The Key Requirements for Performing an MRI - Essay Example

MRI MRI, i.e. Magnetic Resonance Imaging is a tried and tested technique to record the activities of brain. MRI is used to diagnose brain tumours using the functional magnetic resonance imaging (fMRI) scans (Reese, 2005). There are other imaging techniques like Ultrasound, computer-assisted tomography (CT), and X-ray, but MRI is considered the most reliable of them all. MRI is mainly used to diagnose tumours in softer tissues in human body parts. This type of imaging produces a variety of images. These images help in configuring the human body in distinctive manner which in turn helps in detecting pathology. The spatially variable data within the human body which can be measured using the magnetic properties produces a visual image of the object. Slice thickness, is one important parameter, which divides the body part being examined into sections of varying width or thickness. The result of examination depends on the slice thickness. If it happens to be a minute problem, then the slice thickness will be small, because a larger slice thickness might result in erasing small lesions or pathologies. MRI is mainly used to look into the brain and the nervous system of human body to figure out the inconsistencies using the functional magnetic resonance imaging, as these parts happen to be the most delicate parts of human body. Prasad (2005, 292) states that the 'MR images are in effect computer generated visual reconfigurations of physical data such as the relaxation times of hydrogen atoms that are found abundantly in the body.' More than two third of our body is made up of hydrogen atoms. Hydrogen exists in different forms in our body. For example water (H2O), consisting of two H atoms and one O, makes up about 70 percent of human body. In addition Hydrogen atoms are also present in the tissues and fat present in human body. When magnetic energy is sent into our body using radio frequencies, the hydrogen atoms absorb the magnetic rays and send out the same magnetic energy, depending upon the extent of magnetisation. As the nuclei of the hydrogen atoms happen to be in the form of small magnets, so when the magnetic fields change in their surroundings, they respond accordingly. This movement is recorded by the MRI equipment which in turn is analysed by the healthcare professionals to decipher the exact composition. As the experiments started off with an emphasis on nucleus, in early days MRI was known as Nuclear Magnetic Resonance (NMR). Since the process itself is stated to involve very small amount of energy, the normal biochemistry of human body is not affected. The key requirements for performing an MRI include; A strong magnetic field through the primary magnet, emanating out of the a magnetic which could be a permanent, resistive or a superconductive Gradient magnets for fine tuning the magnetic fields The scanning table on which the patient is allowed to lie Attached computers and film for taking the printout of the scan The individual who is to undergo MR imaging is allowed to comfortably lie on a sliding table. The table is then slid into a cylindrical structure which is surrounded by the magnets. Once everything is in position, the magnetic field is turned on; this creates a 2-dimensional image of the particular body part of the person on the computer screens. A coil is placed around the body part/ object which is under observation for MRI, exclusively to pick up the signals for the image. Well, just to make things easier a talk-back system is also in put in place so that after the start of clinging sound the patient doesn't feel uncomfortable, and if required he can communicate with the radiologist. The procedure may last for about 25-50 minutes depending upon the requirements and the complexity of the patient condition. There are in general two to six imaging sequences, each lasting for about 10-15 minutes. Since the process involves magnetic fields, it is therefore advised that anybody having metallic implants in their bodies must inform the radiologist beforehand, so that adequate precautionary measures could be taken. There are some implants like which allow the procedure to be performed, but there are certain implants like those in eyes, heart or brain which might prove hazardous for the safety of the patient. The field strength of the magnets is measured in Tesla. One Tesla is equivalent to 10,000 Gauss. The field strength of MR imaging magnets is far higher than the magnetic field strength of earth. In general the field strength of MRI magnet is 20,000 times stronger than that of earth (Faulkner, 1996, 2). The MRI machine applies radio frequency waves to the human body (or part of body). The proton atoms present in that part of the body start absorbing the energy directed at them. This changes the direction of spin of the proton atoms. It is worth mentioning here that if the spin happens to be in pairs then it cancels out the overall result and the MRI may not be in a position to produce any credible outcome. Therefore, atoms with only odd number of proton or neutrons are preferred for applying this force. Some of the good nuclei for this purpose are 1H, 13C, 19F, 23Na, 31P. Majority of the biological tissues in human body are 12C, 16O, 1H, and 14N. Hydrogen being the most abundant and MR sensitive is used during MRI. As the proton continue to take energy from the RF waves, some of the spins in the lower energy state gather enough energy and jump into higher energy state. This phenomenon is called 'tipping' the net magnetisation towards the transverse plane (Faulkner, 1996, 12). If the energy is enough to produce a 90 degree spin, it is said to be a 90 degree flip angle. Directional magnetic field or magnetic moment associated with charged particles in motion is the basis of MR imaging. The nucleus of an hydrogen atom containing odd number of protons or neutrons results in motion of these particles. This motion is termed as precession, which results in production of a small magnetic field (Mackiewich, 1995, 2). Once the magnetic field is applied these free particles align themselves in the direction of the magnetic field. The Magnetic Resonance can produce a clear image of the human body part when the body part remains in uniform magnetic field for a while. A magnetic moment is produced when the hydrogen nuclei align themselves in one particular direction (Fig-1). Once the 'alignment' is done, a strong RF signal is applied perpendicular to the object/ body part under observation. This causes a tilt in the magnetic moment. This causes a major shakeup in the position of the particles (Fig-2). After a while when the RF force is removed, nuclei tries to realign itself in such a manner that equilibrium is attained with their magnetic moment coming in parallel to the applied magnetic force. This is called relaxation process for the tissues. During this process the nuclei start releasing some of their energy and start emitting out their own signals in the form of RF signals. This signal is called free-induction decay (FID) response signal (Mackiewich, 1995, 2). This signal is in turn measured by the coil placed around the body part under observation and the resultant image is monitored on the computer screens by the radiologists. The relaxation time of hydrogen atoms to come back to their normal positions is generally of two times namely, T1 and T2. The difference in relaxation time is on account of difference in tissue formation. For example the relaxation time of fat is different than that of the water. While preparing the MR image and save some time, the relaxation times are collected while the rest of the numerical array of data is substituted with zero. This type of zero filling does not affect the resolution of the image, instead it smoothens the image (Prasad, 2005, 311). T1 is termed as longitudinal relaxation time indicating the time required for getting magnetised after putting it across the magnetic field i.e. the time to regain longitudinal magnetization after applying the RF signal. On the other hand, T2 is known as the 'Transverse' time. It indicates how long the magnetisation would last along the long axis or transverse axis in a uniform magnetic field. T2 relaxation time happens in case of larger sized proton molecules and due to the presence of a static internal field in the object1. The alignment of hydrogen nuclei and subsequent imaging depends upon a variety of factors like2; The properties of nuclear spins Radio frequency (RF) excitation properties Tissue relaxation properties Magnetic field strength and gradients Timing of gradients, RF pulses, and signal detection The images taken with the help of MRI can be displayed in four main orientations, namely3; Coronal Orientation: It represents the cross sectional i.e. the frontal image of the body in such a manner that the image is taken if the deviation is to be studied from across the shoulders, dividing the body into front and back halves. Sagittal Orientation: It represents the cross-sectional view of the body down the middle, i.e. by dividing the body into left and right halves Axial Orientation: This image is taken along the planer section, perpendicular to the long axis of the body in such a manner that it divides the body into upper and lower halves. Oblique Orientation: A view of the body which is combination of two or three of the above mentioned views is termed as presenting an oblique view. Magnetic Resonance Breast Imaging is another specialised field developed for the imaging of breasts. In this case a special breast array coil is prepared for the person and the rays are applied to this particular region. Sometimes, MRI is taken at different points of time, to figure out how the defective portion inside the body is responding to the medication being offered by the doctors. This way doctors are able to study the life cycle of the disease and find out whether the tumour is reducing, increasing in size or remaining the same after the start of the treatment. Well, subjecting somebody to frequent MRIs has problems of another kind. If the body is subjected to excessive magnetic and RF radiations, that might reflect adversely in some parts of the body. There have been many such studies regarding the safety aspect of MRI and different types of conclusions were drawn from such studies. Taking note of such variations, the US Food and Drug Administration (FDA) came out with the statement in March 1993 which suggested that such concerns need to be probed further before asking the industry for package labelling of the implant device, in regard to its MR compatibility and ferromagnetic properties. For example, not many would readily want to use to the MRI on a pregnant women, because not much research has been done the effect of radio and magnetic waves on the developing foetus. MRI scanning in such cases is decided on case by case basis, considering all aspects and taking into account the health condition of the patient. As technology is evolving and with more inputs from IT enriching the diagnostic tools the federal laws too are being reviewed over time. Conclusion Magnetic Radio Imaging is indeed a technique which has brought about a paradigm shift in the methods of diagnostics. Making use of the available resources within our body to diagnose the condition of our body has proved to be quite effective for the human civilisation. Deciphering the signals too has become quite easier with the dominance of IT and digital techniques. The future appears more promising with the way technology is bringing about the changes around us. References: 1. Bradley, William G. (n.d.). 'Fundamentals of MRI: Part II'. Available online at http://www.e-radiography.net/mrict/fund%20mr2/fundmri%202.htm 2. Faulkner, Wm (1996). 'Basic Principles of MRI'. OutSource, Inc. Available online at http://www.e-radiography.net/mrict/Basic_MR.pdf 3. Mackiewich, Blair (1995). Basic Principles of MRI. Available online at http://www.cs.sfu.ca/people/Faculty/stella/papers/blairthesis/main/node11.html 4. Prasad, Amit (2005). 'Making Images/Making Bodies: Visibilizing and Disciplining through Magnetic Resonance Imaging (MRI)'. Science Technology Human Values. 2005; 30, Sage Publications. 5. Reese, Jordan (2005). MRI lie detector may tell fact from fiction. Temple Times Online Edition. Available online at http://www.temple.edu/temple_times/2-10-05/lies.html (Oct 7, 2007) 6. UC San Diego Medical Centre (2006). How MRI Works. Available online at http://health.ucsd.edu/specialties/mri/ptinfo/how.htm 7. Voyvodic, James (n.d.). Basic Physical Principles of MRI. Available online at http://www.biac.duke.edu/education/courses/fall05/fmri/handouts/2005_Week2_BasicPhysics.ppt Read More