Magnetic resonance
imaging (MRI), nuclear magnetic resonance imaging (NMRI), ormagnetic resonance tomography (MRT) is a medical imaging technique used in radiologyto image the anatomy and the physiological processes of the body in both
health and disease. MRI scanners use strong magnetic fields, radio waves, and field gradients to form images of the body. MRI is based upon the
science of Nuclear
Magnetic Resonance (NMR). Certain atomic nuclei can absorb and emit radio frequency energy when placed in an external magnetic field. In clinical and research MRI, hydrogen atoms are most-often used to generate a detectable
radio-frequency signal that is received by antennas in close proximity to the
anatomy being examined. Hydrogen atoms exist naturally in people and other
biological organisms in abundance, particularly in water and fat. For
this reason, most MRI scans essentially map the location of water and fat in
the body. Pulses of radio waves are used to excite the nuclear spin energy transition and magnetic field gradients
localize the signal in space. By varying the parameters of the pulse sequence, different contrasts can be generated between tissues
based on the relaxation properties of the hydrogen atoms therein. Since its
early development in the 1970s and 1980s, MRI has proven to be a highly
versatile imaging modality. While MRI is most prominently used in diagnostic
medicine and biomedical research, it can also be used to form images of
non-living objects. MRI scans are capable of producing a variety of chemical and physical data, in
addition to detailed spatial images. MRI is widely used in hospitals and
clinics for medical diagnosis, staging of disease and follow-up without
exposing the body to ionizing
radiation. MRI has a wide range of applications in
medical diagnosis and over 25,000 scanners are estimated to be in use
worldwide. MRI affects diagnosis and treatment in many specialties
although the effect on improved health outcomes is uncertain. Since MRI
does not use any ionizing radiation, its use is generally favored in preference
to CT when either modality could yield the same
information. (In certain cases, MRI is not preferred as it can
be more expensive, time-consuming, and claustrophobia-exacerbating). MRI is in
general a safe technique but the number of incidents causing patient harm has
risen. Contraindications to MRI include most cochlear
implants and cardiac
pacemakers, shrapnel and metallic foreign bodies in the eyes. The safety of MRI during the first trimester of
pregnancy is uncertain, but it may be preferable to other options. The
sustained increase in demand for MRI within the health
care industry has led to concerns about cost
effectiveness and over-diagnosis. MRI is the investigative tool of choice for neurological
cancers, as it has better resolution than CT and offers better visualization of
the posterior
fossa. The contrast provided between grey and white matter makes it the best choice for many conditions of
the central
nervous system, including demyelinating
diseases, dementia, infectious
diseases and epilepsy. Since many images are taken milliseconds apart, it
shows how the brain responds to different stimuli; researchers can then study
both the functional and structural brain abnormalities in psychological
disorders.
MRI and Time
Magnetic resonance
imaging was invented by Paul
C. Lauterbur in September
1971; he published the theory behind it in March 1973. The factors leading
to image contrast (differences in tissue relaxation time values) had been
described nearly 20 years earlier by Erik Odeblad (physician and scientist). In
1950, spin echoes were first detected by Erwin
Hahn and in 1952, Herman
Carr produced a
one-dimensional NMR spectrum as reported in his Harvard PhD thesis. In
the Soviet Union, Vladislav Ivanov filed
(in 1960) a document with the USSR State Committee for Inventions and Discovery
at Leningrad for a Magnetic Resonance Imaging device, although this was not approved until
the 1970s. In a March 1971 paper in the journal Science, Raymond
Damadian, an
Armenian-American physician and professor at the Downstate Medical Center State University of New York (SUNY), reported that tumors and normal tissue can be distinguished
in vivo by nuclear magnetic resonance ("NMR"). He suggested that
these differences could be used to diagnose cancer, though later research would find that these
differences, while real, are too variable for diagnostic purposes. Damadian's
initial methods were flawed for practical use, relying on a point-by-point scan
of the entire body and using relaxation rates, which turned out not to be an
effective indicator of cancerous tissue. While researching the analytical
properties of magnetic resonance, Damadian created a hypothetical magnetic
resonance cancer-detecting machine in 1972. He filed the first patent for such
a machine, U.S.
Patent 3,789,832 on
March 17, 1972, which was later issued to him on February 5, 1974. The US National Science Foundation notes "The patent included the idea
of using NMR to 'scan' the human body to locate cancerous tissue." However,
it did not describe a method for generating pictures from such a scan or
precisely how such a scan might be done. Meanwhile, Paul
Lauterbur at Stony Brook University expanded on Carr's technique and
developed a way to generate the first MRI images, in 2D and 3D, using
gradients. In 1973, Lauterbur published the first nuclear magnetic resonance
image and the first cross-sectional image of a living mouse in January
1974. In the late 1970s, Peter
Mansfield, a physicist and
professor at the University of Nottingham, England, developed the echo-planar imaging (EPI) technique that would lead to
scans taking seconds rather than hours and produce clearer images than
Lauterbur had. Damadian, along with Larry Minkoff and Michael Goldsmith,
obtained an image of a tumor in the thorax of a mouse in 1976. They also
performed the first MRI body scan of a human being on July 3, 1977, studies
which they published in 1977. In 1979, Richard S. Likes filed a patent on
k-space U.S.
Patent 4,307,343.
During the 1970s a
team led by John Mallard built the first full body MRI scanner at the University of Aberdeen. On 28 August 1980 they used this
machine to obtain the first clinically useful image of a patient's internal
tissues using Magnetic Resonance Imaging (MRI), which identified a primary
tumour in the patient's chest, an abnormal liver, and secondary cancer in his
bones. It was later used at St Bartholomew's Hospital, in London, from 1983 to 1993. Mallard and
his team are credited for technological advances that led to the widespread
introduction of MRI. In 1975, the University of
California, San Francisco Radiology
Department founded the Radiologic Imaging Laboratory (RIL). With the
support of Pfizer, Diasonics, and later Toshiba America MRI, the lab developed
new imaging technology and installed systems in the US and worldwide. In
1981 RIL researchers, including Leon Kaufman and Lawrence Crooks,
published Nuclear Magnetic Resonance Imaging in Medicine. In the
1980s the book was considered the definitive introductory textbook to the
subject. In 1980 Paul Bottomley joined the GE Research Center in
Schenectady, NY. His team ordered the highest field-strength magnet then
available — a 1.5T system — and built the first high-field device, overcoming
problems of coil design, RF penetration and signal-to-noise ratio to build the
first whole-body MRI/MRS scanner.
In 1982,
Bottomley performed the first localized MRS in the human heart and brain. After
starting a collaboration on heart applications with Robert Weiss at Johns
Hopkins, Bottomley returned to the university in 1994 as Russell Morgan
Professor and director of the MR Research Division. Although MRI is most commonly performed
at 1.5 T, higher fields such as 3T are gaining more popularity because of their
increased sensitivity and resolution. In research laboratories, human studies
have been performed at up to 9.4 T and animal studies have been performed
at up to 21.1T.
Reflecting the
fundamental importance and applicability of MRI in medicine, Paul
Lauterbur of the University of
Illinois at Urbana-Champaign and Sir
Peter Mansfield of
the University of Nottingham were awarded the 2003 Nobel Prize in
Physiology or Medicine for
their "discoveries concerning magnetic resonance imaging". The Nobel
citation acknowledged Lauterbur's insight of using magnetic field gradients to
determine spatial localization, a discovery that allowed rapid acquisition of
2D images. Mansfield was credited with introducing the mathematical formalism
and developing techniques for efficient gradient utilization and fast imaging.
The actual research that won the prize was done almost 30 years before while
Paul Lauterbur was a professor in the Department of Chemistry at Stony Brook University in New
York.
Important Applications
D- MRI
Diffusion
MRI measures the diffusion of water molecules in biological
tissues. Clinically, diffusion MRI is useful for the diagnoses of
conditions (e.g., stroke) or neurological disorders (e.g., multiple
sclerosis), and helps better understand the
connectivity of white matter axons in the central nervous system. In anisotropic medium (inside a glass of water for example), water
molecules naturally move randomly according to turbulence and Brownian motion. In biological tissues however, where the Reynolds number is low enough for flows to be laminar, the diffusion may be anisotropic. For example, a molecule inside the axon of a neuron has a low probability of crossing
the myelin membrane. Therefore, the molecule moves principally
along the axis of the neural fiber. If it is known that molecules in a
particular voxel diffuse principally in one direction, the
assumption can be made that the majority of the fibers in this area are
parallel to that direction.
MAGNETIC RESONANCE ANGIOGRAPHY
Magnetic resonance angiography (MRA) generates pictures of the arteries to
evaluate them forstenosis (abnormal narrowing) or aneurysms (vessel wall dilatations, at risk of rupture). MRA
is often used to evaluate the arteries of the neck and brain, the thoracic and
abdominal aorta, the renal arteries, and the legs (called a
"run-off"). A variety of techniques can be used to generate the
pictures, such as administration of a paramagnetic contrast agent (gadolinium) or using a technique known as "flow-related enhancement"
(e.g., 2D and 3D time-of-flight sequences), where most of the signal on an
image is due to blood that recently moved into that plane, see also FLASH MRI. Techniques involving phase accumulation (known as phase
contrast angiography) can also be used to generate flow velocity maps easily
and accurately. Magnetic resonance venography (MRV) is a similar procedure that
is used to image veins. In this method, the tissue is now excited inferiorly,
while the signal is gathered in the plane immediately superior to the
excitation plane—thus imaging the venous blood that recently moved from the
excited plane.
MAGNETIC RESONANCE SPECTROSCOPY
Magnetic resonance spectroscopy (MRS) is used to measure the levels of
different metabolites in body tissues. The MR signal produces a spectrum
of resonances that corresponds to different molecular arrangements of the
isotope being "excited". This signature is used to diagnose certain
metabolic disorders, especially those affecting the brain, and to provide information
on tumor metabolism. Magnetic resonance spectroscopic imaging (MRSI)
combines both spectroscopic and imaging methods to produce spatially localized
spectra from within the sample or patient. The spatial resolution is much lower
(limited by the available SNR), but the spectra in each voxel contains information
about many metabolites. Because the available signal is used to encode spatial
and spectral information, MRSI requires high SNR achievable only at higher
field strengths (3 T and above).
FUNCTIONAL MRI
Functional
MRI (fMRI) measures signal changes in the brain that are due to changing neural activity. Compared to anatomical T1W imaging, the brain
is scanned at lower spatial resolution but at a higher temporal resolution
(typically once every 2–3 seconds). Increases in neural activity cause changes
in the MR signal via T*
2 changes; this mechanism is referred to as the BOLD (blood-oxygen-level dependent) effect. Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin. Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research. The BOLD effect also allows for the generation of high resolution 3D maps of the venous vasculature within neural tissue.
2 changes; this mechanism is referred to as the BOLD (blood-oxygen-level dependent) effect. Increased neural activity causes an increased demand for oxygen, and the vascular system actually overcompensates for this, increasing the amount of oxygenated hemoglobin relative to deoxygenated hemoglobin. Because deoxygenated hemoglobin attenuates the MR signal, the vascular response leads to a signal increase that is related to the neural activity. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research. The BOLD effect also allows for the generation of high resolution 3D maps of the venous vasculature within neural tissue.
REAL-TIME MRI
Real-time MRI refers to the continuous monitoring
("filming") of moving objects in real time. While many different strategies
have been developed over the past two decades, a recent development reported a
real-time MRI technique based on radial FLASH and iterative
reconstruction that yields a temporal resolution of 20
to 30 milliseconds for images with an in-plane resolution of 1.5 to
2.0 mm. The new method promises to add important information about
diseases of the joints and the heart. In many cases MRI examinations may become
easier and more comfortable for patients.
MAGNETIC RESONANCE GUIDED
FOCUSED ULTRASOUND
In MRgFUS therapy, ultrasound beams are
focused on a tissue—guided and controlled using MR thermal imaging—and due to
the significant energy deposition at the focus, temperature within the tissue
rises to more than 65 °C (150 °F),
completely destroying it. This technology can achieve precise ablation of diseased tissue. MR imaging provides a
three-dimensional view of the target tissue, allowing for precise focusing of
ultrasound energy. The MR imaging provides quantitative, real-time, thermal
images of the treated area. This allows the physician to ensure that the
temperature generated during each cycle of ultrasound energy is sufficient to
cause thermal ablation within the desired tissue and if not, to adapt the
parameters to ensure effective treatment.
MULTINUCLEAR IMAGING
Hydrogen is the most frequently imaged nucleus in MRI
because it is present in biological tissues in great abundance, and because its
high gyromagnetic ratio gives a strong signal. However, any nucleus with a net
nuclear spin could potentially be imaged with MRI. Such nuclei include helium-3, lithium-7, carbon-13, fluorine-19, oxygen-17, sodium-23, phosphorus-31 and xenon-129. 23Na and 31P are
naturally abundant in the body, so can be imaged directly. Gaseous isotopes
such as 3He or 129Xe must be hyperpolarized and then inhaled as their nuclear density is too
low to yield a useful signal under normal conditions. 17O and 19F can be administered in
sufficient quantities in liquid form (e.g. 17O-water) that hyperpolarization is not a necessity.
MOLECULAR IMAGING BY MRI
MRI has the advantages of having very high spatial
resolution and is very adept at morphological imaging and functional imaging.
MRI does have several disadvantages though. First, MRI has a sensitivity of
around 10−3 mol/L to 10−5 mol/L which, compared to
other types of imaging, can be very limiting. This problem stems from the fact
that the population difference between the nuclear spin states is very small at
room temperature. For example, at 1.5 teslas, a typical field strength for clinical MRI, the
difference between high and low energy states is approximately 9 molecules per
2 million. Improvements to increase MR sensitivity include increasing magnetic
field strength, andhyperpolarization via optical pumping or dynamic nuclear
polarization. There are also a variety of signal amplification schemes based on
chemical exchange that increase sensitivity.
BIBLIOGRAPHY
Hollingworth W; Todd CJ; Bell MI; Arafat Q; Girling S; Karia
KR; Dixon AK (2000). "The diagnostic and therapeutic impact of MRI: an
observational multi-centre study". Clin Radiol 55 (11): 825–31.doi:10.1053/crad.2000.0546. PMID 11069736.
Wang PI; Chong ST; Kielar AZ; Kelly AM; Knoepp UD; Mazza
MB; Goodsitt MM (2012). "Imaging of pregnant and lactating patients: part
1, evidence-based review and recommendations". AJR Am J
Roentgenol 198 (4):
778–84. doi:10.2214/AJR.11.7405.PMID 22451541.
Smith-Bindman R; Miglioretti DL; Johnson E; Lee C;
Feigelson HS; Flynn M; Greenlee RT; Kruger RL; Hornbrook MC; et al.
(2012). "Use of
diagnostic imaging studies and associated radiation exposure for patients
enrolled in large integrated health care systems, 1996-2010". JAMA 307 (22):
2400–9. doi:10.1001/jama.2012.5960.PMC 3859870. PMID 22692172.
Nolen-Hoeksema, Susan (2014). Abnormal
Psychology (Sixth ed.). New York, NY: McGraw-Hill Education. p. 67.
Saleh, H; Kassas, B (2015). "Developing Stereotactic
Frames for Cranial Treatment". In Benedict, SH; Schlesinger, DJ; Goetsch,
SJ; Kavanagh, BD. Stereotactic Radiosurgery and Stereotactic Body
Radiation Therapy. Boca Raton: CRC Press. pp. 156–159.
Khan, FR; Henderson, JM (2013). "Deep Brain
Stimulation Surgical Techniques". In Lozano, AM; Hallet, M. Brain
Stimulation: Handbook of Clinical Neurology 116. Amsterdam: Elsevier. pp. 28–30.
Arle, J (2009). "Development of a Classic: the
Todd-Wells Apparatus, the BRW, and the CRW Stereotactic Frames". In
Lozano, AM; Gildenberg, PL; Tasker, RR. Textbook of Stereotactic and
Functional Neurosurgery. Berlin: Springer-Verlag. pp. 456–461.
Sharan, AD; Andrews, DW (2003). "Stereotactic
Frames: Technical Considerations". In Schulder, M; Gandhi, CD. Handbook
of Stereotactic and Functional Neurosurgery. New York: Marcel Dekker.
pp. 16–17.
Apuzzo,
MLJ; Fredericks, CA (1988). "The Brown-Roberts-Wells System". In
Lunsford, LD. Modern Stereotactic Neurosurgery. Boston: Martinus Nijhoff
Publishing. pp. 63–77.
