Magnetic resonance imaging is probably the most important medical imaging modality. MRI images can be acquired using different acquisitions techniques, known as imaging pulse sequences. MR imaging provides an opportunity for specific tissues to give a signal higher or lower than another one, or even to give no signal at all. All these possibilities are the result of the physical principles that underpin the technique. MRI is distinguished from other imaging equipments by an ability to differentiate different soft tissues from each other. Another advantage of MRI over Computed Tomography and X-Ray equipments is that it does not ionize the human body. X-Rays that are used in conventional radiography and Computed Tomography represents electromagnetic radiation with a very high frequency. However electromagnetic radiation is involved in MRI as well, but in a very different way. Radiation during the MRI scan has a much lower frequency and is not attenuated by tissue. This type of radiation is one of the components of nuclear magnetic resonance process, which also requires interaction between atomic nuclei and a strong magnetic field. The nuclei concerned in MRI are hydrogen nuclei, which are abundant in the human body. Measuring the signals coming from hydrogen nuclei gives information about the tissue in which they are situated. Much of the practice of MRI is concerned with ensuring that the signals form hydrogen nuclei in an environment of interest can be clearly distinguished .
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The techniques that are available for medical diagnosis of internal tissues and organs of human body can be divided by invasive and non-invasive ones. X-ray computer assisted tomography (CT X-rays), PET (Positron Photon Tomography) and conventional X-Ray system are part of the invasive techniques due to their use of ionizing radiations. Particularly, it should be noted that for some sectors and/or pathologies MRI reached the highest resolution in images and the fastest acquisition times (e.g. in the case of internal cerebral damages to which, in some cases, CT is blind) . Magnetic Resonance Imaging (MRI) is non-invasive equipment which was rapidly developed in the last 20 years.
Magnetic resonance imaging (MRI) has full 3D capabilities, excellent soft-tissue contrast, and high spatial resolution (1 mm). On the other hand MRI scans tend to last longer compared to that of Computed tomography which typically lasts between 3 and 10 min. Another disadvantage of MRI is that it is more susceptible to patient motion. The major uses of MRI are in the areas of assessing brain disease, spinal disorders, angiography, cardiac function, and musculoskeletal damage . The cost of MRI scanners is relatively high, with the price of 1.5 T system priced on the order of $1.5 million.
The medical use of MRI has developed rapidly since early 1970s. The first MRI equipment for whole human bodies became available at the beginning of the 1980s. In 2002, approximately 22,000 MRI cameras were in use worldwide, and more than 60 million MRI examinations were performed . MRI is now broadly used for medical assessments due to several observable advantages over other imaging techniques. One of the major superiority of MRI is its safety and high image quality. In contrast to Computer Tomography, MRI does not use ionizing radiation thus its use is not restrained by the concern of exposure time. Narrow bore of the MRI system can be unpleasant for persons suffering with claustrophobia. This latter limitation is becoming moot as more open and faster MRI machines come into clinical usage . MRI scans are restricted for the patients with ferromagnetic metal, but not MR-compatible objects inside their bodies, e.g., orthopaedic implants or cardiac pacemakers.
A crucial conceptual breakthrough in the development of MRI was made by Paul Lauterbur. Dr. Lauterbur found out that he could be able to get nuclei in different regions of a sample to wobble at their own individual frequencies by applying a gradient magnetic field. Dr. Lauterbur discovered the imaging principle in 1972 and published his seminal paper on MRI in Nature in 1973, which immediately started a new revolution in research, development, and application of MRI. Nuclei produces signals at different frequencies, the value of each frequency would represent a location from where the signal had been originated. By using gradients in various orientations, he demonstrated the capability plot the structure of a sample inside using MRI equipment. Dr. Peter Mansfield who lately proposed a way to rapidly detect, analyze, and transform such resonance signals, along with Dr. Lauterbur were awarded The Nobel Prize in Physiology on 6th of October 2003 by the Nobel Assembly at Karolinska Institute .
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These discoveries have lead to development of magnetic resonance imaging (MRI), which nowadays plays an important role in medical diagnostics, treatment, and research.
Science of MRI
The origins of the technology lie with a phenomenon of nuclear magnetic resonance, better known as NMR. However due to the frightening nature of the first word, NMR was lately renamed as MRI. MRI uses the magnetic properties of some hydrogen nuclei. When subject to a large static magnetic field, many of the nuclei will line up like compass needles in the direction of the field. The resonance part of MRI emanates when a radio-frequency magnetic field is added at right angles the static field. If the second field has the frequency, that matches the resonant frequency of the hydrogen nuclei; it knocks them out of alignment. When an RF pulse is removed the nuclei return to alignment in specific time. This time is referred as a relaxation time, which is different for nuclei of different tissues. Difference in time can be plotted to give an MR image of interested body region.
The human body is two-thirds water, but different organs contain different amounts of water. MRI system can use these differences in order to reveal the structures of organs. Many diseases lead to the change of water content in organs which is reflected in the image. For example large tumour will have much lower water content than surrounding healthy tissue. This fact will be clearly visible on MR image. MRI is especially used to image brain and the spinal structure of the human body. Images obtained during the scan provide invaluable support for surgery, which is directed from tumours to herniated discs. It also reveals the localized inflammation of multiple sclerosis and can be used in follow-up to treatment to visualize response to treatment with fine detail .
Fundamentals of MRI
As mentioned above the signal coming from human body is essentially due to water protons. These protons exist in identical magnetic environment and resonate at the same frequency. The key innovation for MRI is to impose spatial variations on the magnetic field to distinguish spins by their location. Applying a magnetic field gradient causes each region of the volume to oscillate at a distinct frequency. The most effective non-uniform field is a linear gradient, where the field and the resulting frequencies vary linearly with the distance along the object being studied . Fourier analysis of the signal is performed to plot a map of spatial distribution of spins.
During the scan the patient is placed in the bore, which is surrounded by strong magnet. Magnet produces large static magnetic field that is approximately more than 10,000 times than that of the earths. Each proton can be considered as a small magnet. These protons align in two configurations, with their internal magnetic fields aligned either parallel or unparallel to the direction of the static magnetic field B0, with slightly more found in the parallel state. The protons precess around the direction of the static magnetic field. The frequency of precession is directly proportional to the strength of the static magnetic field. Application of a weak radiofrequency (RF) signals causes the protons to precess and the sum of all of the protons precessing is detected in the form of induced voltage in a tuned detector coil. Spatial information is encoded into the image using variable magnetic field gradients. These impose a linear variation in three dimensions of the magnetic field within the patient. The frequency and the phase of the precessing magnetization is measured by the RF coil, and the analogue signal is then digitized. An inverse two-dimensional Fourier transform is performed to convert the signal into the spatial domain to produce the image . Varying the data acquisition parameters, differential contrast between soft tissues can be obtained.
As mentioned above MRI experiment is based on the precession of a nuclear spin I when it is submersed in a magnetic field. When the patient is placed within the magnetic field B0, the spins Ii of the atomic nuclei inside the human tissues require less energy to be aligned along B0. This gives a total nuclear magnetization, which is expressed by following equation:
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As shown in Figure 1, in a magnetic field spinning nuclei have lower energy when aligned to the magnetic direction then when they are opposed to it.
Fig 1. The basis of NMR in a semi-classical picture: a nuclear spin precesses around body. The two possible directions of the spin vectors determine two energy levels E1 and E2. The frequency difference, Î”Î½ = (E1 - E2) / h, is in the range of radio-frequencies .
The energy difference between the two levels generated by the Zeeman Effect, Î”E = Î³B0, corresponds to a certain frequency Î½ = Î”E/h that represents the range of radio frequencies. After each stimulation by a RF pulse, the atomic nuclei returns to their previous energy level emitting radio waves [7-10]. The detected MR signal contains all the information related to the recovery of the magnetization toward the equilibrium state (i.e. the relaxation time). The relaxation process of the nuclear magnetization is described mainly by two microscopic time constants: the longitudinal one referred to T 1 called nuclear spin-lattice relaxation time and the transversal one T2, which is subsequently called nuclear spin-spin relaxation time.
T 1 - indicates the time required for element to become magnetized as a result of applied magnetic field or time required to retain longitudinal magnetization after applied RF pulse. It is determined by thermal interactions between the resonating protons and other protons. During resonance protons absorb the energy. This energy is then distributed to other nuclei.
T2 is referred as the transverse relaxation time. It measures the period of long transverse magnetization in a uniform external magnetic field. Alternatively, it is a measure of how long the resonating protons remain coherent or rotate in phase following a 90Â° RF pulse. T2 decay occurs because of magnetic interactions that occur between spinning protons. Compared to T1 interactions, T2 interactions is not associated with energy transfer but only a change in phase, which leads to a loss of coherence. Time constants T 1 and T 2 used in MRI appear to be material-dependent that gives the potential to distinguishing different tissues and diseases with a proper analysis of the received RF signal .
Figure 2. Illustration of relaxation times T1 on (a) and T2 and (b) 
Distinctive capabilities that MRI system is due to the great variety of mechanisms that can be achieved to create an image contrast. If MRI technique was restricted to water density, it would posses considerably less usefulness as long as most of the tissues would appear identical. Many different MRI contrast mechanisms can be used to distinguish different tissues and disease processes.
Differences in the T1 time constant can be used to produce image contrast by exciting all magnetization and then imaging before full recovery has been achieved. This is illustrated on the Figure 2 (a). An initial RF pulse destroys all the longitudinal magnetization. The Figure 2 shows recovery of two different T1 components. The short T1 component recovers faster and produces more signals. All this gives a T1 weighted MR image .
Figure 3. Transversal T1 MR Image of Brain (www.mr-tip.com)
Relaxation times of tissues reflect the mobility of hydrogen atoms and molecules that contain hydrogen. Materials with short correlation times will cover a wide range of frequencies, resulting in relatively little spectral density in any particular frequency band and will have long T1 and long T2 relaxation times . Range of mobility of water depends on its environment. Highly ordered structure may be accompanied by reduced mobility. The behaviour of magnetization under the influence of RF pulses and relaxation processes can be described by the Bloch equations. Relaxation times can be prolonged with the use of contrast agents. Ions such as gadolinium are frequently used to cause enhanced T1 relaxation. Such types of compounds are attached to proper pharmaceuticals that make their administration safe.
Magnetic Resonance Imaging System
Typical MRI system is displaced in a room with walls containing RF shielding to create a Faraday cage. Outside the examination room are typically an operator room and a technical room. The technical room holds amplifiers, power supplies, cooling facilities and acquisition systems .
Three types of magnetic fields - main fields or static fields (B2), gradient fields; radiofrequency (RF) fields (B1) are required in MRI scanners. In practice is it also necessary to use coils or magnets that produce shimming fields to enhance the spatial uniformity of the static field (B0). Most MRI hardware engineering is concerned with producing and controlling these various forms of magnetic fields. The special challenge associated with the design and construction of medical scanners was to develop practical of scaling these devices up to sizes capable of safely and comfortably accommodating an entire human patient . The magnetic resonance imaging system comprises a number of major components: the magnet and shim coils, gradient coils and drivers, radiofrequency system and coils, patient handling equipment, measurement control and computer system, together with a range of supporting equipment .
Figure 4. MRI Scanner (www.magnet.fsu.edu)
The magnet is the main component of the MR system. The majority of systems use a superconducting magnet, with field strengths ranging from 0.5-3T (Telsa) for clinical systems. Specially designated clinical research centres use custom made MRI systems which are able to produce magnetic field of approximately 8T or even more. Most superconducting systems are self shielded, significantly reducing the magnetic footprint of the system. Lower field MRI systems are available between 0.1 and 0.3T, which are mainly used for orthopaedic applications. Permanent magnet systems are available with vertical and horizontal field orientations, the latter allowing imaging of standing patients.
In order to provide a very homogeneous field, magnets are shimmed with steel and by adjustable currents by additional field coils. Most shimming equipments are not user accessible. Many modern gradient sets are capable of peripheral nerve stimulation, or even lead to pain, depending on the intensity. Maximum gradient amplitudes are regulated to prevent cardiac fibrillation which is life threatening circumstance. The gradients are driven by signals derived from a digital gradient waveform. . Another practical issue during the scan is the noise, which is generated by gradients when they are constantly switching on and off. Some of the MRI system manufacturers provide specially designated headphones, which significantly decrease the noise of gradients and simultaneously allows communication with the radiologists. Examination of pregnant women is also not recommendable in order to protect fetus.
Figure 5. Basic MRI System Block Diagram (http://www.rell.com)
In order to transmit the planned waveform and significantly reduce the generation of eddy currents within the magnet and shimming coils, gradients are now generally screened. Additive to that special circuits are used to correct deviations from the planned waveform. RF power amplifier is used to amplify and provide transmission of modulated RF signals that are traced from a digital source. Typical RF Power amplifier delivers approximately 15 kW of power. This signal is transmitted to a body coil, and possibly to some other smaller transmits coils. Some advanced systems are now incorporating array coil transmission, to improve RF homogeneity at the high frequencies used in some clinical research systems (e.g., 7 T) . Signal is received from the body by several coils, ranging from a body coil to special multi-element parallel acquisition systems. The detected signal is demodulated, amplified and digitized. Additional coils are used for particular parts of the body, by attaching them on the patients in order to increase the signal. MRI scanner employs sophisticated distributed intelligence, together with one or more consoles which provides control, communication and archive capabilities. Systems also include peripherals like Dicom printers to print images, and standalone computers and servers for picture archival and communications systems (PACS) and Radiology Information Systems (RIS). Specially designed physiological monitors and accessories that are MR compatible may be also present in the scanning room.
Three gradient fields in X, Y and Z direction of Cartesian coordinate system, are used to code position information into the MRI signal that permits the imaging of thin anatomical slices. The direction of static field along the axis of the scanner is conventionally taken as Z direction, There is three relevant gradient fields:
MRI scans are performed by subjecting the spin system to a sequence of pulsed gradient and RF fields. Additionally, it is necessary to have three separate coils - one for each of the relevant gradient fields that have its own power supply and are controlled independently by computer . The generation of MR images requires a rapid sequence of time dependant gradient fields to be applied on patient during the examination. This in turn requires that the currents in each gradient coil should be rapidly switched by computer controlled power supplies. The rate at which the gradient currents can be switched is an important characterization of imaging capabilities of MR scanner. In many pulse sequences, the switching duty cycle is relatively low, and coil heating is not significant. However there are sequences which require very rapid switching of gradient at a high duty cycle .
Figure 6. MRI Scanner Gradient Coils (wwwHYPERLINK "http://www.magnet.fsu.edu/".HYPERLINK "http://www.magnet.fsu.edu/"magnet.fsu.edu)
Radiofrequency (RF) coils are the components of every MR scanner. RF coils are exploited for transmitting and receiving signals at the resonance frequency of the protons within the patients. The operating frequency of widely used 1.5 T MR scanner is 63.86 MHz. Electronic components of MRI transmitter and receiver chains closely resemble corresponding components in radio and television circuitry. There is significant difference of using radiofrequency waves between broadcasting systems and MR Scanners. Transmitting and receiving antennas of broadcast systems compared to MR system operate in the far field of electromagnetic wave, while the MRI system operates in the near field. It should be noted that ideally the RF field should be perpendicular to the static field, last defined as Z direction. RF field can both be linearly polarized on X and Y directions. However, the most efficient RF field results from a quadrature excitation, which requires a coil capable of producing simultaneous X and Y fields with a 900 phase shift between them . Three types of RF coils commonly used in MRI scanners are: Body coils, head coils and surface coils. These coils are placed in the space between the patient and gradient coils. In order to reduce or even eliminate the electromagnetic coupling between RF coils and other components of MR scanner, conducting shields placed in gradient coils are exploited. Body and head coils are large enough to surround the region being imaged and are designed to produce RF magnetic field that should be uniform across the region of interest. Body coils with the diameter (50-60cm) are large enough to surround the patient. Coils designed for head imaging have relatively smaller diameter (typically 28cm) .
Types of Magnets
The magnet is one of the most important part in magnetic resonance imaging (MRI) equipment, thus its performance directly affects the quality of the imaging. Improvement of the homogeneity of the magnetic field accounts signal-to-noise ratio (SNR). A high SNR value ensures a good imaging property. The most important aspects determining the quality of the MRI device are the strength and homogeneity of the magnetic field in the diameter of spherical volume . Magnetic fields are produced by exploiting either electric currents or permanently magnetized materials as sources. In both cases, the field strength, that magnet produces falls off rapidly away from the source. Thus creating a highly uniform magnetic field on the outside of sources is not possible. Consequently, to produce highly uniform field it is obligatory to more or less surround the patient with the magnet. There is three different types of magnet used in the MRI system, which are described below .
Permanent Magnets and Electromagnets - These two types of magnets use magnetized materials to produce the field that is applied to the patient during the scan. In a permanent magnet, the patient is placed in the gap of two permanently magnetized poles. As for electromagnets, they use similar configuration, but the poles are made of soft magnetic materials, which are magnetized only when subjected to the influence of electric current coils. These coils are wound around them. Usage of external power supply is applied to electromagnets. For both type of magnets gap between the poles must be large enough to fit the patient as well as gradient and RF coils.
Permanent magnet materials available for use in MRI scanners are composed by carbon iron, alloys such Alnico, ceramics such as Barium ferrite and Rate Earth alloys such as samarium cobalt . Permanent posses several advantages over the electromagnets:
They produce a relatively small fringing field
They do not require power supplies
However permanent magnets tend to be very heavy and beside that can produce relatively low magnetic fields around 0.3 T. They are also subject to temporal field drift caused by temperature changes. When the pole faces are made from electrically conducting material. Eddy currents induced in the pole faces by the pulsed gradient fields can limit performance of MRI scanner that uses permanent magnets. Neodymium iron has been recently introduced to decrease the weight of permanent magnets but this types of magnets are still not able to produce high magnetic fields .
Resistive Magnets - The first whole body scanners, manufactured in the late 1970s and early 1980, used four to six large coils of copper or aluminum wire surrounding the patient. These coils are energized by powerful (40-100kW) DC power supplies. The heat dissipation increases rapidly with the field strength, thus building resistive magnets that operate at fields higher than 0.3 T is not feasible . The electrical resistance of the coils leads to considerable heating, thus using cooling water that flows through the coils becomes mandatory to prevent overheating.
Superconducting Magnets - Since the early 1980s, the use of cryogenically cooled superconducting magnets [Wilson, 1983] has been the most appealing resolution to the problem of producing the high magnetic field for MRI scanners. The property of exhibiting absolutely no electrical resistance near absolute zero has been know as an exotic property of material since 1911. In the 1950s, a new class of materials (type II superconductors) was discovered. An alloy of niobium and titanium has been used in most superconducting whole-body magnets that have been constructed for use in MRI scanners . These materials withhold the property to carry loss-free electric currents in very high fields.
Figure 6. Magnet which should produce 9.4 Tesla field (Courtesy of "Siemens AG")
The entire length of the magnet wire must be without any flaws - such as imperfect welds - that would interrupt the superconducting properties of the magnet. Superconducting magnets can operate in the continues mode, that means once the current is induced and constant current flow is maintained as long as the temperature of the coils is below the superconducting transition temperature. This temperature is about 10K for niobium-titanium wire. The coils of wires are kept at this low temperature by capsulation in a double walled cryostat that permits them to be bathed in liquid helium at a temperature 4.2K (â‰ˆ -272 0C). Over the time liquid helium begins to boil due to unavoidable heat leakage that take place in the cryostat. Thus helium refill of MR systems equipped with superconducting magnets should be performed as a part of planned maintenance procedure. Due to the ability of super conducting magnets to achieve very strong and stable magnetic field without undue power consumption, this type of magnet is most widely used in modern MR Scanners . Due to the big ampere-turn capability, the superconducting magnets can achieve a high magnetic field in a large volume . Extending the magnetic field over a large gave an opportunity to develop specific scanning sequences that are used to scan the whole body. One of the disadvantages of MR systems equipped with superconducting magnet is that homogeneity of the field becomes hard to control. The homogeneity of the magnetic field in the superconducting magnet is the deciding factor of the imaging quality of a superconducting MRI system .
Magnetic Field Homogeneity
The uniformity of the main magnetic field is measured in ppm. In a defined volume it is the difference between maximum and minimum field strength multiplied by 1 million .
In fact for most imaging techniques homogeneity must be maintained over a large region. Homogeneity of the magnetic files can alter over the time and this imperfection is reduced by the use of shimming fields. There are two types of shimming procedures. Active Shimming - uses additional coils (resistive or superconducting) which are specially designed to produce a magnetic field. When the magnet is installed, its magnetic field is carefully mapped and the current in shim coils are adjusted to cancel out the terms in the harmonic expansion to some defined high order. The second approach is Passive Shimming - which uses small permanent magnets, which are placed at the designated positions along the inner walls of the magnet bore cancel contaminating fields.
Software in MRI Scanners
Software has been a key player for commercial MRI systems. The software typically integrates distinct domains:
MRI physics and real-time conÂtrol software
Software deeply embedded in hardÂware components
Field-Programmable Gate Arrays (FPGAs)
Image reconstruction system
Image processing and viewing
Pulse sequences generate the raw MRI signals and define the image contrast mechanisms, signal-to-noise ratio, and prevention or suppression of imÂage artefacts. Pulse sequence developers must comÂbine a thorough understanding of MRI physics with effective and efficient software engineering. Challenges in this field include timing accuracy on a nanosecond scale and the ability to adapt pulse sequences on a real-time millisecond scale . Apart from the geometrical dimensions, MRI data contain dynamic scans (time series), diffusion weighting, and heart phases. These non standard image dimensions put a challenge on multidimensional viewing and processing software. MRI systems can be integrated within the hospital networks (Figure 8), Standalone image servers (PACS) are used to store and transfer obtained images throughout the hospital network. Images are exported in DICOM (digital imaging and communications in medicine) format. MRI software also includes bunch of diagnostic tools, that are used by service engineers to troubleshoot or asses the state of functioning. Software of modern MRI systems includes PDF readers and Web browsers. They may also be connected to the Internet, which gives a distinctive opportunity of remote service. The MRI system contains information protected under various privacy laws, so firewalls and virus scanners are needed to protect MRI scanners from hackers .
Figure 8. Network environment. The magnetic resonance imaging (MRI) system is connected to a hospital network, which in turn is connected to the Internet .
Different modality equipments (X-Ray, CT, MRI, US) are using different software. The imaging system vendors like (GE Healthcare, AG Siemens, Philips and so forth) use same software interface for different modality equipments. Apart from the cost savings part, it also provides the end user the same look and feel for all of the vendor's systems. This leads to less user training on various systems, which is considered a competitive advantage .
Induced Currents Hazards
MR scanners are widely used in health care centers for diagnostic properties. Examined persons are exposed to static magnetic and time-varying electromagnetic fields (EMF). MRI examinations cause also relatively high level of exposure to static magnetic fields (SMF) of health care workers attending patients before and after examination . Adverse effects of induced currents can be e.g. nerve and muscle tissue excitation, vertigo, phosphenes, difficulties in hand-eye coordination .
MRI scanners produce a very powerful static magnetic field (B0). The most common machines in use today are either 1.5 Tesla (T) or 3.0 T. An MRI scanner also produces a very powerful pulsed RF field (64 or 128 MHz for 1.5T and 3.0T scanners respectively) and also a gradient field in the 1-2 kHz region. The static field can affect implantable medical devices by causing saturation of ferromagnetic electronic components, by inducing force or torque on the AIMD or by inducing image artifact .
Despite the fact that MRI has become one of the medicine's most important diagnostic equipment, patient safety concerns still arise. MRI is contraindicated by both device and MRI equipment manufacturers for patients with active implanted medical devices (AIMDs). The primary concern is overheating of implanted lead wires due to currents induced from the powerful RF fields of the MRI scanner. In pacemaker patients, heating of myocardial tissue has caused increase in pacemaker capture threshold and in some cases complete loss of capture (inability to pace). Permanent damage to an implantable cardioverter defibrillator and at least one patient death and another with severe burns along the wires of deep brain electrodes have also been reported . The authors, with pacemaker lead wires placed in a "worse case" MRI scan condition, have measured distal tip lead wire temperatures of up to 57 degrees C (more than sufficient to cause tissue damage). Another risk is localized myocardial ablation which could result in changes in the action potential vector during Atrial/Ventricular contraction . However, in contrast to reports of problems, there have been several recent anecdotal reports of MRI scans being safely performed on non-pacemaker dependant patients under highly controlled conditions. Proper diagnosis, treatment and management of a number of life threatening diseases such as cancer, neurological and brain disorders are made possible by MRI. Accordingly, the physician, with informed patient consent, must sometimes ignore the legal contraindications, weigh the risk factors, and go ahead and perform an MRI on an AIMD patient. As illustrated in Figure 9, there are many types of AIMDs currently implanted which exclude a large number of patients from MRI examination. These AIMDs shown in Figure.9 may include cardiac pacemakers, implantable cardioverter defibrillators, cardiac resynchronization devices (bi-ventricular), drug pumps, neurostimulators, deep brain stimulators, urinary incontinence devices, left ventricular assist devices, bone growth stimulators, cochlear implants and many more .
Figure 9. Wire frame man with various types implanted AIMDs 
Current and Future of MRI
The primary challenges for an MRI system are patient comfort, imaging performance and cost. Large number of people suffering with claustrophobics decline to take the MR examination, Widening and shortening the "tunnel" will probably help to resolve this problem. Tissue heating, acoustic noise and vibration exhibited during the scan are another major part of MRI disadvantage. Imaging performance involves the strength of the main magnet field, the size of the imaging volume, the strength and switching speed of the gradient coils, and the number of receiver channels . Various technical components of an MRI system are described, and then some future technology development directions are discussed.
Field Strength - The signal to noise ratio (SNR) increases virtually linearly with magnetic field strength. However the radio frequency (RF) power deposited in the patient, for a fixed spin excitation level, increases as the square of the field strength. Limiting power deposition is usually achieved by imaging more slowly; hence at high field the SNR per unit time may in certain cases be reduced. A further problem at higher field strengths is that wavelength effects in the body cause image inhomogeneities. Hence most imaging is performed at 1.5 Tesla (T), or about 64MHz for the proton nucleus. At this frequency the wavelength in tissue (Z=70) is about 56cm. Nevertheless more and more clinical imaging is being performed at 3T .
Length, Inner Diameter, Imaging Volume and Cost Factors - From a claustrophobia perspective it would be nice if the magnet length could be reduced significantly. In order to reduce the magnet length by 20cm while maintaining spherical imaging volume of 48cm, internal forces should be increased by employing more super-conducting wire. These types of wire will inevitably increase the manufacturing cost of MR scanner.
Another option for reducing claustrophobia is to increase the magnet bore diameter. However to keep the internal stresses similar, the length needs to be increased proportionally. Further, the costs increase almost as the third power of the diameter. Hence adding 1Ocm to the diameter would increase the magnet cost by about 33% .
Peripheral Nerve Stimulation - Switching gradient fields, with a simultaneous high slewrate and amplitude, can induce peripheral nerve stimulation (PNS - experienced as a mild muscle twitching). Current high performance gradient sets (slewrates -200T/m/s and amplitudes of -45mT/m) are capable of this. If the maximum slewrate is used then the amplitude has to be restricted - and vice-versa. For fixed amplitude there is a slewrate below which PNS doesn't occur. The lower the gradient amplitude the higher is this value. MRI scanners are constrained to operate in this "no-stimulation" regime. The slewrate at which PNS occurs is also roughly inversely dependent on the diameter of the imaging volume. Hence systems with smaller imaging volumes can typically be operated in a faster imaging mode .
Integrated Gradient and RF coils - With some reduction in imaging volume, it is possible to integrate an RF coil within the gradient coil, such that more radial space is available for the patient. This can be coupled with a significant shortening of the magnet in order to produce a much less "claustrophobic" system .
Integrated PET/MR or X-Ray/MR - The anatomical detail given by MRI and spectroscopy available with Magnetic Resonance Spectroscopy (MRS) complement the quantitative physiological imaging obtained with PET. Such a device has not become a reality because of the incompatibilities of photomultiplier tubes (PMTs) and their associated electronics with MRI's high magnetic fields . Combining PET and MRI scanning into one system will give potential of comprehensive imaging of the body. Such system would be a great asset to nuclear medicine.
Focused Ultrasound - This is technology for ablating tumors using a focused beam of ultrasound, whereby the temperature is monitored in real-time with MRI. This method of temperature monitoring is possible because the water NMR resonance is very slightly frequency dependent, and the associated phase shift can be detected with MRI .