Depth Pulse Sequences for Surface Coils - Essay Example

Specialised Nursing Name: Course: Professor: Institution: April 28, 2012 Qs 1. The use of surface coil was introduced in 1980. Surface coil consist of either one or two copper loops of wire. They are positioned under the patient’s organ close to the location of interest. Surface coils are placed in a way that the major component of B1 RF field that is generated by the coil is orthogonal to B0. They can be used for both RF transmission and detection of NMR signals (Lean et.al, 2007 pp 552). One challenge with surface coils is the failure to be precise with the localization. This is caused by the in homogeneity of the RF fields of the surface coil. Surface coils also have a challenge of inhomogeneous transverse magnetic field which is associated with localisation and assessment of VOI. Surface coils have an advantage as they reduce the noise levels. Orbital surface coils have a small field of view. Surface coil location is a simple way of achieving a good signal to noise ratio short measurement time. MRI surface coils are normally placed on the skin of the patient and they obtain images of tissues which are in proximity to the coil. They have very high sensitivity and thus can produce a much larger signal to noise ratio than what is obtained with whole volume coils. Small surface coils can be used in diffuse metabolic effects of superficial structures for studies. Surface coil performance can be improved if combined by adiabatic pulses (Katz-BrullR, Lavin, & Lenkinski, 2002, pp.1199). The sensitivity of the surface coil decreases with the distance from the coil. As a result of Lorentz reciprocity sensitivity of the coil is directly proportion to the magnetic field intensity which is created inside the body by the coil for a standard value of the coil current. One disadvantage of surface coils is that it suffers from inadequate spatial localization and it also has non uniform radio frequency excitation. Decrease in sensitivity with increased distance from the coil makes the coil useless in obtaining images for those tissues that are located deeper in the body than the dimensions of the coil. Surface coils are preferred due to their better reception of signal. This is because they are more sensitive to signal that are at close proximity in the body of interest. In areas where the body tissues that are being imaged are close, they produce a very clear image. However, they amplify noise signals too efficiently. This is advantageous because the sensitivity of noise signals that are far from the area of interest is reduced. In some cases a body coil can be used as a receive coil. If these two coils are different the latter have to be detuned and the preamplifier blocked to prevent burning from the larger transmit signal. Transmit coil is separated from gradient coils by use of an RF shield. RF power that is received from the amplifier is in the form of two signals with a 900 phase difference. The RF transmits coil functions to convert power into circularly RF magnetic field. Qs 2. In steam technique there is the excitation of three successive slices by the use of three selective pulses. Any of these three pulses is applied in the presence of orthogonal x, y and z gradients. At the intersection of the three slices there is the VOI localization which is chosen by the pulses of the three gradients. Half of the resonating spins are usually at the transverse plane after every two pulses. The rest are in a longitudinal plane. After the two pulses there is a mixing period. After the third pulse, there is detection of the stimulated echo signal which follows a time interval of TE/2. Longitudinal magnetization prepared by the phase decays in relation to T1 during the mixing period. The third pulse brings magnetization in the transverse plane. This is the magnetization that produces the STEAM signal (Jacobs et al, 2004 pp.69). The other magnetization is dephased by crusher gradient of mixing period. This does not contribute in any way to the stimulated echo. As a result of this, there is only one half of the spin magnetization in VOI that results to STEAM signal. STEAM sequencing is conceptually simple. RF pulses used in this technique produce a stimulated echo as well as undesirable spin coherences. After the third pulse four signal echoes are formed. One of them occurs during the mixing time after the second and the third pulses. STEAM has an advantage of detecting H resonances of metabolites which has a short T2 relaxation (Lehman et.al, 2007 pp 1297). In this technique, there can be high achievement of water suppression .This is because the CHESS pulses can be applied both in preparation and during the sequence without affecting the echo time. RF depositions can be lowered by use of small flip angles. Magnetisation in STEAM is usually along the longitudinal axis. Due to this relaxation is only caused by T1 relaxation. Relaxation of T2 occurs only during the TE periods. This means that by use of T2 only very small signal is lost. PRESS technique utilizes both T1 and T2 relaxation throughout the process. STEAM has a disadvantage due to loss by a factor of two in relation to signal intensity. In contrast to STEAM, PRESS has a gain of Signal intensity by a factor of two. In PRESS volume localization is not as sensitive to the subject motion as in STEAM. In times when a long echo time is desired PRESS is preferred to STEAM. However, those techniques that are more efficient in water suppression cannot be applied in the PRESS technique. PRESS is not considered suitable for metabolites that have a short T2. In STEAM technique the sensitive volume used in localization is achieved by applying three 90°pulses which are frequency selective consecutively (Mountford, et.al 2001 pp 1236). These generate a steam echo which originates from ROI. In case of full localization this can be achieved in a single acquisition and it does not require phase cycling. Some of the advantages of STEAM technique are the well delineated ROI which results from the frequency selective 90° pulses that generate good section profile (Saslow et al. 2007, pp 79). There are very minor signal losses as a result of T2. Water suppression excellent, and the 90°RF pulses used are of more broadband than 180° pulses. The resulting localized signal is less dependent on RF in homogeneity. PRESS Technique: In this technique sensitive volume in localization is selected by an application of pulses that are 90°frequency selective which are then followed by two 180° frequency selective pulses in order to generate a spin echo from ROI. PRESS has an advantage over STEAM technique because it has an additional gain in SNR and a collection of spin echo. This is in contrast to stimulated echo (Sitter et al, 2002 pp 330). Qs. 3 Metabolism is a sequential number of chemical reactions that occurs n cells of organisms for the sustenance of life. The chemical processes enable the growth, reproduction and propagation of life. Metabolites refer to the products and the intermediary products of metabolism though the term is conventionally restricted to use in description of metabolic processes in small molecules. Different metabolites have different functions. Some of these functions include signalling, physical structure, enzyme inhibition and energy. The quantification of metabolic concentrations is best determined by the numerical integration of peak areas which the concentrations adopt. Customarily peak areas are used for these calculations rather than peak heights as the areas represent more accurate and realistic concentrations tendencies of metabolites. In addition to this, peak area has the advantage of being less sensitive or dispersed by mechanisms of peak broadening. These mechanisms tend to broaden, shorten and cause peaks to be less symmetrical. Despite these changes, the mechanisms have little or no effect on the peak’s total area. Peak area measurements are thus more reliable and accurate than height measurements. The peak’s height is measured as the metabolic concentration. It is measured as the ratio of height against the peak’s central creatine. For example, in the endogenous marker method of concentration measurement in peak areas, a singular measured peak emanating from metabolite that is endogenous by nature is taken as the standard of concentration. Systematically, peak ratios obtained after corrections of factors/ mechanisms such as relaxation and converted by the use of a reference value obtained from literatures regarding the reference metabolite into concentrations. The most commonly used metabolites for referencing are total creatine, choline and NAA. The estimation of peak areas is an essential part of metabolic profiling in the biological systems measurement. Metabolites of low molecular weight and the metabolites intermediaries lead to the reflection of the dynamic response nature of modifications of genetics. Physiological and patho-physiological also result. Interpretation and precise measurement of the metabolites endogenous profiles from biological samples can be accurately performed. This test has in the past provided numerous for the investigation of the changes that are induced by the presence of external stimuli for the enhancement of academic and medical knowledge. With this knowledge biological variations existing in sub populations can be better understood. Methods closely related and applied in peak area determination of metabolite concentrations include nuclear magnetic resonance, mass spectrometry and liquid chromatography. These methods are especially of importance in pathology and toxicology. Profiling of metabolites has been determined to complement methodologies and technologies. They help in the investigation of preclinical development problems of drugs is possible with these techniques. Research further indicates that metabolite profiling has specific value in the improvement and comprehension of perxisome and phospholipidosis and proliferation (Bendall, Glickson & Thian 2005, p.453). Through metabolite profiling greater potential to identify and highlight biomarkers is possible and the mechanicals that support pathological and toxicological investigations are possible in the preclinical development of drugs. Qs.4 Stroke is an example of a neurological disorder that can be treated by the use of Magnetic Resonance Signals. A stroke is a complication that results in the rapid loss of functions of the brain. Stroke is caused by a cerebrovasular accident and causes an interruption of blood supply. According to (Ackerstaff, Glunde & Bhujwalla 2003, p.528), “There are many pulse sequences that can be used in MRS. Some of the most common pulse sequences are spin echo sequence, inversion recovery sequence, gradient echo sequence, diffusion pulse sequence, saturation recovery sequence, echo planar pulse sequence and spiral pulse sequence.” The most superior way of diagnosing stroke is by use of MRI imaging that is sensitive to neurological disorder and strokes by use of spinal sequence like perfusion and diffusion.EPI is very common in diagnoses of stroke because of its superior qualities. One of the characteristics is speed.EPI is faster than FSE in allowing more slices per TR. This can be attributed to the speed as well as the fewer RF pulses that it possesses thus it generate less absorption rate. It also increases patient comfort and throughput. The use of EPI makes it possible to image patients with reduced sedation.EPI has a high performance system that enables performance within a very short time thus it avoids off resonance artefacts. This makes it superior to the conventional system. Another characteristic is variety of contrast.EPI allows one to get access to tissues that are of T1 T2 weighing. The T2 contrast in EPI is similar to the one in spin echo image. EPI is flexible in terms of solution available. Flexibility is every useful in EPI when one wants to image a small body structure like pituitary gland. Minimizing the field of view can increase the resolution at a very high rate in a single shot. With EPI it is very possible to only get half of the data and be able to calculate the rest of the data by conjugate synthesis. In other situations, parallel EPI or segmentation can be used to reduce geometric distortions while resolution is increased (Glunde & Bhujwalla 2004, p.4274). Acute stroke as a neurological disorder has dire clinical ramifications when detected and treated early. Its intervention by continuously improving technologies offers enhanced and extended patient function and reductions in morbidities associated with strokes. The role played by Magnetic resonance imaging (MRI) has both expanded and integrated to clinical detection, assessment and determination of the prevalence of acute stroke in patients for early thrombolytic therapy. DWI and DSC are more readily available and accessible to patients. This has a direct and positive impact to patients to whom the service might not have been available. The patients would suffer neurological impairments that are permanent and not measurable by the current conventional means. Furthermore, other advantages accrue from the use of MRI in the diagnosis, brain attack and treatment of neurological disorders. A salient issue like the life quality long/ extended term care required by the patients of stroke complications should be addressed and resolved. Insurers on the other hand would prefer to cover the costs of early detection; diagnosis and treatment rather than costs resulting from advanced cases of stroke that have gone on for long durations without treatment. These latter costs are more expensive as the disorder is in its chronic stages. Techniques used in MRI of diffusion and perfusion are viewed with scepticism by some medical professionals. However, research has shown that and strong statistical evidence proves that these methods are very supportive in the imaging of acute strokes. In addition, these methods are conducted with relative speed and ease therefore quite advantageous in time management and efficiency for clinicians. Localized Magnetic Resonance Imaging can and is used in the determination of the status of neurological health and future recovery prediction. Anatomic and functional ideals with addition of biochemical contributions of data lead to the management, detection and diagnosis of stroke. MRS provides precision in stroke cases in the definition of penumbra. Active therapy is therefore enhance and aided by this accuracy. Bibliography: AckerstaffE, Glunde K, Bhujwalla ZM, 2003, “Choline phospholipid metabolism: a target in cancer cells?” J Cell Biochem; 90: 525–533. 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