Magnetic resonance imaging has developed over the last 20 years as an effective method for medical diagnostics. The procedure consists of exposing the human body to a short duration pulse of a very strong magnetic field. Hydrogen protons in the water and fat molecules in the human body which are spontaneously oriented turn themselves in the direction of the magnetic field and rotate on their axes, under the influence of this magnetic field. By extracting the external magnetic pulse, the spinning protons trigger an electrical signal in the pickup coils from which an image of the internal tissue is digitally generated.
MRI diagnostics produce clearer images of soft tissue and blood flow inside arteries and veins than ultrasound images and are also easier to interpret for the medical technician. The very large cost of MRI machines and their large size and specialized installation requirements act as a deterrent to wider use of this technology. There is work underway that promises reduced costs and size of MRI machines, especially in the form of specialized machines for the scan of extremities such as wrists and ankles. The success of this effort could lead to wider use of the technology.
Keywords: Magnetic Resonance Imaging, diagnostics, magnetic pulse
The first Magnetic Resonance Image (MRI) was created in 1973, and the technique has now become for the medical profession a rapidly growing medical diagnostic device. Over 30 million MRI procedures were done in the US in 2010, and new advances in technology are making specialized MRI procedures available for screening for a wider range of diseases and medical conditions each year.
The human body can be considered to be essentially made up of three types of material; bone which is hard and made up of minerals such as calcium, soft tissue including muscles, flesh, blood vessels and organs such as liver, kidney, heart and lungs and fluids including blood and air. The field of diagnostic imaging began with Wilhelm Rontgen’s discovery of X-rays in 1895. Even today, over two-thirds of medical diagnostics, are done using X-rays. X-rays are ionizing radiations, and the image is captured on a photographic film. X-ray images are ideal for viewing bones but the resolution is also insufficient for viewing soft tissue. The invention of Computed Tomography (CT) sought to address this limitation by using digital images in place of the photographic plate and to manipulate the images for contrast and Brightness for differentiating between different forms of soft tissue (Ostensen, 2001).
Ultrasound or ultrasonography was developed as the safer technology for viewing soft tissue and body fluids in the 1950s and 1960s. In this technique, sound waves of frequency between 3.5 MHz and 7 MHz are generated using a transducer or “probe”. Sound waves passing through the human body get reflected when it passes from one type of tissue to the other. The reflected sound wave is picked by a microphone built into the same probe housing as the signal generator, and a computer image of the internal tissue is created in real-time. The medical professional can move the probe over the area of the body to see the image changes and also to freeze the image for recording. Using ultrasound, it is possible to see images of blood flow through arteries and veins and see heart valves opening and closing. Ultrasound equipment is comparatively inexpensive and is safe as compared to X-rays. The interpretation of the ultrasound images, however, requires trained medical professionals, and there is a high risk of incorrect diagnosis (Ostensen, 2001). Ultrasound has no known side effects and is safely used even for the examination of an unborn fetus. Ultrasound waves do not pass through the air and are therefore not effective for the examination of the stomach or the intestines. They also cannot penetrate bone and therefore are not used for areas with bone covering such as the skull. In obese patients, excess body fat sometimes makes ultrasound examination difficult as the reflected sound waves get attenuated.
The MRI is the newest of the diagnostic imaging tools available to the medical profession. The MRI machine is very expensive with costs in the range of $ 1 million to $ 1.5 million. The machine weighs 3-4 tons and has special requirements relating to the room where it is installed and the maintenance it needs. The high costs and complexity have limited the total number of installed MRI machines to only about 35,000 with over 80% of these being in the US, Europe and Japan. GE Healthcare in the US, Siemens and Philips Medical in Europe and Toshiba and Hitachi in Japan are the major manufacturers. Refurbished old MRI machines from US, Europe and Japan are often sent to emerging countries such as India.
The Basic Principle of Magnetic Resonance Imaging
When an external magnetic field is applied to the human body, some of the positively charged protons from the Hydrogen atoms align in the direction of the magnetic field while others align in the opposite direction. The opposing protons are fewer in number than the aligning protons, and there is a net magnetizing force. The protons also spin on their axes like a gyroscope creating an electric current in sensor coils located around the body. When the external field is removed, the protons return to their state of random orientation (Pooley, 2005).
In the short time interval between removal of the external magnetic field and the return of the protons to their random state, a current is induced in the sensor coils at a frequency called the Larmor precession frequency is shown below (Pooley, 2005).
In an MRI machine, a large circular electromagnet is used to create a magnetic field pulse around the patient. The Radio Frequency sensing coils pick up the precession signal.
The external magnetic field is also applied as a gradient with a stronger field at one point that weakens around the periphery using the magnetic gradient coils in the X, Y and Z axes. The RF signal emitted by the protons decay and the rate of decay indicates the position of the protons in the human body relative to the gradient coils providing a 3D image.
The two images above are from the MRI of a section of the brain where the rates of decay from different sections of the spinal column are shown, separately for longitudinal magnetization and transverse magnetization. Over the years, the medical profession has developed decay characteristics for each part of the human body which serve to determine the magnitude and duration of magnetic pulses, the gradients to be applied and the resultant images that will be generated. This data is used to program the MRI machine to look for specific medical diagnostic information based on each patient’s medical history or symptoms (Pooley, 2005).
The Continuing Evolution of the MRI Machine
The MRI machine’s most popular configuration is with cylindrical magnets, and in scores, 1.5 T (for Tesla = 10,000 Gauss) and 3 T. In study some 7 T higher power machines are used. The magnetic field of the planet is just 0.5 Gauss, and the small magnet used on a fridge door is 1000 Gauss, indicating that the magnetic fields of the MRI are many orders of magnitude greater. These fields are created by cooling the electromagnetic coils in liquid helium to make them superconducting. Otherwise, the heat generated by the current flowing through these coils would be impossible for the equipment to handle. The MRI magnet makes up over 30% of the cost of the MRI machine. To direct the magnetic field precisely in the desired direction on the human body, the magnetic field is “self-shielded” by placing it inside counteracting magnets that prevent leakage (Steckner, 2006). The powerful magnetic fields are strong enough to cause any ferromagnetic material in the room to become a projectile. MRI rooms are designed to be without any such materials, and the patient is also checked to ensure that he is not wearing any jewellery or has any ferromagnetic implants.
The major problems experienced by patients being placed inside the MRI machine is the claustrophobic feeling of being in a confined space and due to the loud thumping and knocking noises heard when the magnetic coils are pulsed.These noises can be as loud as 125 decibels, similar to the noise of a door being slammed or a balloon being burst close to the ear. Efforts are being made to make the cylindrical opening larger and to dampen the noises (Steckner, 2006).
The quality of the MRI image depends on “Signal to Noise Ratio” between the signals from the Hydrogen protons and the noise from other sources. Advanced signal filters and digital electronics technologies are being applied for MRI image processing. One significant problem that remains is the need for accurate comparison of MRI scan images taken several weeks apart. It is important for medical professionals treating to say, a tumour growth with medication or radiology, to monitor if the growth is slowing in response to the treatment. The scanning photos taken at various times need to be precisely matched to be able to do this. This requires exceptional skill on the part of the medical professional. Efforts are underway for the MRI image to take references from bone formations near the site of the tumour so that the second and later scans have the same orientation as the first scan (Stecker, 2006).
One important benefit with MRI and other computer-based diagnostic tools is the possibility for the images to be transmitted to a remote hospital for review by a specialist doctor. The scan itself can be performed by remote access to the local MRI machine.Patients exposed to powerful magnetic fields often complain of dizziness and disorientation after the procedure. Many patients cannot lie still inside the machine, and since any movement can cause a blurred image, they need to be sedated before an MRI scan (Stecker, 2006).
New smaller MRI machines have been developed that are suited for the scan of body extremities such as hands, wrists, feet and ankles that would be much smaller in size weighing 300 to 400 kg and significantly lower in cost, perhaps under $100,000. This would enable the wider application of such machines to areas such as the orthopaedic departments of hospitals (Magnetica, 2013).
The high cost of an MRI scan (over $ 1,000 per procedure in the US) has limited the use of this useful diagnostic tool only to cases where alternative diagnostic procedures are ineffective. Over 29% of all MRI procedures in the US are for the brain, head and neck and 25% for the spine. MRI scans of extremities account for another large share of 24%, which is the reason a specialized machine has been developed for this application. Procedures for a pelvic examination, mammography or vascular diagnosis are usually done by ultrasonic or X-ray, mainly due to costs. In breast cancer screening, for example, the MRI is superior as it can distinguish between a benign lesion and a tumour and avoids the need for a needle biopsy (Magnetica, 2013).
One interesting new application reported is to use MRI images to direct focused ultrasound waves to the brain to cure ultra lateral tremors that afflict an estimated 10 million Americans. The MRI capability is being used in this application not only for diagnosis but also in disease control (Shantouf, 2011).GE Global Research reports working on a new magnesium- diboride superconductor that would not need the helium cryogenic cooling. That would make the MRI system smaller and more accessible, probably even for consulting room for a general practitioner (Xu, 2012). For such a conceptual product, the GE website has this interesting image, which seems to be one for the extremities rather than for the whole body.
Summary and Conclusions
Magnetic resonance imaging has strong benefits to test for a range of human diseases over other diagnostic procedures. The very high costs of the equipment have slowed the adoption of this technology, especially in the economically backward regions of the world where it could contribute to improved health for the large populations who live there. The new advances, including the development of low-cost MRI scanners for extremities, suggest important new thinking in this area.
The digitally generated MRI image has the possibility of being transmitted through the internet for review by a specialist doctor and advice to the local doctor on the treatment procedure. This feature would make it possible for MRI technicians to handle the scan process at hospitals that may not have specialist doctors for interpretation of results. It is also conceivable in the future for software-based expert systems to be developed that could help make a preliminary interpretation of the digitally scanned images and filter out only the cases that need the specialist interpretation.
The development of low-cost MRI machines and the transmittal of images to remote doctors for diagnosis have immense benefits for the use of MRI machines for diagnostics in the poorer parts of the world where specialized doctors are unavailable in the smaller towns and villages. Patients in such places have to travel to large cities merely to make a diagnosis.
- Magnetica. (2013) Today’s MRI Market. Retrieved 4 April 2013 from: http://www.magnetica.com/page/innovation/todays-mri-market/
- Ostensen, H. (2001). Diagnostic Imaging: What is it? When and how to use it when resources are limited? World Health Organization, 2001. Retrieved 4 April 2013 from: http://whqlibdoc.who.int/hq/2001/WHO_DIL_01.1.pdf
- Pooley, R.A. (2005, April 25). Fundamental Physics of MR Imaging. Radio Graphics. Retrieved 4 April 2013 from: http://radiographics.rsna.org/content/25/4/1087.full
- Shantouf, R. (2011, Oct. 24). MRI guided focused ultrasound shows promise for essential tremors treatment. Retrieved 4 April 2013 from:http://www.medgadget.com/2011/10/mri-guided-focused-ultrasound-shows-promise-for-essential-tremor-treatment.html
- Steckner, M.C. (2006). Advances in MRI equipment design, software and imaging procedures. AAPM. Retrieved 4 April 2013 from: http://www.aapm.org/meetings/amos2/pdf/26-5961-46702-744.pdf
- Xu, M., (2012). A More mobile future for MRI. GE Global Research. Retrieved 4 April 2013 from: http://ge.geglobalresearch.com/blog/more-mobile-future-for-mri/