SPECIALIZED MR SEQUENCES

SPECIALIZED MR SEQUENCES

In addition to spin echo, inversion recovery, and GRE sequences, thereare few sequences that can generate specific information about thetissue, molecular nature and structure. They include (i) magnetic resonance angiography, (ii) perfusion imaging, (iii) diffusion imaging,

(iv) MR spectroscopic imaging

MAGNETIC RESONANCE ANGIOGRAPHY

The magnetic resonance angiography (MRA) employs blood flow enhancement as the basis. The flow of blood depends on velocity,flow profile, direction relative to slice, pulse sequence and its parameters and slice acquisition and direction of flow. Signal from flow can be used to produce MR angiographic images. The relative saturation of the surrounding tissue and the blood entering the slice volume decides the signal. Blood entering the slice contain unsaturated spins and blood within the slice contains saturated spins.Blood leaving the slice removes spins of different saturation conditions.The blood outside the volume does not interact with RF field and enter the volume and produce large signal, compared to spins within the

volume. This effect is higher in slow laminar flow and hence veins appear bright. Since aorta has fast flow, it appears dark or void. Turbulent flow cause rapid loss of coherence and appear as dark.Sometimes, flow related enhancement is undesirable. Presaturation pulses are applied to volumes above and below the imaging volume.This can be achieved by GRE sequence and hence veins, artery and CSF appear bright. This is helpful to avoid motion artifacts and depends

on blood velocity, slice thickness and TR. MRA technique can be done either by phase contrast angiography or time of flight angiography.

PERFUSION IMAGING

Perfusion facilitates delivery of oxygen, nutrients and removal of waste such as CO2. Perfusion measurement reveals the rate of blood delivery to the capillary bed. It is a measure of metabolic activity and it can be done either by using bolus of contrast or arterial spin labeling. The first method uses paramagnetic contrast agent, such as diethylenetriaminepenta-acetic acid (Gd-DTPA) to carry out measurements. The contrast agent modify the relaxation of protons in the blood and shortenT2*, which causes changes in signal. Thus, pre- and post-contrast images reveal the level of perfusion. This procedure is contraindicated in patients with renal dysfunction.In arterial spin labeling perfusion techniques, two images are obtained.The first image is obtained by spin inversion technique (spin labeled

image) and the second image without inversion (control image).Subtraction of the two images gives perfusion image, which is a measure

of blood flow.

Functional Imaging

Functional imaging (fMRI) is based on increased blood flow in the brain area due to natural activity. This reduces deoxyhemoglobin levels, which is a para-magnetic agent that alters T2*. Hence, blood oxygen level is compared between stimulus and rest, to study brain function. The effects are short lived, and require rapid imaging sequences, e.g. EPI,

fast GRE. At rest oxyhemoglobin and deoxyhemoglobin levels are equal. During activity, more oxygen is extracted from the capillary, resulting increased blood flow that causes change in deoxyhemoglobin. Oxyhemoglobin (fully oxygenated blood) is a dia magnetic and has no effect on signal.

Deoxyhemoglobin (reduced hemoglobin) is a para-magnetic agent, due to unpaired electrons and produce magnetic in homogeneities in tissues and increases T2*. Thus, deoxyhemoglobin serve as an in vivo positive contrast agent in functional MR study. Its concentration is influenced

by variation in oxygenation and tissue metabolism. The acquisition is named as blood oxygen level dependent (BOLD). In BOLD technique, multiple T2* weighted images of head are produced at rest. Later, the patient is subjected to stimulus and again multiple images are obtained. The rest image data set is subtracted from the stimulus data, by voxel by voxel. The brain activity is represented by change in signal in a specified area of the brain. The stimulus may be finger movement, light flashes or sound. The areas of activity in the brain are statistically analyzed and color coded. The other areas are not color coded. The resultant image is superimposed on a gray scale brain image, to obtain functional map of the patient

DIFFUSION WEIGHTED IMAGING

Water mostly gives MRI signal from normal and diseased tissue. In normal state, water has random motion due to thermal energy (Brownian motion). In tissue, restriction of molecular motion is there and hence the damping effect called diffusion exist. In many tissues, diffusion is

isotropic. In some tissues, the diffusion is in preferred direction, and hence, it is an isotropic (e.g. muscle, white /gray matter). In diffusion weighted imaging (DWI) strong MR gradients are applied to produce signal differences based on the mobility and directionality of water diffusion. Normal tissue water molecule has more moloss of signal. Abnormal or injured tissue water molecules have lesser mobility, resulting in less signal loss. Thus, DWI technique can estimate

diffusion coefficient, which is a measure of molecular motion. Diffusion imaging techniques includes (i) echo planner imaging and (ii) navigator imaging techniques.

Echo planner imaging (EPI) acquires a complete image in a single shot and it is sensitive to magnetic field inhomogeneities. Image distortionartifacts are present due to susceptibility variation at interfaces, e.g. air, bone and soft tissue. The image is noisy and spatial resolution

is limited, and hence signal averaging is required.Navigator methods acquire images in multiple shots, uses navigator

MR signals for each shot, to detect and correct the bulk motion. Itgives improved spatial resolution with minimal image distortion artifactsand SNR. The required acquisition time is about 10 minutes and mainly used in cardiac study with ECG gating. It is less prone to ghosting artifacts, due to patient motion.The perfusion imaging distinguishes areas of rapid and slow proton diffusion. Volume in which protons are more mobile will show increased signal, compared to that of less mobile. It is a valuable diagnostic tool in stroke patients, neurology and the patient is held by straps firmly during imaging.bility, have greater  loss of signal. Abnormal or injured tissue water molecules have lesser mobility, resulting in less signal loss. Thus, DWI technique can estimate diffusion coefficient, which is a measure of molecular motion. Diffusion imaging techniques includes (i) echo planner imaging and (ii) navigator imaging techniques.Echo planner imaging (EPI) acquires a complete image in a single

shot and it is sensitive to magnetic field inhomogeneities. Image distortion artifacts are present due to susceptibility variation at interfaces, e.g.air, bone and soft tissue. The image is noisy and spatial resolution is limited, and hence signal averaging is required. Navigator methods acquire images in multiple shots, uses navigator MR signals for each shot, to detect and correct the bulk motion. It

gives improved spatial resolution with minimal image distortion artifacts and SNR. The required acquisition time is about 10 minutes and mainly used in cardiac study with ECG gating. It is less prone to ghosting artifacts, due to patient motion. The perfusion imaging distinguishes areas of rapid and slow proton diffusion. Volume in which protons are more mobile will show increased signal, compared to that of less mobile. It is a valuable diagnostic tool in stroke patients, neurology and the patient is held by straps firmly during imaging.

MR SPECTROSCOPIC IMAGING

MR spectroscopic imaging (MRS) is a method of measuring tissue chemistry. It provides frequency spectrum of the tissue based on the molecular motion and composition. The peak intensities and position in the spectrum indicate how the atom is bounded to a molecule. The metabolites peaks, caused by frequency shifts are analyzed. The electron cloud shielding around a nuclei causes slightly different resonance frequencies. This is compared with standard frequency and

the shift, known as chemical shift is obtained. The chemical shift lies between water and fat. Since the amplitudes of water and fat are greater, they need to be suppressed. Then, the signal is Fourier transferred and plotted as frequency spectrum.MRS can be performed either with single voxel or multiple voxel with a minimum volume of about 1 cm3. In a single voxel, stimulated echo acquisition mode (STEAM) or point resolved spectroscopy sequence (PRESS) is used. The study reveals the presence of pathology,relationship between choline (Cho) concentration, creatine (Cr) and N-acetyle-aspartate (NAA) as ratios. Higher Cho concentration with depressed NAA and Cr reveals presence of tumor. Lipid peak indicates hypoxia and high grade tumor.

Thus, MRS is useful to identify metabolic disorders, infections, andtreatment evaluation. To study the individual metabolism, sequential imaging with phase encoding gradient is used. High magnetic field (3 T) is required for good spectral resolution and the field must be uniform,

about 1 ppm. To reduce imaging time, larger pixels (1 cm) are used.