17.2 Magnetic Resonance Imaging

Transmutation of the nucleus from one element to another is associated with radioactivity.

When radium emits an alpha particle, the nucleus is transformed into the element radon.
Most physics texts discuss the details of the process.

A random event is the decay or transmutation of a radioactive nucleus.

Some nuclei decay sooner than others.
The laws of probability can be used to predict the decay rate for the aggregate if we deal with a large number of radioactive nuclei.
The half-life is the time interval for half of the original nucleus to be transliterated.

The half-life of radioactive elements varies greatly.

Some decay very quickly and have a half-life of less than a few microseconds.

Others have a half-life of thousands of years.

The Earth's crust is home to the very long-lived radioactive elements.

The short-lived radioactive isotopes can be produced by bombarding certain stable elements with high-energy particles.

The nucleus of naturally occurring phosphorus has 15 protons and 16 neutrons.

A half-life of 14 days is what this radioactive phosphorus has.

Radioactive elements can be produced in a similar way.
In biological and clinical work, many of these isotopes have been very useful.

The shapes of internal organs can be seen with a computerized X-ray tomography.

Information about the internal structure of tissue is not provided by X-rays.
Changes in tissue structure and pathological alterations inside internal organs can be missed by CT scans.
This technique uses the magnetic properties of the nucleus to provide images of internal body organs with information about soft-tissue structure.

The techniques we have discussed so far are relatively easy to use.

They use reflected or transmitted energy to see internal structures.

The principles are relatively easy to explain, but a detailed description is beyond the scope of this text.

An introduction to the principles of nuclear magnetic resonance is what begins the discussion.

The quantum mechanical property of spin is found in the nucleus of atomic nuclei.

As if they were small spinning tops, we can imagine these particles.
Small bar magnets are created by the spin of the nuclear particles.
The small magnets associated with the nucleons line up inside the nucleus to cancel each other's magnetic fields.
The nucleus has a net magnetic moment if the number of nucleons is odd.
Tiny magnets are created by nuclei with an odd number of nucleons.
Hydrogen has a nucleus with a single protons and has a nuclear magnetic moment.
The human body is made of mostly water and hydrogen.
Magnetic resonance images of structures within the body can be produced using the magnetic properties of the hydrogen nucleus.
Nuclear magnetic properties of hydrogen will be the focus of our discussion.

Small arrows are represented by the nuclear magnets.

The situation is changed by an external magnetic field.

The parallel configuration has a lower energy.

The Mag netic fields can be found in the range 1 to 4 T.

The Larmor Frequency is given by Eq.

The population of the spin up and spin down states is equalized by a displacement of 90*.
An external source of energy is needed to reverse the alignment of antiparallel spins.

The magnetic moment is displaced from the direction of the external field by the use of a short radio Frequency driving pulse at the Larmor Frequency.

The magnetic moment from the external magnetic field is displaced by an angle determined by the strength and duration of the driving pulse.

The magnetic moment is displaced from the external magnetic field by an angle determined by the strength and duration of the driving pulse.

The displaced magnetic moment produced by the radio Frequency driving pulse, precesses around the external magnetic field and itself produces a radio Frequency signal at the Larmor Frequency of rotation.

The signal can be detected by a separate coil or the driving coil.

More of the nuclei are lined up parallel to the field in the presence of an external field.

The parallel spins are flipped into the antiparallel configuration by the radio Frequency pulse.
When the driving pulse ends, the nuclear spins and magnetic moment return to the original equilibrium alignment.
The exchange of energy between the nuclear spins and the surrounding molecule causes the equilibration.
The precession angle decreases with the return of the magnetic moment to the original alignment.
The spin lattice relaxation time is called.

The local magnetic field is not perfect.

The magnetic field is caused by the magnetic properties of the molecule next to the nuclear spins.

The Larmor Frequency of the individual nuclear magnetic moments differ slightly from each other because of the variations in the local magnetic field.

The total NMR signal decreases when the precessions of the nuclei get out of phase.
The time is called the spin-spin relaxation time.

Information about the material being studied is contained in the NMR signal detected after the driving pulse.

The magnitude of the emitted NMR signal is a function of the number of hydrogen nuclei in the material.
A relatively low NMR signal can be produced by bone, which contains relatively few water or other hydrogen-based molecules.
The post-pulse radiation is much higher.

Information about the nature of the material within which the precessing nuclei are located can be provided by the rate of decay of the emitted NMR signal.

An analogy can be provided by the spinning top.

A well-designed vacuum top will spin for a long time.

The duration of the spin in air will be shorter because of the air molecule interactions.
The top will spin less in water where the losses are greater.
Information about the nature of the medium surrounding the top is provided by the decay rate of the spinning top.

Information about the matter surrounding the precessing nuclei can be provided by 2.

2 57 msec is seen in [16-4].

Since the 1940s, the NMR principles have been used to iden tify molecule in various physics, chemistry, and biological applications.

The entire volume exposed to the magnetic field is used to derive the detected NMR signal.
Information about the location of the signal within the volume studied can't be provided by the technique.

In order to get a three-dimensional image using nuclear magnetic resonance, we need to identify the location of signals from small sections of the body and then build the image from these individual signals.

Tomographic spatial images can be obtained from intersection points of narrowly focused X-ray beams.
The long wavelength of the radio frequencies cannot be collimated into the narrow beams needed to examine small regions of interest.

In the 70s, several new techniques were developed to create two-dimensional tomographic images similar to those provided by computed tomographic scans.

P. C. Lauterbur described the first one in 1973.
The two tubes have the same Larmor frequencies.

The Larmor Frequency is now characterized by the axis.

The slice will be examined with direction to select within the body.

A lot of signals have to be collected to make an image.
Two of the signal are needed.
The process is more complex and requires more sophisticated computer programs.

It has been useful in neurology.

All parts of the brain can be seen inside.
Information about the functions performed by the brain is not provided by conventional magnetic resonance.

Post mortem studies of brain tumors and injuries were the main source of information about the brain's specific functions.

In 1861, a French physician, Paul Pierre Broca, determined that a patient had a left cerebral hemisphere abnormality and that this part of the brain controlled speech production.
The development of fMRI has made it possible to observe a wide range of neural functions.

The energy requirement of a specific region of the brain increases when that region is activated.

Oxygenated blood flow to that part of the brain increases in order to meet the increased energy requirements.