mri: a noble diagnostic medical tool and

8/14/2019 MRI: A Noble Diagnostic Medical Tool And

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MRI: A Noble Diagnostic Medical Tool and a Window to the Brain and Body

By

Shivaprasad M Khened,

Curator, Nehru Science Centre, Mumbai

Introduction

Beginning in the late 1950s, but accelerating at an ever-faster pace by the end of the 90s,technology has dramatically transformed modern medicine. Before World War II thetypical physician had a modest toolbox consisting of a thermometer, stethoscope,sphygmomanometer, and occasional access to x-ray machines and electrocardiograph.Along with these medical devices a limited cabinet of pharmaceuticals assisted thephysician of the 1940s, including the sulfa drugs and penicillin. After the War biologicalresearch was transformed through a new armamentarium of biophysics instruments in the

likes of Electron Microscopes, Ultracentrifuges, Mass Spectrometers and new agentssuch as radioactive isotopes. A revolution in microelectronics and semiconductorsinitiated during the War together with the development of computers led the way to newfields of biomedical imaging such as Ultrasound, Computerized Tomography (CT) andPositron Emission Tomography (PET) scanners, Nuclear Magnetic Resonance Imaging(MRI). The focus of this article will however be on the discoveries concerning theMagnetic Resonance Imaging.

MRI

Magnetic Resonance Imaging (MRI) is perhaps the most important non-invasive

diagnostic tool in today's medicine. The basic components of any MRI system are themagnet, RF transmitter, gradient coil, and the receiver coil, along with a computer toanalyse the incoming signal and produce image. MRI is a diagnostic technique that givesa picture of the inside of the body without using X-rays or other potentially harmfulradiation. It is mostly used to image the brain and spinal cord, the eye and ear, the jointsand heart. Within a few minutes the MRI scan can reveal whether the patient has had astroke and also can predict which part of the brain is in danger of a stroke. It is thesespecial diagnostic features that have made MRI an indispensable tool in any modernhospitals. The technique of magnetic resonance imaging has proven to be invaluable forthe diagnosis of a broad range of conditions in all parts of the body, includingneurological and behavioral disorders, musculoskeletal injuries, cancer, heart andvascular diseases. MRI is also used to create maps of biochemical compounds within anycross section of the human body. These maps give basic biomedical and anatomicalinformation that provides new knowledge and may allow early diagnosis of manydiseases. Since MRI has the ability to provide information about the state of health oforgans and tissues in addition to giving details of their shape and appearance, thisimaging technique has major advantages over other diagnostic methods. And in all thesecases, MRI works with no harmful intervention.


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MRI has been a boon especially for the sports persons since it can give clear pictures ofsoft-tissue structures near and around bones, it is often the best option for spine and joint problems which helps in early diagnosis of sports related injuries, especially thoseaffecting the knee, shoulder, pelvis, and hip, elbow and wrist. The images allow thephysicians to see even the very small tears and injuries to ligaments and muscles.

Principles of MRI

The MRI is significant and applicable to the human body because we are all filled withsmall biological magnets, the most abundant and responsive of which is the nucleus ofthe hydrogen atom, the proton. The principles of MRI take advantage of the randomdistribution of protons, which possess fundamental magnetic properties. This processinvolves three basic steps. First, MRI generates a steady-state condition within the bodyby placing the body in a strong (30,000 times stronger than the Earth's magnetic field)steady magnetic field. Secondly, it changes the steady-state orientation of protons bystimulating the body with radio frequency energy. Thirdly, it terminates the radio

frequency stimulation and listens to the body transmitting information about itself at thespecial resonant frequency using an appropriately designed antenna coil. The transmittedsignal is detected and serves as the basis of the construction of internal images of thebody using computer principles similar to those, which were already developed for CAT(Computerized Axial Tomography) or CT scanners.

Nobel Prize in Physiology or Medicine for 2003

The Nobel Assembly at Karolinska Institutet has decided to award the Nobel Prize inPhysiology or Medicine for 2003 jointly to U.S. chemist Paul C. Lauterbur, Ph.D. andBritish physicist Sir Peter Mansfield, Ph.D. for their seminal discoveries concerning theuse of Magnetic Resonance Imaging to visualize different structures.

Lauterbur, 74, was cited for his seminal experiment that showed how 2D images can beproduced using the relative positions of magnetic resonant behaviors among protons in aphantom. He had published the results in Nature in March 1973 while working at theState University of New York at Stony Brook. Lauterbur is director of the BiomedicalMagnetic Resonance Laboratory at the University of Illinois in Urbana.

Mansfield, 69, was honoured for research conducted at the same time as Lauterbur'sachievements. His findings, also published in 1973, applied a field gradient strategy tomap the structure of crystalline material. Following this line of research, Mansfielddeveloped the first MRI echo-planar pulse sequence in 1976. He was a leader of theUniversity of Nottingham team that developed a prototype for modern gradient-coilsystems and a whole-body MR scanner designed for British electronics firm EMI. He isemeritus professor of physics at the University of Nottingham.

The Nobel Committee chose not to recognize Dr. Raymond V. Damadian, who was alsoresponsible for key discoveries leading to the development of the first machine toproduce an MR image of a human being in 1977.


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This article describes the history that over the last 70 years has led from the work ofscientists simply investigating the nature of matter to applications that have ultimately ledto the development of MRI that can now save many lives. It would also examine thecontroversy associated with its invention claims. The controversy surrounding the

invention of MRI is centred on the contribution of Raymond Damadian whose nameincidentally does not figure in the Nobel Prize.

The history of MRI

X-Rays

The history of magnetic resonance imaging has its roots planted in the late 1800s.William Roentgen discovered X-rays On November 5, 1895. Roentgen studied hisdiscovery for seven weeks and learned that the newly found rays that he named X-rays,traveled in straight lines, could not be reflected or refracted, and did not respond to

magnetic or electric fields. The X rays passed through most substances, including the softtissues of the body, but left bones and most metals visible. Despite the hazards that the X-rays produced, they greatly contributed to the medical field. It was a discovery thatcreated a rise in the quality of health care because it provided a means of noninvasiveresearch of the human body. The discovery of X-rays was responsible for the decrease inexploratory surgery and the increase in quality of overall health care because it provideda map of the human body for physicians. For his discoveries concerning the X raysRoentgen was awarded the very first Nobel Prize in physics for the year 1901.

Nuclear Magnetic Resonance

Scientists tried to improve and expand on the amazing images produced by X-raysthrough the discovery of nuclear magnetic resonance imaging (NMR). The firstsuccessful nuclear magnetic resonance (NMR) experiment was made in 1946,independently by two scientists in the United States. Felix Bloch, working at StanfordUniversity, and Edward Mills Purcell, from Harvard University, found that when certainnuclei were placed in a magnetic field they absorbed energy in the radio frequency rangeof the electromagnetic spectrum, and re-emitted this energy when the nuclei transferredto their original state. Both men studied the hydrogen atom, because of its favorablenuclear properties. They chose to study the proton - the nucleus of the hydrogen atom(H). Because the hydrogen nucleus is composed of a single proton, it has a significantmagnetic moment. Hydrogen would turn out to be the most important element for MRIbecause of its favourable nuclear properties, nearly universal presence, and abundance inthe human body as part of water (H2O). Bloch and Purcell shared the 1952 Nobel Prizefor physics, and their discoveries have lead to the nuclear magnetic resonance (NMR) incondensed matter. There have been two other Nobel prizes associated with thefundamental discoveries arising from NMR. In 1991, Richard Ernst, Switzerland, wasawarded the Nobel Prize in Chemistry for his contributions to the development of highresolution NMR spectroscopy, an important analytical tool in chemistry. In 2002, KurtWuthrich also of Switzerland, was awarded the Nobel Prize in Chemistry for his work


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using NMR spectroscopy to determine 3D structures of biological macromolecules insolution.

Relaxation Times

During the 50's and 60's NMR spectroscopy became a widely used technique for the non-destructive analysis of small samples. Many of its applications were at the microscopiclevel using small (a few centimeters) bore high field magnets. One of the first advancesthat came in the course of NMR spectroscopy work was the measurement of twoquantities called relaxation times, T1 and T2. T1 is the time it takes the nuclei in testsamples to return to their natural alignment; T2 is the duration of the magnetic signalfrom the sample. Relaxation is a very important process in MR imaging as it determinesthe type of signal obtained greatly impacting the type of image generated. NicolaasBloembergen (1981 Nobel laureate in physics for development of laser spectroscopy)was the first researcher to measure the two relaxation times accurately. He alsocollaborated with Purcell in measuring the changes in the relaxation times in a variety of

liquids and solids. The technological developments in computing and advent of high-speed computers facilitated the improved measurement of relaxation times in fractions ofseconds there by making NMR a practical research tool.

Computerised Tomography

MRI also owes a debt to a technique called Computerised Tomography (CT) as it wasdeveloped initially on the back of CT but quickly outpaced that technique. The Science ofimaging began with the work of British electronics engineer with the EMI Company,Godfrey Hounsfield. Between 1969 and 1972 Hounsfield built a working model of amachine, which was a combination of an x-ray machine and a computer. He used certainprinciples of algebraic reconstruction in his machine and was able to scan the body frommany directions and also manipulate the images to produce a cutaway view of the bodysinterior. Unknown to Hounsfield, South African born nuclear physicist Allan Cormack,had published a paper describing the mathematical basis for the construction of aComputerised Axial Tomography (CAT) essentially with the same idea in 1963, using areconstruction technique called the Radon transform. In recognition of their workspertaining to the development of Computerized Tomography (CT), and the principlesunderlying CT both Hounsfield and Cormack were awarded the Nobel Prize inphysiology or medicine in 1979.

Dr. Raymond Damadian.

In the late 60s and early 70s Raymond Damadian demonstrated that a NMR tissue parameter (termed T1 relaxation time) of tumour samples, measured in vitro, wassignificantly higher than normal tissue. Although not confirmed by other workers,Damadian intended to use this and other NMR tissue parameters not for imaging but fortissue characterization (i.e., separating benign from malignant tissue). Damadian is acontroversial figure in NMR circles. Although criticism has been leveled at his scientificacumen it should not overshadow the fact that his description of relaxation time changes


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in cancer tissue was one of the main impetuses for the introduction of NMR intomedicine.

Raymond Damadian, a medical doctor and the founder of the company Fonar has alsolaid claim to the "inventor" label of MRI. In 1978, Damadian formed FONAR

Corporation (from "Field fOcused Nuclear mAgnetic Resonance"), which produced thefirst commercial scanner in 1980. Later the company developed the first FDA-approved,first mobile, and first whole-body MR scanners. Fonar's patented Iron Circuit technologyhas enabled the company to develop seven different MRI products. Raymond V.Damadian, inventor of the method known today as magnetic resonance imaging or MRI,was born in Forest Hills, New York in 1936. He received a BS in mathematics in 1956and then turned to medicine, earning an MD in 1960 from the Albert Einstein College ofMedicine (Bronx, NY). After his internship, residency, and Fellowships at WashingtonUniversity and Harvard, Dr. Damadian served for some time in the Air Force, and thenjoined the faculty of SUNY Downstate Medical Center. There, his research into sodiumand potassium in living cells led him to his first experiments with nuclear magnetic

resonance (NMR), which caused him to first propose the MR body scanner in 1969. Earlyin his medical training, Damadian chose internal medicine because of its analyticalnature. By the time he graduated, he decided to do his detective work in the researchlaboratory rather than the clinic because of the prospects that his research, if successful,might help millions of people rather than the thousands he could reach dispensingmedical treatments. Damadian said "I didnt know if anything would come of any researchI might do, but I knew I wanted the chance to try to help many more than I couldpersonally reach in a lifetime."

MRI Scan of human body using Indomitable

Experimenting on rats, Damadian discovered dramatic differences in the quality andduration of NMR signals emitted by cancerous versus healthy tissues that confirmed hisidea of the MR body scanner. His 1971 paper, "Tumor Detection by MagneticResonance," was met with skepticism from the scientific community, but Damadianforged ahead, filing the first of his patents for an MRI scanner the next year. The scannerused liquid helium to supercool magnets in the walls of a cylindrical chamber; the nucleiof hydrogen atoms in the water, which all cells contain, reacted to the resultant magneticfield, and a three-dimensional spatial localization method coordinated the signals into thescan. Damadian spent the next few years working with teams of graduate students tomake his scanner a reality. Meanwhile, many scientists had decided that Damadian'sideas were not so misguided after all and began to compete to develop the first workablescanner. Finally, in 1977, Damadian's team produced the first MRI scan of the humanbody; using a prototype device he called "Indomitable" (now installed in the NationalMuseum of American History a Smithsonian Institution). Damadian filed for the patentof a NMR body scanner entitled "Apparatus and Method for Detecting Cancer in Tissue"on March 17, 1972, (one full year before Lauterburs publication appeared in Nature onMarch 16, 1973), Damadian has earned over 40 patents (including the one for his MRIscanner), as well as the 2001 Lemelson-MIT Program's Lifetime Achievement Award.


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In 1988, Damadian and Lauterbur shared the National Medal of Technology, issued bypresidential decree, for their independent contributions in conceiving and developing theapplication of MR technology to medical uses. He was also inducted into the USAsNational Inventors Hall of Fame (1989). Damadian also has won a $128 million suitagainst GE by defending his MRI patents.

Sweet spot Technique

Damadian and his team constructed the first Human scanner and used the sweet spottechnique of his patent to accomplish the first human scan and proved to the world thatthe whole body NMR scanning was achievable in near future. It is thus unambiguous thatDamadian is the originator of the NMR body scanner concept, preceding Lauterburs firstconceptualization of an NMR scanner by years. While history has shown the Lauterburmethod superior to the Damadian method in speed and efficiency, there is no doubt thatDamadian is the originator of the NMR scanning concept and that he uncovered the tissueNMR signals that made it possible. The contributions of both pioneers to the creation of

MRI have been essential to its genesis. The President of the United States, for example,in a ceremony at the White House on July 15, 1988, awarded the nations highest honor intechnology, the National Medal of Technology, jointly to Lauterbur and Damadian. Hecited them "for their independent contributions in conceiving and developing theapplication of magnetic resonance technology to medical uses included whole-bodyscanning and diagnostic imaging".

It was partly in recognition of these works that the draft article on MRI commissioned bythe National Academy of Sciences in its "Beyond Discovery" series stated that Dr.Damadian was "crucial" to the invention of the MRI and it also had attributed four of 12MRI milestones to him. However when the article finally appeared in print Damadiansname was conspicuously missing from the article and the article played down his roledismissing that the method he discovered had not proved clinically reliable in detectingor diagnosing cancer. It is perhaps this article and many other controversies associatedwith the findings of Damadian that may have prompted the Nobel committee to over lookhis claims on MRI and exclude his name for the Nobel Prize.

Paul C Lauterbur

Paul C Lauterbur, born 1929, is a professor of Chemistry, Biophysics and ComputationalBiology, and Bioengineering at the University of Illinois at Urbana-Champaign. Duringthe early 1960s, he used nuclear magnetic resonance devices to develop carbon-13spectroscopy. Damadians observation of T1 and T2 differences in cancerous tissue wasan important event for Paul Lauterbur. After seeing Dr. Damadian's experiment repeatedby a graduate student, Mr. Lauterbur had a flash of brilliance. He realized he couldsubject the nuclei to a second magnetic field that varied in strength in a precise way. This prompted him to discover the possibility of creating a two-dimensional image byintroducing gradients in the magnetic field. Though the idea of a "magnetic fieldgradient" was not new, Lauterbur was the first to see how it could be used to reconstructan image. He wrote his idea in a notebook and had it witnessed the next day. His work,


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with later contributions from Peter Mansfield, forms the basis for modern MRI imaging.Lauterbur's gradient approach quickly gained favour over Damadian's sweet spottechnique scanner method. Lauterbur while working as a Professor of Chemistry at theState University of New

York at Stony Brook added to the CAT scan idea and obtained the first magneticresonance image (MRI) in 1972. Lauterbur's groundbreaking idea was to superimpose onthe spatially uniform static magnetic field a second weaker magnetic field that variedwith position in a controlled fashion, creating what is known as a magnetic field gradient.At one end of a sample the graduated magnetic field would be strong, becoming weakerin a precisely calibrated way down to the other end. Because the resonance frequency ofnuclei in an external magnetic field is proportional to the strength of the field, differentparts of the sample would have different resonance frequencies. Thus, a given resonancefrequency could be associated with a given position. Moreover, the strength of theresonance signal at each frequency would indicate the relative size of volumes containingnuclei at different frequencies and thus at the corresponding position. Subtle variations in

the signals could then be used to map the positions of the molecules and construct animage.

Zeugmatography

Lauterbur published a short paper in Nature On 16th March 1973 titled "Image formation by induced local interaction; examples employing magnetic resonance" the articleincluded an image of his test sample: a pair of small glass tubes immersed in a vial ofwater. It did not appear from the title that the article represented the foundation for arevolution in imaging. The paper in fact was nearly not published having been initiallyrejected by the editor as not of sufficiently wide significance for inclusion in Nature. Inthis seminal paper Lauterbur described a new imaging technique that he termedzeugmatography (from the Greek word zeugmo meaning yoke or a joining together). Thisreferred to the joining together of a weak gradient magnetic field with the stronger mainmagnetic field allowing the spatial localization of two test tubes of water. He used a backprojection method to produce an image of the two test tubes. This imaging experimentmoved from the single dimension of NMR spectroscopy to the second dimension ofspatial orientation being the foundation of MRI. Lauterbur further presented his approachon zeugmatography at the International Society of Magnetic Resonance meeting inJanuary 1974 held in Bombay, Raymond Andrew, William Moore, and Waldo Hinshawfrom the University of Nottingham were in the audience and took note of the approach.As a result, Hinshaw and Moore developed their own approach to MRI with theirSensitive Point method. An alternative imaging method was proposed in 1974 by Alan N.Garroway, Peter K. Grannell, and Peter Mansfield, with a second research group at theUniversity of Nottingham. By the late 70's and early 80's a number of groups, includingmanufacturers, in the US and UK showed promising results of MRI in vivo. This was,and still is, a technological challenge to produce wide bore magnets of sufficientuniformity to image the human body.


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Some of the current research of Lauterbur is directed toward the development of newmathematical techniques and algorithms for NMR imaging, including spectroscopicimaging, and toward NMR microscopy for the observation of very small objects and forhigh resolution imaging of selected regions in animals and humans, with applications tohistology, the characterization of nonbiological materials and the study of structure,

transport, and motion within complex objects.

Peter Mansfield

Peter Mansfield, born 1933, is Emeritus Professor of Physics at the University of Nottingham. The Nobel committee recognized him for further developing the use ofgradients in the magnetic field and showing how the signals could be mathematicallyanalyzed, which made it possible to develop a useful imaging technique. He also showedhow extremely fast imaging could be achieved, a theory that became technically possiblewithin medicine a decade later. Mansfield utilized gradients in the magnetic field in orderto more precisely show differences in the resonance. He showed how the detected signals

rapidly and effectively could be analysed and transformed to an image. This was anessential step in order to obtain a practical method. Mansfield also showed howextremely rapid imaging could be achieved by very fast gradient variations (so calledecho-planar scanning).

Peter Mansfield and his colleagues created the first MRI of a human body part, a finger in1976. These images were termed MRI instead of NMR. The difference between the twois that nuclear magnetic resonance is the resonance that occurs when a nucleus is placedin a magnetic field and is swept by a radio frequency that causes the nuclei to flip. Thiscauses the radio frequency to be absorbed, which is what is measured. Magneticresonance imaging is very similar to NMR but is a more complex application in whichthe geometric source of the resonance is detected and calculated by Fourier transformsanalysis. This type of transform analysis states that adding a series of sine waves withappropriate amplitude and phase can make any signal or waveform.

Mansfield graduated with First Class Honours in physics in 1959 from Queen MaryCollege, University of London. He had been involved at an undergraduate level with thedesign and construction of an Earth's field NMR magnetometer. It was during this earlyperiod that he developed interest in the field of NMR imaging. The success of this projectled to an invitation from Jack Powles to join his NMR research group in order to tacklethe challenging problem of pulsed NMR in solids. This work started in October 1959 andlasted the usual three-year Ph.D. study period. Mansfield continued to pursue his interestin NMR and worked on the subject including during the period of his sabbatical atHeidelberg in the early 70s.

Echo Planar ImagingIn 1976, Mansfield introduced the ultra-high-speed Echo Planar Imaging (EPI) technique.The idea of introducing a new imaging technique arose out of a growing concern ofMansfield at the time that if NMR imaging were ever to be of clinical value, imagingtimes, then measured in many tens of minutes, would have to be drastically reduced. At


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the time of its conception EPI was difficult to implement by virtue of the extremerequirements of gradient amplitude and fast switching rates. Nevertheless, Ian Pykettimplemented an early version of EPI in 1977. He succeeded in obtaining crude snapshotimages of a test-tube phantom and later a live human finger. EPI and the whole conceptof ultra-high-speed imaging and spectroscopic imaging developed by Mansfield were

ahead of their time by some 15 years or so and it is this technique that has paved the wayfor the modern MRI equipments.

Since the very concept of ultra high-speed imaging used by Mansfield was a break awayapproach, recognition for its utility was slow to come by. Except at Nottingham, EchoPlanar Imaging languished worldwide but received a slight impetus when Ljunggrenpublished his paper in 1983 on the comparison of NMR imaging methods. By then themajor MRI companies had launched a series of medical scanners based on slowerimaging methods and were clearly not inclined to upgrade machine specifications for theimplementation of EPI before recouping their investments. Thus the EPI languished for afurther 10 years. In this intervening period, newcomers to the business began to

rediscover Mansfields techniques and applied them to the process of speeding up theslower MRI methods. This class of fast imaging, known as FLASH imaging, waspreferred by manufacturers who saw it as a stepwise approach to the implementation ofhigh and hopefully ultra-high-speed imaging.

Mansfield while working at Nottingham in the mid-1980s acquired a super conductive0.5-Tesla whole body magnet. Mansfield says It was clear to me, even before wereceived the magnet, that if EPI were to function correctly within the close confines ofthe magnet, something had to be done to decouple the rapidly switched gradient fieldsfrom the metal structures comprising the magnet cryostat. Work started on gradientscreening in 1984 and culminated in the concept of active magnetic screening during thecourse of 1985. General Electric and other major manufacturers quickly took up activemagnetic screening of gradient coils, which now forms part of the standard range offeatures offered on all MRI imaging machines. Peter Mansfield was knighted in 1993.

Conclusion

Magnetic Resonance Imaging (MRI) has grown rapidly as a reliable technique for non-invasive diagnosis and is constantly being improved. MRI has revolutionized the standardof medical care around the world, largely because it provides highly detailed three-dimensional images of human anatomy. A versatile, powerful, and sensitive tool, MRIcan generate thin-section images of any part of the body from any angle and direction,without surgical invasion, all in a relatively short period of time. The impact of MRI has been enormous. Surgical planning, real time cardiac imaging, musculoskeletalcharacterization, and brain imaging are now routine MRI applications. As on date,Magnetic Resonance Imaging is used primarily as a diagnostic tool, but it has grown tobe a powerful research tool, as in the area of brain mapping. It is currently crossing overinto the therapeutic side of medicine with the emergence of MRI-guided surgery. MRIalso is showing potential to image on the cellular level. MRI has also played major role inthe Visible Human Project. The Visible Human Project is another stepping stone in the


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advancement of technology and medicine. The National Library of Medicine, USA, hasused MRI in its research to develop a digital imaging database to aid in the advancementof education in the medical field. The first MRI equipment for applications in medicinehad made their appearance in the 1980s. Today however it is routinely used the worldover. In 2002, about 22,000 MRI cameras were in use worldwide and more than 60

million MRI examinations were performed.

Types of MRI

A variety of different techniques are used in MRI applications today. Following are someof the commonly used techniques of MRI used in modern medical diagnostics.

Functional MRI (fMRI):Functional magnetic resonance imaging, or fMRI for short, utilizes echo-planar imagingand involves very rapid scans of approximately 20 ms or less. Activity in the patients

brain is induced by some form of stimulation, either directly through the senses or byinvoking some form of thought, such as memory. Blood flow increases in the activatedareas and the rapid scanning is necessary to capture these changes. Sometimes chemicalagents are used to increase the contrast in the MR images. These agents are injected intothe blood before stimulation.

Interventional MRI (iMRI):Interventional MRI enables real-time scanning of the brain and spinal cord duringsurgery. The traditional equipment had to be modified to allow surgeons access to a viewof the scanned images while they performed. The modifications include two holes iniMRIs magnet where monitors displaying the scanned images are mounted.

MR Angiography (MRA):Magnetic Resonance Angiography produces images of blood flow within the circulatorysystem. One method in which this is done makes use of the spin-echo technique.Different frequency pulses excite spins in plane slices at 90 and 180 degrees. In thepresence of flow, the two spins will run into each other and produce an echo. Theintensity of the echo signal, and thus the intensity of the image, is proportional to thevelocity of the flow. The spins will not run into each other in the absence of flow and anecho will not be produced. Another, more complicated, method uses a bipolar gradientpulse. This pulse is only able to affect spins that have a velocity.

MR Microscopy (MRM):Magnetic Resonance Microscopy is a form of high-resolution imaging. It works under thesame principle as traditional MRI, only modified for smaller specimens. Speciallydesigned coils are used in conjunction with higher magnetic fields and stronger gradientsto enable imaging of small objects. The special coils are designed to fit the small sizes.An encapsulated specimen fits inside a custom-made coil and subjected to the higherintensity procedures.


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MR Spectroscopy (MRS):Magnetic Resonance Spectroscopy can utilize any form of resonance imaging, such asthe spin-echo technique. However, instead of using the feedback from these techniques toprovide information about position or spatial orientation, resonance spectroscopy suppliesoutput concerning the molecular composition of an object. How does this work? Electron

clouds surrounding molecular compounds within the object block resonant atoms. Thisblocking occurs in varying degrees for different chemicals and different positions ofcompounds, allowing for the assessment of chemical shifts within the tissue.

Volume Imaging (3-D):Three-dimensional imaging uses the spin-echo technique to provide images of volumesinstead of the conventional 2-D planar slice of organs or parts. To make a 3-D imageusing MR technology, scanned slices of the object are taken in multiple planes. Theinformation from these scans is then conglomerated into a single image of 3-D spatialarrangement.

mri: a noble diagnostic medical tool and

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