Echo Planar Image Sequences Data - Assignment Example

Specialized Nursing Name: Course: Professor: Institution affiliation Topic: 7005 April 7, 2012. Qs 1. Discuss the BLIP EPI method presented in Module 1 in detail. How does it work? (7 marks) Echo planar image (EPI) sequences data collection has different ways of comparing with sequences in standard pulse. One of them is phase encoding in EPI which has a constant gradient in phase encoding. The other method is BLIP which is normally applied at every end of read out gradient in short durations.EPI is an imaging method that is fats and flexible. It has good contrast and therefore has a potential of being applied in many clinical situations like brain imaging.EPI does not produce multiple echoes by using RF pulse. It utilizes quick switching gradients to produce echo train. In EPI the frequency of the encoding gradient moves from positive to negative and ends up creating echoes that are odd and also even (Lu et.al, 2009) In FSE every refocused echo in EPI makes one line in K- space. The gradient either positive or negative read changes the direction of reading the line. BLIP which is a phase makes a new line to be sampled by causing a shift in Ky. The encoding gradient in blip phase does not change in amplitude. There is only a single excitation and there is no 1800 RF pulses in between echoes. The large phase gradient that occurs before the readout starts is the BLIP. In order to make a shift from one row to another a DKy is required which is a comparison of a small kick. This is the same change that occurs in TR in spin echo sequencing. Every echo is affected by encoding gradients that are next to it, either previous or the one immediately before it (Lufkin, 1989). Echo planar is superior to conventional in investigations of the human physiological processes in the body. It has an ability of reducing the motion artifacts and the scan time. This is because in EPI a single RF pulse acquires multiple lines of K space. In EPI BLIP is used to encode phases. It is normally applied inb very small amplitudes by putting in signal in K and a new line is recorded. In EPI, shares in every line of k-space read both positive and negative gradient and this result to changes in direction. BLIP is applied when the read out gradient is at zero. In K space every echo is acquired in a reverse line and before an image is reconstructed an alternative line in K-space must be recorded. The following echoes are affected by BLIP and thus the necessity of a large negative phase gradient for encoding before the start of read out. Due to the fact that the pre-phase gradient is large, the K-space in the signal is kicked to the edge. In pre phase the read gradient kicks the signal to the right and left side in the K-space while kick signal to in the upper and lower directions. Spin echo EPI excites the signal by 90 and then refocuses the signal by 180 RF. The in EPI flips the spine by not more than 90 RF and the gradient in the prophase must be closer to the RF pulse. Q-2 what is Nyquist ghosting? What feature of MRI gives rise to the Nyquist or N/2 ghosting that is seen with EPI? (3 marks) Nyquist ghosting is the asymmetry between the odd and even echoes. There are a number of artifacts that occur in echo planar imaging. Nyquist usually occur when two signals that have shifted in K space are superimposed and they have an interferogram effect. The direction of FOV in phase encoding adds one half ghost artifacts. When there is a ghost in one half of the size of view during phase encoding this is known as Nyquist or N/2 ghost. This can be eliminated by a method in which a pair of echoes from both the odd and even can be phase encoded. These pairs are then separately reconstructed into two different images that are then put together. Certain MRI features results to Nyquist that is often seen with the EPI. This includes Increase in induction effects of eddy currents that are a result of improper design of coils. Reduction in the time factor of even adding extra time may cause a problem of software that may lead to improper shift on the raw data. Any incorrect location of signal from their original location can cause a problem. Another cause is lack of shift parameters during reconstruction. A hardware problem may be as a result of incorrect amendment between digitization of signal and GR standardization (Partain, 2007). To avoid and reduce N/2 ghosting appropriately designed coils should be used to reduce the induction of eddy currents. The sampling clock in the hardware should be adjusted to ensure that the signal digitization and gradient activity timing are well calibrated .To incorporate a shift in the raw data an extra timing factor should be added within the software. Shift parameters should be used to construct a free artifact image during reconstruction. Q3- One of the major disadvantages to EPI is its sensitivity to off resonance effects. Why is EPI so sensitive to these effects? What are these effects? (3 marks) EPI is very sensitive to off resonance effect like magnetic susceptibility and chemical shift. This is because EPI lacks RF pulses. In fast spin echo (FSE) the RF pulses are the ones that correct any build up of errors in echoes. This is done by refocusing any signal delay which may be caused by chemical shift and magnetic susceptibility. This correction depends on the difference in time between pulses to echo. In FSE this difference is relatively smaller and this is what makes it insensitive to effects of magnetic susceptibility. In EPI and in the presence of gradient echo these effects accumulate. Time taken in between any two adjacent points that are along the ky axis can be long. This is due to the absence of RF pulse that can intervene and rephrase the MR signal. This long duration of time that is taken between the two points, can result to low effect bandwidth that in the phase direction. All reconstructions are interpreted in the same manner and do not matter whether it is from phase encoding or accumulated error and so any reconstruction cannot be termed as legitimate. Spins from off resonance with accumulated error are misregistered in the encode direction phase. In the human brain there are some blind spots where the tissues have magnetic susceptibility. To reduce this susceptibility, a hybrid EPI sequence echo can be applied that usually modifies or even improve both the gradient and RF pulse during the single repetition time with between two to four in each excitation time. These Effects Are: Magnetic Susceptibility Chemical Shift Magnetic susceptibilities: Human tissues in the body are different in magnetic properties and this can only be characterized by ones magnetic susceptibility. Tissues those that are adjacent to one another and they have a difference in magnetic susceptibility result to disruption of the local field. A good example is the interfaces between bones and tissues located in the spongy bone. Iron tissues like in those cavities that are air filled or filled with metal materials cause geometric distortion. This geometric distortion can be reduced by decreasing the ETL time and then swapping the phase and read directions. Another example is the air tissue interface that is located in the lungs and also in sinuses like maxillary, ethmoid and mastoid sinuses. Any protons of water that may be present in this inhomogeinity field will cause off resonance. The frequency shift of the mentioned spins will be different in the different regions and from person to person but the resonance remains the same as that caused by fat. This means that they are positioned in the wrong place in the direction of phase encoding. One key thing to note here is that, large inhomogeinity within the local field especially that caused by metal braces present on the teeth can cause very large distortions even when the sequence is perfectly optimized. Chemical shift: fat and water resonance are different in frequency. For a chemical that is off resonance this change in frequency results to shifts in pixels. In EPI method this usually happens in phase and read directions. in order to determine the chemical shift of pixels frequency is divided by the bandwidth for every pixel. Q 4-The use of EPI sequences is quite common in the clinical imaging of stroke victims. Why? What clinical benefits can be achieved? (7 marks) A person suffers from stroke when a blood vessel especially an artey gets obstructed by a blood clot. It can also result when the flow of blood to the brain is interrupted and it is even worse if bleeding occurs into the brain. In most of the patients that suffer from stroke, 85% are ischemic. Symptoms of stroke differ from one person to another depending on the type of stroke. Some of the people have mild symptoms while others have severe symptoms. This difference is also due to the size of the damage in the brain tissues. A damager in one side of the brain affects the different side of the body. The two types of stroke are: A hemorrhagic stroke or ischemic stroke (Leifer, 2009). A hemorrhagic stroke occurs when a blood vessel within the brain is damaged. Ischemic stroke occurs when; an artery that is carrying oxygenated blood to the brain is blocked. This is the most common type of stroke. 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 artifacts. 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. EPI has the following clinical benefits: It can allow perfusion imaging of the brain. Perfusion tests are performed to allow access of blood delivery to a vascular bed. The test is an injection of contrast agent for example gadolinium and this is followed by rapid imaging which follows the contrast bolus to the organ that is focused. When the contrast is finally attained one can get information concerning the delivery of blood to the organ of interest. This is possible by doing an analysis of temporal behavior of the MR. EPI is very good in this test due to its rapidity. This is because it has temporal resolution that is well suited for this when following the contrast. In addition EPI has a capability of T2 weighted imaging. The intensity of EPI image decreases until when the bolus is drained into veins. The EPI signal intensity reduces due to shortening of T2 tissues by contrast bolus (Edelman, 1994). Time series created by images can be used to create a relative contrast against time which can be used to give the volume of cerebral blood (CBV).It can also be used to create the cerebral flow of blood (CBF) maps. These two can be utilized as indicators of brain functioning. Perfusion can be applied to show the presence of tissue and how viable that tissue is in ischemic penumbra. The ability to detect any risk of stroke shows that EPI may be helpful in acute stroke patient’s therapy. Diffusion-Weighted EPI Diffusion is the random movement of thermal molecules. It plays a vital role in cerebral vascular accidents. Spin echo EPI can be made sensitive to water motion in a tissue by inserting a gradient that is sensitive either before or after the pulse. This causes diminishing of the signal which is in relation to the velocity of water diffusion of molecules. If the diffusion is at a high rate this results to hypo intensity while slow rates cause hyper intensity (Carmichael et.al,2009). Benefits of diffusion weighed EPI are the temporal resolution and its insensitivity to physiological motion. This is the reason why DW-EPI is normally used as a single shot procedure because of the insensitivity to movement. Diffusion pulses are very sensitive to small velocities and as such there is loss of signals in MR which results to artifacts.EPI speed is beneficial to diffusion tensor imaging where there is repetition of sequence to give room for diffusion gradient application in different directions and to increase accuracy (Amar, 2011). Diffusion Weighting Application Acute cerebral vascular accidents can be diagnosed early enough by use of diffusion imaging. This is possible when routine images are unremarkable and when CVA can be distinguished from other processes (Burkhardt et.al, 2003) In DW EPI method there is presence of hyper intense signal which is caused by reduction of diffusion of water molecules in the ischemic region. This normally happens in acute cerebral ischemia. This is considered to result from shutting down of energy transport in the affected cells. This occurs in the early stages of ischemia. When the affected cells shut down, they cause a swelling and this hinders random movement of water molecules during diffusion which results to hyper intense signal (Fisher, 1995). The density of protons, T1 and T2 images do not show much indication of an ischemic region. On the other hand, DW EPI shows a hyper intense region. Localizing the affected region helps in effective intervention in early stages of stroke. Bibliography: AMAR, A. P. 2011. Brain and Vascular Imaging of Acute Stroke. World Neurosurgery, 76, S3-S8. 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