MAGNETIC RESONANCE IMAGING

Magnetic resonance imaging

Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualize internal structures of the body in detail. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body.
MRI can create more detailed images of the human body than possible with X-rays.

An MRI scanner is a device in which the patient lies within a large, powerful magnet where the magnetic field is used to align the magnetization of some atomic nuclei in the body, and radio frequency magnetic fields are applied to systematically alter the alignment of this magnetization.
This causes the nuclei to produce a rotating magnetic field detectable by the scanner—and this information is recorded to construct an image of the scanned area of the body.
Magnetic field gradients cause nuclei at different locations to precess at different speeds, which allows spatial information to be recovered using Fourier analysis of the measured signal. By using gradients in different directions, 2D images or 3D volumes can be obtained in any arbitrary orientation.
MRI provides good contrast between the different soft tissues of the body, which makes it especially useful in imaging the brain, muscles, the heart, and cancers compared with other medical imaging techniques such as computed tomography (CT) or X-rays. Unlike CT scans or traditional X-rays, MRI does not

use ionizing radiation.

[size=34]Magnetic Resonance Imaging

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المزيد عن MRI
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NMRI or MRI ?

Magnetic resonance imaging (MRI) is an imaging technique used primarily in medical settings to produce high quality images of the inside of the human body. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The technique was called magnetic resonance imaging rather than nuclear magnetic resonance imaging (NMRI) because of the negative connotations associated with the word nuclear in the late 1970's.

MRI started out as a tomographic imaging technique, that is it produced an image of the NMR signal in a thin slice through the human ****. MRI has advanced beyond a tomographic imaging technique to a volume imaging technique. This package presents a comprehensive picture of the basic principles of MRI.

Before beginning a study of the science of MRI, it will be helpful to reflect on the brief history of MRI. Felix Bloch and Edward Purcell, both of whom were awarded the Nobel Prize in 1952, discovered the magnetic resonance phenomenon independently in 1946. In the period between 1950 and 1970, NMR was developed and used for chemical and physical molecular analysis.

In 1971 Raymond Damadian showed that the nuclear magnetic relaxation times of tissues and tumors differed, thus motivating scientists to consider magnetic resonance for the detection of disease. In 1973 the x-ray-****d computerized tomography (CT) was introduced by Hounsfield.

This date is important to the MRI timeline because it showed hospitals were willing to spend large amounts of money for medical imaging hardware. Magnetic resonance imaging was first demonstrated on small test tube samples that same year by Paul Lauterbur. He used a back projection technique similar to that used in CT.

In 1975 Richard Ernst proposed magnetic resonance imaging using phase and frequency encoding, and the Fourier Transform. This technique is the basis of current MRI techniques. A few years later, in 1977, Raymond Damadian demonstrated MRI called field-focusing nuclear magnetic resonance. In this same year, Peter Mansfield developed the echo-planar imaging (EPI) technique. This technique will be developed in later years to produce images at video rates (30 ms / image).

Edelstein and coworkers demonstrated imaging of the **** using Ernst's technique in 1980. A single image could be acquired in approximately five minutes by this technique. By 1986, the imaging time was reduced to about five seconds, without sacrificing too much image quality.

The same year people were developing the NMR microscope, which allowed approximately 10 µm resolution on approximately one cm samples. In 1987 echo-planar imaging was used to perform real-time movie imaging of a single cardiac cycle. In this same year Charles Dumoulin was perfecting magnetic resonance angiography (MRA), which allowed imaging of flowing blood without the use of contrast agents.

In 1991, Richard Ernst was rewarded for his achievements in pulsed Fourier Transform NMR and MRI with the Nobel Prize in Chemistry. In 1992 functional MRI (fMRI) was developed. This technique allows the mapping of the function of the various regions of the human brain. Five years earlier many clinicians thought echo-planar imaging's primary applications was to be in real-time cardiac imaging.

The development of fMRI opened up a new application for EPI in mapping the regions of the brain responsible for thought and motor control. In 1994, researchers at the State University of New York at Stony Brook and Princeton University demonstrated the imaging of hyperpolarized 129Xe gas for respiration studies

[size=34]MR Terminology

TR = time interval between the 90 degree RF pulse & subsequent RF pulse applied to same s

TE = time interval between the 90 degree RF pulse & first echo

presaturation = RF pulses used to saturate out hydrogen nuclei before excitation

Acquisition time = affected due to change in TR

TI = time between the initial 180 & 90 degree pulse in an IR

180degree RF pulse = rephases the dephasing hydrogen nuclei in a SE sequence

Echoplanar imaging = fill all of K space in one repletion using very long echo trains

magnetization transfer = used to suppress background tissue enhancing certain disease processes

dynamic imaging = rapid acquisition of images after contrast enhancement

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How MRI works

MRI machines make use of the fact that body tissue contains lots of water, and hence protons (1H nuclei), which get aligned in a large magnetic field. Each water molecule has two hydrogen nuclei or protons. When a person is inside the powerful magnetic field of the scanner, the average magnetic moment of many protons becomes aligned with the direction of the field.

A radio frequency current is briefly turned on, producing a varying electromagnetic field.
This electromagnetic field has just the right frequency, known as the resonance frequency, to be absorbed and flip the spin of the protons in the magnetic field. After the electromagnetic field is turned off, the spins of the protons return to thermodynamic equilibrium and the bulk magnetization becomes re-aligned with the static magnetic field. During this relaxation, a radio frequency signal (electromagnetic radiation in the RF range) is generated, which can be measured with receiver coils.

Information about the origin of the signal in 3D space can be learned by applying additional magnetic fields during the scan. These additional magnetic fields can be used to generate detectable signal only from specific locations in the body (spatial excitation) and/or to make magnetization at different spatial locations precess at different frequencies, which enables k-space encoding of spatial information.

The 3D images obtained in MRI can be rotated along arbitrary orientations and manipulated by the doctor to be better able to detect tiny changes of structures within the body. These fields, generated by passing electric currents through gradient coils, make the magnetic field strength vary depending on the position within the magnet.

Because this makes the frequency of the released radio signal also dependent on its origin in a predictable manner, the distribution of protons in the body can be mathematically recovered from the signal, typically by the use of the inverse Fourier transform.

Protons in different tissues return to their equilibrium state at different relaxation rates.
Different tissue variables, including spin density, T1 and T2 relaxation times, and flow and spectral shifts can be used to construct images. By changing the settings on the scanner, this effect is used to create contrast between different types of body tissue or between other properties, as in fMRI and diffusion MRI.
MRI is used to image every part of the body, and is particularly useful for tissues with many hydrogen nuclei and little density contrast, such as the brain, muscle, connective tissue and most tumors.

Magnetic field

MRI scans require a magnetic field with two properties, uniform field density and strength. The magnetic field cannot vary more than 1/10,000 of 1% and field strength ranges (depending on the scanner) from 0.2 to 3 teslas in strength in currently clinically used scanners, with research scanners investigating higher field strengths such as seven teslas.

The lower field strengths can be achieved with permanent magnets, which are often used in "open" MRI scanners, for claustrophobic patients. Higher field strengths can be achieved only with superconducting magnets.
An MRI with a 3.0 tesla strength magnet may be referred to as a "3-T MRI" or "3-tesla MRI"
Since the gradient coils are within the bore of the scanner, there are large forces between them and the main field coils, producing most of the noise that is heard during operation. Without efforts to damp this noise, it can approach 130 decibels (dB) with strong fields (see also the subsection on acoustic noise).

Applications

In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor) from normal tissue.
One advantage of an MRI scan is that it is harmless to the patient. It uses strong magnetic fields and non-ionizing electromagnetic fields in the radio frequency range, unlike CT scans and traditional X-rays, which both use ionizing radiation.

While CT provides good spatial resolution (the ability to distinguish two separate structures an arbitrarily small distance from each other), MRI provides comparable resolution with far better contrast resolution (the ability to distinguish the differences between two arbitrarily similar but not identical tissues).

The basis of this ability is the complex library of pulse sequences that the modern medical MRI scanner includes, each of which is optimized to provide image contrast based on the chemical sensitivity of MRI.

The typical MRI examination consists of 5–20 sequences, each of which is chosen to provide a particular type of information about the subject tissues. This information is then synthesized by the interpreting physician
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Functional magnetic resonance imaging
or functional MRI (fMRI
is an MRI procedure that measures brain activity by detecting associated changes in blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region also increases.

The primary form of fMRI uses the blood-oxygen-level-dependent (BOLD) contrast, discovered by Seiji Ogawa. This is a type of specialized brain and body scan used to map neural activity in the brain or spinal cord of humans or other animals by imaging the change in blood flow (hemodynamic response) related to energy use by brain cells.

Since the early 1990s, fMRI has come to dominate brain mapping research because it does not require people to undergo shots, surgery, or to ingest substances, or be exposed to radiation.
Another method of obtaining contrast is arterial spin labeling.

The procedure is similar to MRI but uses the change in magnetization between oxygen-rich and oxygen-poor blood as its basic measure. This measure is frequently corrupted by noise from various sources and hence statistical procedures are used to extract the underlying signal.

The resulting brain activation can be presented graphically by color-coding the strength of activation across the brain or the specific region studied. The technique can localize activity to within millimeters but, using standard techniques, no better than within a window of a few seconds.

FMRI is used both in the research world, and to a lesser extent, in the clinical world. It can also be combined and complemented with other measures of brain physiology such as EEG and NIRS. Newer methods which improve both spatial and time resolution are being researched, and these largely use biomarkers other than the BOLD signal.

Some companies have developed commercial products such as lie detectors based on fMRI techniques, but the research is not believed to be ripe enough for widespread commercialization.

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How MRI Works

Dr. Raymond Damadian, a physician and scientist, toiled for years trying to produce a machine that could noninvasively scan the **** with the use of magnets. Along with some graduate students, he constructed a superconducting magnet and fashioned a coil of antenna wires.

Since no one wanted to be the first one in this contraption, Damadian volunteered to be the first patient.
When he climbed in, however, nothing happened. Damadian was looking at years wasted on a failed invention, but one of his colleagues bravely suggested that he might be too big for the machine.

A svelte graduate student volunteered to give it a try, and on July 3, 1977, the first MRI exam was performed on a human being. It took almost five hours to produce one image, and that original machine, ****d the "Indomitable," is now owned by the Smithsonian Institution.

In just a few decades, the use of magnetic resonance imaging (MRI) scanners has grown tremendously.
Doctors may order MRI scans to help diagnose multiple sclerosis, brain tumors, torn ligaments, tendonitis, cancer and strokes, to **** just a few. An MRI scan is the best way to see inside the human **** without cutting it open

.That may be little comfort to you when you're getting ready for an MRI exam. You're stripped of your jewelry and credit cards and asked detailed questions about all the ****llic instruments you might have inside of you. You're put on a tiny slab and pushed into a hole that hardly seems large enough for a person. You're subjected to loud noises, and you have to lie perfectly still, or they're going to do this to you all over again.

And with each minute, you can't help but wonder what's happening to your **** while it's in this machine. Could it really be that this ordeal is truly better than another imaging technique, such as an X-ray or a CAT scan? What has Raymond Damadian wrought?

Let the magnets of this mighty machine draw you to the next page, and we'll take a look at what's going on inside.
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MRI Magnets: the Major Players

HowStuffWorks.com

The components of an MRI system

MRI scanners vary in size and shape, and some newer models have a greater degree of openness around the sides. Still, the basic design is the same, and the patient is pushed into a tube that's only about 24 inches (60 centimeters) in diameter [source: Hornak].

But what's in there?
The biggest and most important component of an MRI system is the magnet. There is a horizontal tube -- the same one the patient enters -- running through the magnet from front to back. This tube is known as the bore. But this isn't just any magnet -- we're dealing with an incredibly strong system here, one capable of producing a large, stable magnetic field.

The strength of a magnet in an MRI system is rated using a unit of measure known as a tesla. Another unit of measure commonly used with magnets is the gauss (1 tesla = 10,000 gauss). The magnets in use today in MRI systems create a magnetic field of 0.5-tesla to 2.0-tesla, or 5,000 to 20,000 gauss.

When you realize that the Earth's magnetic field measures 0.5 gauss, you can see how powerful these magnets are.
Most MRI systems use a superconducting magnet, which consists of many coils or windings of wire through which a current of electricity is passed, creating a magnetic field of up to 2.0 tesla. Maintaining such a large magnetic field requires a good deal of energy, which is accomplished by superconductivity, or reducing the resistance in the wires to almost zero.

To do this, the wires are continually bathed in liquid helium at 452.4 degrees below zero Fahrenheit (269.1 below zero degrees Celsius) [source: Coyne]. This cold is insulated by a vacuum. While superconductive magnets are expensive, the strong magnetic field allows for the highest-quality imaging, and superconductivity keeps the system economical to operate.
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The Other Parts of an MRI Machine

MRI Developments

MRI machines are evolving so that they're more patient-friendly. For example, many claustrophobic people simply can't stand the cramped confines, and the bore may not accommodate obese people. There are more open scanners, which allow for greater space, but these machines have weaker magnetic fields, meaning it may be easier to miss abnormal tissue. Very small scanners for imaging specific body parts are also being developed.

Other advancements are being made in the field of MRI. Functional MRI (fMRI), for example, creates brain maps of nerve cell activity second by second and is helping researchers better understand how the brain works. Magnetic
resonance angiography (MRA) creates images of flowing blood, arteries and veins in virtually any part of the body

Two other magnets are used in MRI systems to a much lesser extent. Resistive magnets are structurally like superconducting magnets, but they lack the liquid helium. This difference means they require a huge amount of electricity, making it prohibitively expensive to operate above a 0.3 tesla level. Permanent magnets have a constant magnetic field, but they're so heavy that it would be difficult to construct one that could sustain a large magnetic field.

There are also three gradient magnets inside the MRI machine. These magnets are much lower strength compared to the main magnetic field; they may range in strength from 180 gauss to 270 gauss. While the main magnet creates an intense, stable magnetic field around the patient, the gradient magnets create a variable field, which allows different parts of the body to be scanned.

Another part of the MRI system is a set of coils that transmit radiofrequency waves into the patient's body. There are different coils for different parts of the body: knees, shoulders, wrists, heads, necks and so on.

These coils usually conform to the contour of the body part being imaged, or at least reside very close to it during the exam. Other parts of the machine include a very powerful computer system and a patient table, which slides the patient into the bore. Whether the patient goes in head or feet first is determined by what part of the body needs examining. Once the body part to be scanned is in the exact center, or isocenter, of the magnetic field, the scan can begin.
What goes on during a scan? Find out on the next page.
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Hydrogen Atoms and Magnetic Moments

The steps of an MRI

When patients slide into an MRI machine, they take with them the billions of atoms that make up the human body. For the purposes of an MRI scan, we're only concerned with the hydrogen atom, which is abundant since the body is mostly made up of water and fat. These atoms are randomly spinning, or precessing, on their axis, like a child's top. All of the atoms are going in various directions, but when placed in a magnetic field, the atoms line up in the direction of the field.

These hydrogen atoms have a strong magnetic moment, which means that in a magnetic field, they line up in the direction of the field. Since the magnetic field runs straight down the center of the machine, the hydrogen protons line up so that they're pointing to either the patient's feet or the head.

About half go each way, so that the vast majority of the protons cancel each other out -- that is, for each atom lined up toward the feet, one is lined up toward the head. Only a couple of protons out of every million aren't canceled out. This doesn't sound like much, but the sheer number of hydrogen atoms in the body is enough to create extremely detailed images. It's these unmatched atoms that we're concerned with now

What Else Is Going on in an MRI Scan?

Next, the MRI machine applies a radio frequency (RF) pulse that is specific only to hydrogen. The system directs the pulse toward the area of the body we want to examine. When the pulse is applied, the unmatched protons absorb the energy and spin again in a different direction. This is the "resonance" part of MRI. The RF pulse forces them to spin at a particular frequency, in a particular direction. The specific frequency of resonance is called the Larmour frequency and is calculated ****d on the particular tissue being imaged and the strength of the main magnetic field.

At approximately the same time, the three gradient magnets jump into the act. They are arranged in such a manner inside the main magnet that when they're turned on and off rapidly in a specific manner, they alter the main magnetic field on a local level. What this means is that we can pick exactly which area we want a picture of; this area is referred to as the "slice." Think of a loaf of bread with slices as thin as a few millimeters -- the slices in MRI are that precise.

Slices can be taken of any part of the body in any direction, giving doctors a huge advantage over any other imaging modality. That also means that you don't have to move for the machine to get an image from a different direction -- the machine can manipulate everything with the gradient magnets.

But the machine makes a tremendous amount of noise during a scan, which sounds like a continual rapid hammering. That's due to the rising electrical current in the wires of the gradient magnets being opposed by the main magnetic field. The stronger the main field, the louder the gradient noise. In most MRI centers, you can bring a music player to drown out the racket, and patients are given earplugs.

When the RF pulse is turned off, the hydrogen protons slowly return to their natural alignment within the magnetic field and release the energy absorbed from the RF pulses. When they do this, they give off a signal that the coils pick up and send to the computer system. But how is this signal converted into a picture that means anything?
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MRI Images and How They're Made

Ron Levine/The Image Bank/Getty Images

Doctors examine the contrasts on an MRI scan.

The MRI scanner can pick out a very small point inside the patient's **** and ask it, essentially, "What type of tissue are you?" The system goes through the patient's **** point by point, building up a map of tissue types.

It then integrates all of this information to create 2-D images or 3-D models with a mathematical formula known as the Fourier transform. The computer receives the signal from the spinning protons as mathematical data; the data is converted into a picture. That’s the "imaging" part of MRI.

The MRI system uses injectable contrast, or dyes, to alter the local magnetic field in the tissue being examined. Normal and abnormal tissue respond differently to this slight alteration, giving us differing signals.

These signals are transferred to the images; an MRI system can display more 250 shades of gray to depict the varying tissue [source: Coyne]. The images allow doctors to visualize different types of tissue abnormalities better than they could without the contrast. We know that when we do "A," normal tissue will look like "B" -- if it doesn't, there might be an abnormality.

An X-ray is very effective for showing doctors a broken bone, but if they want a look at a patient's soft tissue, including organs, ligaments and the circulatory system, then they'll likely want an MRI. And, as we mentioned on the last page, another major advantage of MRI is its ability to image in any plane.

Computer tomography (CT), for example, is limited to one plane, the axial plane (in the loaf-of-bread analogy, the axial plane would be how a loaf of bread is normally sliced). An MRI system can create axial images as well as sagitall (slicing the bread side-to-side lengthwise) and coronal (think of the layers in a layer cake) images, or any degree in between, without the patient ever moving.

But for these high-quality images, the patient can't move very much at all. MRI scans require patients to hold still for 20 to 90 minutes or more. Even very slight movement of the part being scanned can cause distorted images that will have to be repeated. And there's a high cost to this kind of quality; MRI systems are very expensive to purchase, and therefore the exams are also very expensive.

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[size=34]MR Imaging Procedures
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1. Which anatomical plane is oriented 90 degrees to midline and divides the body into front and back halves?
Your Answer:
Correct Answer: Coronal

2. All of the following techniques should be employed EXCEPT _______ when imaging the pituitary gland.
Your Answer:
Correct Answer: Fat saturation

3. To visualize the internal auditory canal, high resolution images are acquired in which planes?
Your Answer:
Correct Answer: Axial & coronal

4. Which pulse sequence is most useful in identifying hemorrhage?
Your Answer:
Correct Answer: Gradient echo

5. Lymph nodes are frequently ________ and vessels are _______ on T2-weighted images of the neck
Your Answer:
Correct Answer: isointense, dark

6. ______ images best demonstrate disk degeneration or dehydration.
Your Answer:
Correct Answer: Sagittal T2

7. The spinal region LEAST affected by CSF pulsation is the _______ region.
Your Answer:
Correct Answer: Lumbar

8. The CSF in the spinal cord is _______ on T1-weighted images and _______ on T2-weighted images.
Your Answer:
Correct Answer: Dark, bright

9. It is difficult to define gray/white matter structures in the pediatric population so _____pulse sequences should be utilized because of the low development of myelin.
Your Answer:
Correct Answer: Inversion recovery

10. Name the condition where part of the cerebellar tonsil is displaced below the foramen magnum.
Your Answer:
Correct Answer: Chiari malformation
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[size=34]MR Physics

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1. The central area of the magnetic field in MR imagers is known as __________.
Correct Answer: isocenter

2. 1T = __________.
Correct Answer: 10,000 Gauss

3. The precessional frequency for hydrogen can be calculated from __________.
Correct Answer: Larmor Equation

4. The term "slew rate" refers to the __________ of the gradient magnetic field.
Correct Answer: strength/speed of the slope

5. In order to achieve "resonance," the transmitted RF pulse must match the __________.
Correct Answer: precessional frequency

6. Following excitation by the RF pulse, the spins relax back toward __________.
Correct Answer: equilibrium

7. The main purpose of the static magnetic field (known as the BO field) is to __________.
Correct Answer: magnetize the tissue

8. After RF excitation, the spins _________.
Correct Answer: relax

9. Most superconductive magnets are solenoids and, thus, exhibit a(n) ______magnetic field.
Correct Answer: horizontal

10. The signal induced in a receiver coil immediately following an RF excitation pulse is known as __________.
Correct Answer: FID
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[size=34]MR Pulse Sequences

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1. In general, inversion sequences begin with a 180 degree RF pulse to invert Mz. After the inversion, there is an operator-specified delay time known as the ____.
Your Answer: flip angle
Correct Answer: TI

2. FLAIR sequences are generally used to suppress the signal from ____.
Your Answer:
Correct Answer: water

3. On T2-weighted images, the cerebral spinal fluid is ___________.
Your Answer:
Correct Answer: white

4. On a T1-weighted image in the brain, CSF will appear ___________.
Your Answer:
Correct Answer: black

5. Which MR imaging sequence produces T2-weighted or T1-weighted images with a fat signal similar to spin echo, but the fat is brighter?
Your Answer:
Correct Answer: Turbo Spin Echo

6. The _______ is produced when a region of pathology may be missed in the final image if too thick a slice is used, since it is averaged together with several millimeters of other tissue.
Your Answer:
Correct Answer: Partial volume effect

7. Fat suppression techniques like SPIR (Spectral Presaturation with Inversion Recovery) and SPAIR (Spectral Attentuated Inversion Recovery) utilize the difference in resonance frequencies of water and fat. That difference is about ______ ppm.
Your Answer:
Correct Answer: 3.4

8. The T1 time of water at 1.5T is approximately ____.
Your Answer:
Correct Answer: 2000ms

9. The T1 time of fat at 1.5T is approximately ____.
Your Answer:
Correct Answer: 150ms

10. In a gradient echo pulse sequence, as the flip angle is increased (will all other factors remaining the same), the resultant image will yield more __________.
Your Answer:
Correct Answer: T1-weighting[/size]
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[size=34]MR Angiography

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