancer exhibits wide international variation in incidence, which has led to the identification of several factors correlating with the risk of the disease. Magnetic resonance imaging (MRI) techniques can provide quantitative information about the biological and physical properties of tissue. PURPOSE: This work tested several magnetic resonance tissue parameters for their ability to distinguish quantitatively between breast tissues in subjects at substantially different risk for breast cancer as defined indirectly by their parenchymal pattern on mammograms. METHODS: Quantitative MRI parameters (relative water content, longitudinal relaxation time [T1], and transverse relaxation time [T2]) were measured for breast tissue using newly developed techniques in two groups of women with mammographic parenchymal appearance associated with high (Dy pattern [i.e., extensive nodular or diffuse density]; n = 12) or low (N1 pattern [i.e., breast containing mainly fat]; n = 11) risk of breast cancer. RESULTS: The two groups have significantly different average relative water content (P less than .0001) and average T1 (P less than .0001). Pixel histograms of T2 values show marked differences between the two groups which can be characterized with a fourth moment parameter. CONCLUSIONS: Quantitative MRI techniques exhibit good potential for assessing tissue characteristics in the breast that are associated with risk of breast cancer. IMPLICATIONS: Future work will address the direct correlation of MRI parameters with risk of breast cancer.

Adult↗
PubMed1992

RF current density imaging in homogeneous media.

MRI has proven capable of imaging quasistatic volume current densities in electrolytic and biological media. In this paper, the feasibility of extending the method to image RF current density at the Larmor frequency is studied. RF current imaging could be relevant to MR power absorption and safety and to hyperthermia analysis, as well as creating dielectric and conductivity-dependent tissue contrast. The approach is to deliberately induce or inject RF currents in a sample synchronous with an MR pulse sequence and measure the resulting transverse RF magnetic field components. Current density is extracted by computing the curl of the magnetic fields. The preliminary theory has been developed for uniform media where both displacement and conduction currents exist while skin effects or eddy currents are absent. If the derivative in the B0 direction of the RF magnetic field component parallel to B0 is negligible, then sufficient information exists to reconstruct the RF current density component that is parallel to B0 without rotating the sample. The relative phase of the current can also be estimated. The method has been proven feasible by successfully imaging a uniform 85.6-MHz current density in a salt water phantom. The experiment conforms closely to capacitively coupled hyperthermia heating.

Artifacts↗
PubMed1992

New imaging technologies: prospects for target definition.

Developments in medical imaging over the past 2 decades are providing significant improvement in tumor definition. Improvements in soft tissue visualization with tumor specific contrast combined with direct 3D data acquisition and millimetre spatial resolution set new standards in tumor definition. Such precision in tumor imaging provides a new level of challenge for precision radiation treatment. Significant further improvement over the next decade in geographic definition is unlikely. The challenge for imaging research is that of extracting tissue specific information about tumors such as perfusion and response to therapy rather than simply anatomical clarity.

Humans↗
PubMed1991

Data extrapolation for truncation artifact removal.

Clinicians typically obtain high-resolution clinical MR images in an effort to avoid the truncation artifacts that often arise in Fourier transform reconstruction of limited data. A method for reducing these artifacts in MR images, at the reconstruction stage, would allow for reduced imaging times, through the collection of fewer phase encode steps and increased signal-to-noise ratios, through increased pixel size. The approach to reducing truncation artifacts in MR images is developed and a simple algorithm is presented which significantly reduces truncation artifacts in images with as few as 96 phase encode steps. The algorithm is compared with a more sophisticated method of reconstructing truncation-free images and is shown to be equivalently effective. Three clinical examples are shown illustrating the success of the method.

Algorithms↗
PubMed1991

On the transverse relaxation rate enhancement induced by diffusion of spins through inhomogeneous fields.

The design of magnetic particles as a magnetic resonance contrast agent will rely on the prediction of their ability to induce transverse relaxation among the surrounding protons. There exists several divergent predictions of the contribution of these agents to 1/T2. This article points out a problem, commonly overlooked, in the development of expressions for the relaxation enhancement which has led some workers to the derivation of results inappropriate for large magnetic particles. The size of the magnetic inhomogeneity created by the particle precludes the averaging of the interaction over a single proton unless it experiences an average field in the time tau between pulses. Computer simulations following the trajectories of diffusing water molecules in inhomogeneous fields are shown to be the correct approach to dealing with large inhomogeneities.

Computer Simulation↗
PubMed1991

A quantitative comparison of the TERA modeling and DFT magnetic resonance image reconstruction techniques.

The resolution of magnetic resonance images reconstructed using the discrete Fourier transform (DFT) algorithm is limited by the effective window generated by the finite data length. The transient error reconstruction approach (TERA) is an alternative reconstruction method based on autoregressive moving average (ARMA) modeling techniques. Quantitative measurements comparing the truncation artifacts present during DFT and TERA image reconstruction show that the modeling method substantially reduces these artifacts on "full" (256 X 256), "truncated" (256 X 192), and "severely truncated" (256 X 128) data sets without introducing the global amplitude distortion found in other modeling techniques. Two global measures for determining the success of modeling are suggested. Problem areas for one-dimensional modeling are examined and reasons for considering two-dimensional modeling discussed. Analysis of both medical and phantom data reconstructions are presented.

Algorithms↗
PubMed1991

Pulsed NMR relaxometry of striated muscle fibers.

The longitudinal (T1) and transverse (T2) proton (1H) nuclear magnetic resonance (NMR) relaxation of fast- and slow-twitch muscle fibers are examined using rat muscle tissues in which one fiber type predominates. Both continuum and discrete exponential component fits are made to Carr-Purcell-Meiboom-Gill (CPMG) and inversion recovery pulse sequence measurements. In addition, experiments which illustrate the large sources of variability that have led to apparent conflicts in the literature are presented. Based on the results of this study, unique NMR features that distinguish fast- and slow-twitch muscle fibers are presented. The feasibility of differentiating fast- and slow-twitch muscle fibers using magnetic resonance (MR) imaging is briefly discussed.

Analysis of Variance↗
PubMed1991

Practical implementation and optimization of one-shot T1 imaging.

Longitudinal relaxation times (T1) can be measured rapidly in an imaging context using a "one-shot" method based on the pulse sequence originally proposed by D. C. Look and D. R. Locker (Rev. Sci. Instrum. 41, 250 1970). This sequence is significantly faster than either repeated inversion recovery or repeated saturation recovery methods. The method uses a 180 degrees inversion pulse followed by multiple small-angle alpha pulses that sample the longitudinal magnetization during its recovery. Choices of inversion pulse, tip angle, and time intervals are discussed for optimal clinical use. We can produce 29 images sampling the full T1 recovery curve with a 256 x 256 resolution in about 10 min. From this data, T1 images can be calculated with a precision of 10%.

Humans↗
PubMed1991

Quantitative two-dimensional time correlation relaxometry.

An experimental and analytical method is presented for obtaining two-dimensional NMR time correlation spectra of T1 (or T1p) and T2 exponential relaxation in heterogeneous samples. The numerical algorithms used in this study do not bias the solution by any a priori assumptions as to the number of required components. The constraints used to overcome the ill-posed and ill-conditioned nature of this inverse problem are also described. With this method, the correlations of T1 (or T1p) and T2 exponential components in systems of nonexchanging standards and rat muscle tissues are obtained.

Algorithms↗
PubMed1991

High signal intensity in MR images of calcified brain tissue.

Calcified lesions of the brain occasionally appear bright on T1-weighted MR images. This report shows that particulate calcium can reduce T1 relaxation times by a surface relaxation mechanism. Calcium particles with greater surface area show greater T1 relaxivity. Reduced proton density and reduced T2 tend to diminish signal intensity, but reduced T1 increases signal intensity. Thus, for concentrations of calcium particulate of up to 30% by weight, the signal intensity on standard T1-weighted images increases but subsequently decreases.

Adolescent↗
PubMed1991

Optimization of prostatic magnetic resonance imaging technique.

With a 1.5-T magnetic resonance imager the authors systematically varied a large number of technical factors to obtain an optimum balance between high image quality and reasonable imaging time for the prostate gland. Each parameter was adjusted relative to benchmark images of very high quality to achieve a reasonable acquisition time with as little loss of the signal-to-noise ratio (SNR) as possible. Image quality was judged subjectively by magnetic resonance radiologists and objectively by measurements of SNR for the prostate. The authors recommend multislice, multiecho spin-echo pulse sequences with dual surface coils, fat suppression, reduced bandwidth, a repetition time of 1500 ms, echo times of 30 and 60 ms, a flip angle of 60 degrees, two excitations, a slice thickness of 5 mm with a 1.5-mm gap and 192 phase-encoding steps. The acquisition time for one such series was 9.6 minutes.

Humans↗
PubMed1990

Optimization of survey protocols for MRI.

A method for evaluating the sensitivity of MRI protocols to changes in an arbitrary number of tissue parameters over a broad range of parameter values is presented. This analysis is useful for choosing an optimal basis set of images for either a "survey" protocol or tissue segmentation algorithms. A survey protocol is required when searching for a lesion of unknown type or location. Segmentation requires unique signal signatures for tissues that may cover a broad range of tissue parameter values. Data acquisition is modeled as a mapping of a domain of tissue parameter values into a signal manifold in a signal strength space defined by the MRI protocol. The efficacy of the protocol is evaluated by investigating the characteristics of the signal manifold. A figure of merit which maximizes the probability of discriminating each point in the domain of tissue parameters from all others is developed.

Clinical Protocols↗
PubMed1990

Why MEM does not work in MR image reconstruction.

This paper discusses the theory and application of the Maximum Entropy Method (MEM) to the reconstruction of Magnetic Resonance (MR) images. It is shown that the MEM is inappropriate for MR image reconstruction and that the usual heuristic justification is invalid in this case. The application of the MEM in MR image reconstruction is characterized as merely one of many constrained regularization approaches.

Fourier Analysis↗
PubMed1990

Does IVIM measure classical perfusion?

MR measurements based on motion encoding gradients provide interesting information about diffusion in tissues and have also been advanced as a way to measure tissue perfusion. This communication shows why IVIM cannot measure perfusion in the classical sense. Attempts to do so result from an unclear understanding of classical perfusion measurements and from confusion between terminal deposition (or uptake) and blood volume flow.

Blood Flow Velocity↗
PubMed1990

Cause of signal loss in MR images of old hemorrhagic lesions.

Old hemorrhagic lesions in the brain are characteristically surrounded by a band of hemosiderin-containing tissue. This region is typically of low signal intensity on long-echo-time (TE) radio-frequency (RF) spin-echo magnetic resonance (MR) images and on gradient-echo MR images. To determine the cause of signal loss in this band, the authors measured the signal that arises from imaging such a region with use of an RF spin-echo technique with a 180 degrees pulse incrementally displaced from TE/2. The incremental loss of signal was small. Using an agar phantom containing iron particles, the authors also showed that signal loss results primarily from diffusion in magnetic gradients. They conclude that most signal loss in the dark band surrounding areas of late-stage hemorrhage arises from diffusion in areas of magnetic inhomogeneity.

Adult↗
PubMed1990

Making magnetic resonance images and beyond.

Magnetic resonance (MR), a comparatively new imaging method in Canada, is based on physical principles that are different from all current imaging methods. An understanding of these principles is therefore essential to appreciate the clinical capabilities and limitations of MR. Because MR is a multiparameter imaging method, the challenge of choosing appropriate imaging sequences must be addressed. The technical development of anatomical image formation is now almost complete, so future research will focus on the use of MR to obtain more than just spatial information. The quantitative measurement of relaxation times, chemical composition, flow and spectra hold great promise in extending the capabilities of MR beyond those of other imaging methods.

Humans↗
PubMed1989

Spin locking for magnetic resonance imaging with application to human breast.

The dependence of rotating frame spin-lattice relaxation, T1 rho on locking field frequency, f1, was measured for phantom materials and human breast tissues. These data were used to predict the relative signal strengths obtainable in a spin-locking imaging sequence. This imaging sequence was implemented on a 0.15-T imaging system and measurements of phantom and tissue signal strength for various imaging parameters agreed with predicted signal strengths. Compared to T1 and T2, T1 rho appears to have unique capability to distinguish tumor from normal fat and fibrous breast tissues. The applications of T1 rho to tissue characterization and imaging at high static field strengths are discussed.

Adipose Tissue↗