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

---

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.