E. Raman

Watershed-based segmentation of 3D MR data for volume quantization

The aim of this work is the development of a semiautomatic segmentation technique for efficient and accurate volume quantization of Magnetic Resonance (MR) data. The proposed technique uses a 3D variant of Vincent and Soilles immersion-based watershed algorithm that is applied to the gradient magnitude of the MR data and that produces small volume primitives. The known drawback of the watershed algorithm, oversegmentation, is strongly reduced by a priori application of a 3D adaptive anisotropic diffusion filter to the MR data. Furthermore, oversegmentation is a posteriori reduced by properly merging small volume primitives that have similar gray level distributions. The outcome of the proceeding image processing steps is presented to the user for manual segmentation. Through selection of volume primitives, the user quickly segments of first slice, which contains the object of interest. Afterwards, the subsequent slices are automatically segmented by extrapolation. Segmentation results are contingently manually corrected. The proposed segmentation technique is tested on phantom objects, where segmentation errors less than 2% are observed. In addition, the technique is demonstrated on 3D MR data of the mouse head from which the cerebellum is extracted. Volumes of the mouse cerebellum and the mouse brains in toto are calculated.

Parameter estimation from magnitude MR images

This article deals with the estimation of model‐based parameters, such as the noise variance and signal components, from magnitude magnetic resonance (MR) images. Special attention has been paid to the estimation of T1‐ and T2‐relaxation parameters. It is shown that most of the conventional estimation methods, when applied to magnitude MR images, yield biased results. Also, it is shown how the knowledge of the proper probability density function of magnitude MR data (i.e., the Rice distribution) can be exploited so as to avoid (or at least reduce) such systematic errors. The proposed method is based on maximum likelihood (ML) estimation.

An energy-based beam hardening model in tomography

As a consequence of the polychromatic x-ray source, used in micro-computer tomography (microCT) and in medical CT, the attenuation is no longer a linear function of absorber thickness. If this nonlinear beam hardening effect is not compensated, the reconstructed images will be corrupted by cupping artefacts. In this paper, a bimodal energy model for the detected energy spectrum is presented, which can be used for reduction of artefacts caused by beam hardening in well-specified conditions. Based on the combination of the spectrum of the source and the detector efficiency, the assumption is made that there are two dominant energies which can describe the system. The validity of the proposed model is examined by fitting the model to the experimental datapoints obtained on a microtomograph for different materials and source voltages.

Quantification and improvement of the signal-to-noise ratio in a magnetic resonance image acquisition procedure

A procedure is developed to quantify and improve the signal-to-noise ratio (SNR) of magnetic resonance images. The image SNR is quantified using the correlation function of two independent acquisitions of an image. To test the performance of the quantification, SNR measurement data are fitted to theoretically expected curves. The proposed correlation technique is also used to improve the SNR by estimating the amplitude of the signal spectrum. The technique is applied to a set of MR images, and its performance in terms of gain in SNR, contrast-to-noise ratio (CNR), and resolution loss is compared to that of classical noise filters. The SNR as well as the CNR is improved significantly with minor loss of resolution. Finally, it is shown that the correlation technique can be implemented in a highly efficient way in almost any acquisition procedure of a magnetic resonance imaging system.

In vivo noninvasive determination of abnormal water diffusion in the rat brain studied in an animal model for multiple sclerosis by diffusion-weighted NMR imaging

In vivo NMR images of the rat brain were obtained using a NMR microscope (7 T) from SMIS (England). Four animals were imaged every 3-4 days during a pathological cycle (starting after induction and up to 37 days) of experimental allergic encephalomyelitis (EAE), an animal model for multiple sclerosis. The EAE rats were weighted and clinically scored daily. We aimed at measuring the apparent diffusion coefficient (ADC) or the mean diffusivity (D) with a high accuracy, and within a reasonable experimental time frame, because of the clinical situation of the animals. Therefore, we fitted the ADC value from five diffusion-weighted images--with an experimental time of 17 min/image--and chose to apply diffusion-sensitizing gradients in a direction intersecting all fiber directions of the external capsule. With this, we also obtained high b-values. For the control rats, we obtained a statistical mean value of ADC = (388 +/- 16) 10(-12) m2/s for gray matter and a statistical mean value of (D) of (750 +/- 30) 10(-12) m2/s for white matter, measured in the external capsule. For the EAE rats, no alterations in ADC values of gray matter with increasing clinical scores were observed. Concerning white matter, as determined in the external capsule, there were no significant differences in (D) values between controls and EAE rats before clinical signs occurred. However, when clinical signs were observed, we could demonstrate a significant positive correlation between the clinical score and the (D) values in the external capsule. As the clinical signs became more severe, we measured a rise in water diffusion (increase in (D)) in the external capsule, which was accompanied by the occurrence of interstitial edema as revealed by a complementary histological study.

Adrenoceptor and local modulator control of cutaneous blood flow in thermal stress

Blood flow to the skin is controlled by body temperatures in two ways: core and mean skin temperature combine in the central nervous system to form a reflex mechanism that controls the frequency of activity in sympathetic nerves to the cutaneous blood vessels; and local mechanisms independent of reflex effects control contractile response to the sympathetic transmitter norepinephrine (NE) at different temperatures. Cutaneous vessels differ in responsiveness to NE across temperatures: in limbs and tails, the superficial vessels constrict more strongly to NE when cooled, while the deep vessels show weaker responses to NE when cooled. This allows the limb to dissipate heat when warm and to conserve heat when cool. The mechanism for this difference in thermal response of deep and superficial vessels is not completely known, but may relate to differences in the adrenoceptors on which NE acts, and/or to the actions of locally produced substances that modulate the responses to NE in different ways at different temperatures. This paper discusses the alpha1- and alpha2-adrenoceptors involved in contraction of deep and superficial cutaneous vessels and also describes the roles of the local modulator nitric oxide, which interacts with adrenoceptors to affect cutaneous blood flow.

Optimal estimation of T2 maps from magnitude MR images

A Maximum Likelihood estimation technique is proposed for optimal estimation of Magnetic Resonance (MR) T2 maps from a set of magnitude MR images. Thereby, full use is made of the actual probability density function of the magnitude data, which is the Rician distribution. While equal in terms of precision, the proposed method is demonstrated to be superior in terms of accuracy compared to conventional relaxation parameter estimation techniques.

The effect of beam hardening on resolution in x-ray microtomography

Spatial resolution describes the ability to distinguish adjacent objects. It is often used as a measure of the quality of an image. By studying the edge of a cylindrical object a value for the spatial resolution is obtained. However, as a consequence of the polychromatic source used in micro-CT, beam hardening will occur which has the effect of enhancing the edges of the objects in the reconstruction images. These artefacts will have an influence on the resolution measure. In this paper, the spatial resolution in the reconstruction images is determined with and without beam hardening correction. In addition, the effect of the geometric magnification and the dependence on the type of sample material is studied.

A bimodal energy model for correcting beam hardening artefacts in X-ray tomography

As a consequence of the polychromatic X-ray sources, used in micro-computer tomography (/spl mu/CT) and in medical CT, the attenuation is no longer a linear function of absorber thickness. If this nonlinear beam hardening effect is not compensated, the reconstructed images will be corrupted by cupping artefacts. Here, a bimodal energy model for the energy spectrum is presented, which may be used for reduction of artefacts caused by beam hardening. In essence, this correction method is a linearization technique based on a physical model, where no a priori knowledge about the spectrum of the source is required.

In-vivo study of the thermoregulation of the rat tail using magnetic resonance angiography (MRA)

In the rat, almost 20% of the total body heat-loss occurs by sympathetically mediated increases in blood flow through a system of arteriovenous anastomoses (AVAs) in the skin of the tail which are absent at the base and abundant at the tip. To study the mechanisms of thermoregulation in the rat tail we monitored online the blood vessel temperature and the arterial and venous vessel size and their mutual vascular volume interactions using in vivo MRA. During a gradual rise in rectal temperature from 36 degrees Celsius to 40 degrees Celsius, tail surface temperatures were measured at ventral (Ta) and lateral (Tv) sits overlying the respective vascular bundles. At the base, middle and tip, diameter of the ventral artery and the lateral veins of the heat-loaded animal increased clearly upon rising body temperature. Calculation of (Ta - Tv) in function of the rectal temperature during heating showed that at the tail base (Ta - Tv) was maximum at rectal temperature of 38 degrees Celsius and minimum at 39 degrees Celsius. At the middle and the tip of the tail, a steady rise of (Ta - Tv) was observed. If we assume that vasodilatation is a synchronical process along the length of the tail, then the difference in (Ta - Tv) is due to the presence of AVAs.