FAT AND WATER ON MRI

Water, organ parenchyma (tissues of the hydrous type) and fat are the main constituents of the organism. Their molecules contain many hydrogen atoms whose nucleus is useful for the NMR signal. In a multitude of situations it is important to differentiate between the signal coming from water and organ parenchyma, and that of fat tissue. MRI can provide an excellent contrast between soft tissues by exploiting the particular characteristics of hydrogen atoms, according to whether they are bound to water or to lipid molecules.

Fat signal

As fat tissues have a short relaxation time T1 they appear as a hypersignal in T1-weighted sequences.
The relaxation time T2 of fat is also short, but the fat still appears as a relative high signal intensity in multi-echoes T2-weighted sequences (TSE, FSE).
This fat high signal intensity can hide enhancement after Gadolinium injection in T1-weighted sequences, or an edematous hypersignal in fatty organs in T2-weighted sequences. It may be difficult to distinguish fat from other tissues with high T1 and T2 signal intensities (blood degradation products in a hematoma for example).
Furthermore fat is responsible for chemical shift artifacts and is also clearly visible in motion artifacts.

Fat signal suppression

There are two families of techniques to reduce or even suppress the signal from fat tissue, whatever the signal weighting:

• those based on the particular T1 of fat: inversion-recovery with short inversion time (STIR)
• those based on the differences in hydrogen resonance frequency in fat molecules compared to hydrogen resonance frequency in water and other soft tissues: fat saturation and selective excitation of water.

There are many applications of fat suppression methods: Identification of fat tissue, differentiation from blood clots, edema detection, enhancement after Gadolinium injection, reduction of chemical shift artifacts, MR spectroscopy, background suppression in MR angiography…

MR ANATOMY OF KNEE JOINT

SONG OF PROTON

THE SONG OF PROTON

MRI BASICS

MAGNETIC NUCLEI

These were discovered in the 1940's by Purcell and Bloch, who received a Nobel prize for this discovery in 1953. Any nucleus with an odd number of protons and neutrons will have an intrinsic spin. This will induce a very small magnetic field around the nucleus which then behaves like a tiny magnet. A large number of nuclei including hydrogen, phosphorus, carbon 13, and sodium have this property. Of all these elements hydrogen is by far the most abundant in the human body. It is for this reason that hydrogen mapping is being used for NMR imaging at the present time.
THE MR IMAGE IS A "MAP" OF HYDROGEN ATOMS IN THE BODY.

GENERAL PRINCIPLES

Hydrogen atoms may be considered as tiny magnetic needles. If the body is placed in a very strong uniform magnetic field all the tiny hydrogen magnets will tend to align themselves along the axis of the magnetic field. They rotate (or wobble) around this axis at a fixed frequency (for any magnetic field strength). They behave much like a compass needle held on the earth's surface. If the aligned hydrogen protons are bombarded with radiowaves of the same frequency as the wobble frequency then they will absorb energy and flip to a higher energy level. When the radiowaves are turned off, the hydrogen protons will return to their resting state and release energy. This released energy is also in the form of radiowaves. It is detected, amplified, and then used to generate images.
THE HYDROGEN ATOMS ARE LOCATED BY DETECTING RADIO FREQUENCY ENERGY RELEASED BY THEM.

T1 AND T2 RELAXATION TIMES

T1

T1 is the time taken for protons which have been flipped to return to their original state.

T2

This is the transverse relaxation time. After a 90 degree flip the net magnetic field resonates about the transverse plane and generates a current (the MR signal). The protons which are originally in phase, slowly go out of phase and eventually neutralize out each other. The current stops. The T2 relaxation time is the time taken for this to happen.

Notes on T1 and T2

1. T1 is always greater than T2. For pure water they are of similar values approximately 3 seconds. In pure solids T1 will increase dramatically and T2 will shrink to fractions of a second. For normal body tissued T1 relaxation times are in the range of 300 - 1000 msec and T2 relaxation times are between 30 - 90 msec.
2. RECOVERY TO THE RESTING STATE IS SEPARATED INTO T1 AND T2 COMPONENTS.

IMAGES

MR images are slices of the body. They can be obtained in any plane and in almost any thickness. we usually use 5-10 mm slice thickness.
In x-rays the final image depends basically on one parameter which is attenuation of an x-ray beam by tissues. Different tissues attenuate the x-ray beam differently thus enabling an image to be formed. On an x-ray the order of the gray scale is fixed, with bone appearing white, soft tissue gray, fat a darker gray and air black.
Unlike x-ray imaging, the MR image depends not on one but on four separate parameters. This makes life particularly complicated. The NMR image depends on:

1. the density of hydrogen atoms
   
2. the T-1 relaxation time
   
3. the T-2 relaxation time
   
4. blood flow.

In clinical imaging we generate two separate sets of images, one dependent on T1 relaxation and the other on T2 relaxation.
The signal intensity on T1 weighted images depends on:

T1 relaxation times H2 concentration blood flow.

The signal intensity on T2 weighted images depends on:

T2 relaxation times H2 concentration blood flow.

SIGNAL INTENSITY

We get a strong signal from tissues with:

short T1 relaxation time long T2 relaxation time high H2 concentration.

We get a weak signal from tissues with:

long T1 relaxation time short T2 relaxation time low concentration of H2 flow. and from flowing blood.

	Length of T1 Relaxation Time	Signal Strength on T1 Images	Length of T2 Relaxation Time	Signal Strength on T2 Images
Fat Soft tissues Most acute patholog Fluid Bone Cortex	+ +++ ++++ +++++ ++++++	+++++ +++ ++ + 0	+++++ +++ +++++ +++++ +	+++++ +++ +++++ +++++ 0

MR IMAGES DEPEND ON MULTIPLE TISSUE CHARACTERISTICS

RADIOFREQUENCY PULSES

The radiowaves sent into the body are short pulses of very precise strength and frequency. It is by changing the strength, frequency and timing of the radiowave pulses that we produce T1 or T2 weighted images.
RADIOWAVE STRENGTH AND DURATION DETERMINES T1 OR T2 WEIGHTING OF THE IMAGE.

BASIC DESIGN OF AN MR MACHINE

The magnets and radiowave coils are arranged in layers in a large circle. The patient lies within this circle. The outer-most structure is the large magnet which produces a very strong uniform magnetic field. The strength of this magnet is kept constant during operation. Within this magnet are arranged a series of smaller magnets. These are the gradient coils. Their strengths are changed repeatedly during scanning. They are able to generate small gradient magnetic fields in any plane. The reason for these gradients is explained later. Lying closer to the center of the circle are the radiofrequency coils. They generate and receive radiowaves.

ADVANTAGES AND DISADVANTAGES OF MR

Advantages

1. No radiation.
2. Noninvasive.
3. No patient positioning or movement.
4. No steak artifacts.
5. High soft tissue contrast differentiation.
6. No moving parts.
7. Imaging construction in any plane.
8. Gives an anatomical image as well as additional information about the physical and chemical properties of tissues being evaluated.

Disadvantages

1. One cannot identify calcium well.
2. Very expensive ($500->1000 per study).
3. Slow: 30 to 60 minutes per patient.

PULSE SEQUENCE

This is a series of pulsed radiowaves beamed into the patient. The intensity, frequency and timing of the pulses can all be varied. We mainly vary TE (time to echo) and TR (pulse repitition time).

	Short	Long
TETR	20 - 40 msec300-700 msec	60 - 120 msec1000-2500 msec

	TE	TR
T1 weighted image hasT2 weighted image had H concentration weighted image has	shortlong short	shortlong long

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MAGNETIC RESONANCE IMAGING QUESTIONS FOR SOPHOMORE TEST

The following questions should be answered as true or false.

T/F
1. Because the frequency of wobble (precession) of hydrogen protons is dependent only on the magnetic field strength, we can obtain spatial information by superimposing small gradient magnetic fields onto the uniform main magnetic field. T/F
2. In magnetic resonance the order of whiteness or blackness of different tissues (eg. bone, muscle, air) on the gray scale is fixed like in CT and conventional radiography. T/F
3. T1 relaxation time means that the magnet is rested at 1:00 PM each day whilst the staff drink tea. T/F
4. Hydrogen, sodium, or phosphorus could all be utilized to create MR images. Hydrogen is routinely utilized because of its abundance in the body. T/F
5. There are many advantages of MR. Two of these are that MR is extremely safe, and that MR images can be obtained in any body plane. T/F
6. An x-ray image depends only on one parameter, which is the differential attenuation of x-rays by different tissues. MR images depend on multiple different factors, including hydrogen concentration, T1 and T2 relaxation time and blood flow. T/F
7. On MR imaging strong signals are obtained from tissues with short T1 and long T2 relaxation times. T/F
8. The T1 and T2 relaxation times of hydrogen are the same in different body tissues. T/F
9. The MR image is formed by radio frequency energy that is released from hydrogen atoms.

MENISCEAL TEAR

There are two menisci in your knee; each rests between the thigh bone (femur) and shin bone (tibia). The menisci are made of tough cartilage and conform to the surfaces of the bones upon which they rest. One meniscus is on the inside of your knee; this is the medial meniscus. The other meniscus rests on the outside of your knee, the lateral meniscus. What does the meniscus do?
These meniscus functions to distribute your body weight across the knee joint. Without the meniscus present, the weight of your body would be unevenly applied to the bones in your legs (the femur and tibia). This uneven weight distribution would cause excessive forces in specific areas of bone leading to early arthritis of knee joint. Therefore, the function of the meniscus is critical to the health of your knee.
The meniscus is C-shaped and has a wedged profile. The wedged profile helps maintain the stability of the joint by keeping the rounded femur surface from sliding off the flat tibial surface. The meniscus is nourished by small blood vessels, but the meniscus also has a large area in the center of that has no direct blood supply (avascular). This presents a problem when there is an injury to the meniscus as the avascular areas tend not to heal. Without the essential nutrients supplied by blood vessels, healing cannot take place.

How does the meniscus work?

The knee joint is very important in allowing people to go about performing almost any activity. The joint is made up of three bones: the femur (thigh bone), the tibia (shin bone), and the patella (knee cap). The surfaces of these bones within the joint are covered with a layer of cartilage. This important surface allows the bones to smoothly glide against each other without causing damage to the bone. The meniscus sits between the cartilage surfaces of the bone to distribute weight and to improve the stability of the joint.

Meniscus Tear or Cartilage Tear?

Both the covering of the bone within the joint and the meniscus are made of cartilage--this makes the issue a little confusing. People often say 'cartilage' to mean the meniscus (the wedges of cartilage between the bone) or to mean the joint surface (so-called articular cartilage which caps the ends of the bone). When people talk about a cartilage tear, they a talking about a meniscus tear. When people talk about arthritis and wear of cartilage, they are talking most often about the articular cartilage on the ends of the bone.
What happens with a meniscus tear (torn cartilage)?
The two most common causes of a meniscus tear are due to traumatic injury (often seen in athletes) and degenerative processes (seen in older patients who have more brittle cartilage). The most common mechanism of a traumatic meniscus tear occurs when the knee joint is bent and the knee is then twisted.
It is not uncommon for the meniscus tear to occur along with injuries to the anterior cruciate ligament (ACL) and the medial collateral ligament (MCL)-these three problems occurring together are known as the "unhappy triad," which is seen in sports such as football when the player is hit on the outside of the knee.

Symptoms of a Meniscus Tear?

Individuals who experience a meniscus tear usually experience pain and swelling as their primary symptoms. Another common complaint is joint locking, or the inability to completely straighten the joint. This is due to a piece of the torn cartilage physically impinging the joint mechanism of the knee. The most common symptoms of a meniscus tear are:

• Knee pain
• Swelling of the knee
• Tenderness when pressing on the meniscus
• Popping or clicking within the knee
• Limited motion of the knee joint

Diagnosis of a Meniscus Tear

Any patient who has knee pain will be evaluated for a possible meniscus tear. A careful history and physical examination can help differentiate patients who have a meniscus tear from patients with knee pain from other conditions. Specific tests can be performed by your doctor to detect meniscus tears. X-rays and MRIs are the two tests commonly used in patients who have meniscus tears. An x-ray can be used to determine if there is evidence of degenerative or arthritic changes to the knee joint. The MRI is helpful at actually visualizing the meniscus. However, simply 'seeing' a torn meniscus on MRI does not mean a specific treatment is needed. Treatment of meniscus tears depends on several factors, as not all meniscus tears require surgery.

Treatment of a Meniscus Tear

Treatment of a meniscus tear depends on several factors including the type of tear, the activity level of the patient, and the response to simple treatment measures. When surgical treatment of a meniscus tear is required, the usual treatment is to trim the torn portion of meniscus, a procedure called a meniscectomy. Meniscus repair and meniscal transplantation are also surgical treatment options. Determining the most appropriate meniscus tear treatment is something you can discuss with your doctor.

SLAP TEAR SHOULDER JOINT

he shoulder joint is considered a 'ball and socket' joint. However, in bony terms the 'socket' (the glenoid fossa of the scapula) is quite small, covering at most only a third of the 'ball' (the head of the humerus). The socket is somewhat deepened by a circumferential rim of fibrocartilage which is called the glenoidal labrum. Previously there was some argument as to the structure (it is fibrocartilaginous as opposed to the hyaline cartilage found in the remainder of the glenoid fossa) and function (it was considered a redundant evolutionary remnant, but is now considered integral to shoulder stability). Most authorities agree that the tendon of the long head of the biceps brachii muscle proximally becomes fibrocartilaginous prior to attaching to the superior aspect of the glenoid. Similarly the long head of the triceps brachii inserts inferiorly.^[1] Together these cartilaginous extensions of the tendon are termed the 'glenoid labrum'. A SLAP tear or lesion occurs when there is damage to the superior or uppermost area of the labrum. SLAP lesions have come into public awareness with their increasing frequency in overhead and particularly throwing athletes. The increased frequency relates to the relatively recent description of labral injuries in throwing athletes ^[2] and the initial definitions of the 4 SLAP sub-types^[3] all happening since the 1990s. The identification and treatment of these injuries continues to evolve, however it is safe to say that a baseball pitcher suffering a 'dead arm' caused by a SLAP lesion today is far more likely to recover such that he can return to the game at its highest level than was the case previously.

Sub-types

At least ten types of this injury are recognized with varying degrees of damage,^[4] seven of which are listed here

1. Degenerative fraying of the superior portion of the labrum, with the labrum remaining firmly attached to the glenoid rim
2. Separation of the superior portion of the glenoid labrum and tendon of the biceps brachii muscle from the glenoid rim
3. Bucket-handle tears of the superior portion of the labrum without involvement of the biceps brachii (long head) attachment
4. Bucket-handle tears of the superior portion of the labrum extending into the biceps tendon
5. Anteroinferior Bankart lesion that extends upward to include a separation of the biceps tendon
6. Unstable radial of flap tears associated with separation of the biceps anchor
7. Anterior extension of the SLAP lesion beneath the middle glenohumeral ligament

MRI OF SHOULDER IMPINGEMENT SYNDROME

 

Fig. 1 —20-year-old man with supraspinatus tendinosis. Oblique coronal fast spin-echo T2-weighted MR image shows supraspinatus tendon has increased signal near its insertion on greater tuberosity (arrow). Cystic changes are incidentally noted in humeral head.

Fig. 2 —20-year-old man with infraspinatus tendinosis. Oblique coronal fast spin-echo T2-weighted MR image shows infraspinatus tendon has increased signal near it insertion on greater tuberosity (arrow). Cystic changes are incidentally noted in humeral head near attachment of infraspinatus tendon.

Fig. 3 —17-year-old girl in abduction external rotation. Fat-suppressed proton density image shows infraspinatus being impinged by posterosuperior glenoid labrum (arrow).

Fig. 4 —20-year-old man with cystic changes in humeral head. Axial fast spin-echo T2-weighted MR image shows cystic changes in posterosuperior humeral head near attachment sites of supraspinatus and infraspinatus tendons (arrow).