The graphics window displays a schematic representation of the major systems on a magnetic resonance imager and a few of the major interconnections. This overview briefly states the function of each component. Some will be described in detail later in this chapter.
At the top of the schematic representation you will find the components of the imager located in the scan room of a magnetic resonance imager. The magnet produces the Bo field for the imaging procedure. Within the magnet are the gradient coils for producing a gradient in Bo in the X, Y, and Z directions. Within the gradient coils is the RF coil. The RF coil produces the B1 magnetic field necessary to rotate the spins by 90o or 180o. The RF coil also detects the signal from the spins within the body. The patient is positioned within the magnet by a computer controlled patient table. The table has a positioning accuracy of 1 mm. The scan room is surrounded by an RF shield. The shield prevents the high power RF pulses from radiating out through the hospital. It also prevents the various RF signals from television and radio stations from being detected by the imager. Some scan rooms are also surrounded by a magnetic shield which contains the magnetic field from extending too far into the hospital. In newer magnets, the magnet shield is an integral part of the magnet.
The heart of the imager is the computer. It controls all components on the imager. The RF components under control of the computer are the radio frequency source and pulse programmer. The source produces a sine wave of the desired frequency. The Pulse programmer shapes the RF pulses into apodized sinc pulses. The RF amplifier increases the pulses power from milli Watts to killo Watts. The computer also controls the gradient pulse programmer which sets the shape and amplitude of each of the three gradient fields. The gradient amplifier increases the power of the gradient pulses to a level sufficient to drive the gradient coils.
The array processor, located on some imagers, is a device which is capable of performing a two-dimensional Fourier transform in fractions of a second. The computer off loads the Fourier transform to this faster device.
The operator of the imager gives input to the computer through a control console. An imaging sequence is selected and customized from the console. The operator can see the images on a video display located on the console or can make hard copies of the images on a film printer.
The next three sections of this chapter go into more detail on the magnet, gradient coils, RF coils, and RF detector on magnetic resonance imagers.
The imaging magnet is the most expensive component of the magnetic resonance imaging system. Most magnets are of the superconducting type. This is a picture of a 1.5 Tesla superconducting magnet from a magnetic resonance imager.
A superconducting magnet is an electromagnet made of superconducting wire. Superconducting wire has a resistance approximately equal to zero when it is cooled to a temperature close to absolute zero (-273.15o C or 0 K) by emersing it in liquid helium. Once current is caused to flow in the coil it will continue to flow as long as the coil is kept at liquid helium temperatures.
(Some losses do occur over time due to infinitely small resistance of the coil. These losses are on the order of a ppm of the main magnetic field per year.)
The next animation window contains a cross sectional view of a superconducting imaging magnet.
The length of superconducting wire in the magnet is typically several miles. The coil of wire is kept at a temperature of 4.2K by immersing it in liquid helium. The coil and liquid helium is kept in a large dewar. This dewar is typically surrounded by a liquid nitrogen (77.4K) dewar which acts as a thermal buffer between the room temperature (293K) and the liquid helium.
The gradient coils produce the gradients in the Bo magnetic field.
They are room temperature coils which because of their configuration create the desired gradient. Since the horizontal bore superconducting magnet is most common, the gradient coil system will be described for this magnet.
Assuming the standard magnetic resonance coordinate system, a gradient in Bo
in the Z direction is achieved with an antihelmholtz type of coil. Current in the two coils flow in opposite directions creating a magnetic field gradient between the two coils. The B field at one coil adds to the Bo
field while the B field at the center of the other coil subtracts from the
Bo field.
The X and Y gradients in the Bo field are created by a pair of figure-8 coils. The X axis figure-8 coils create a gradient in Bo in the X direction due to the direction of the current through the coils.
The Y axis figure-8 coils provides a similar gradient in Bo along the Y axis.
RF coils
create the B1 field which rotates the net magnetization in a pulse sequence. They also detect the transverse magnetization as it precesses in the XY plane. RF coils can be divided into three general categories; 1) transmit and receive coils, 2) receive only coils, and 3) transmit only coils. Transmit and receive coils serve as the transmitter of the B1 fields and receiver of RF energy from the imaged object. A transmit only coil is used to create the B1
field and a receive only coil is used in conjunction with it to detect or receive the signal from the spins in the imaged object. There are several varieties of each. The RF coil on an imager can be likened unto the lens on a camera. A photographer will use one lens for a close up shot and a different one for a wide angle long distance shot. Just as a good photographer has several lenses, a good imaging site will have several imaging coils to handle the variety of imaging situations which might arise.
An imaging coil must resonate, or efficiently store energy, at the Larmor frequency. All imaging coils are composed of an inductor, or inductive elements, and a set of capacitive elements. The resonant frequency, n, of an RF coil is determined by the inductance (L) and capacitance (C) of the inductor capacitor circuit.
Some types of imaging coils need to be tuned for each patient by physically varying a variable capacitor.
The other requirement of an imaging coil is that the B1 field must be perpendicular to the Bo magnetic field.
Some of the more common imaging coils are depicted in the animation screen. The depictions show the direction of the B1 field, the mode of operation, and applications.
Multi Turn Solenoid
Surface Coil
Surface coils are very popular because they are a receive only coil and have a good signal-to-noise ratio for tissues adjacent to the coil. Here is an example of a image of the lower human spine obtained with a surface coil.
Here is a picture of a flat circular surface coil with its connecting cable.
The cable will connect to the imager. This is a picture of a surface coil molded to conform to the back of the knee.
Bird Cage Coil The bird cage coil is the coil of choice for imaging the head and brain. Here is a picture of the human head in a bird cage coil.
All of the head images in this hypertext book were obtained using a bird cage coil.
Single Turn Solenoid The single turn solenoid is useful for imaging extremities, such as the breasts and the wrist. This animation window entry shows a single turn solenoid imaging coil around the human wrist.
The detail icon will provide you with more information on the construction of a single turn solenoid.
Saddle Coil
Phased-Array Coil
Litz Coil (Doty Scientific, Inc., Columbia, SC)
The multiturn solenoid, bird cage coil, single turn solenoid, and saddle coil are typically operated as the transmitter and receiver of RF energy. The surface and phased-array coils are typically operated as a receive only coil. When a surface or phased-array coil is used, a larger coil on the imager is used as the transmitter of RF energy to producing the 90o and 180o pulses.
The quadrature detector
is a device which separates out the Mx'
and My'
signals from the signal from the RF coil. For this reason it can be thought of as a laboratory to rotating frame of reference converter. The heart of a quadrature detector is a device called a doubly balanced mixer (DBM). The doubly balanced mixer has two inputs and one output. If the input signals are Cos(A) and Cos(B), the output will be 1/2 Cos(A+B) and 1/2 Cos(A-B). For this reason the device is often called a product detector since the product of Cos(A) and Cos(B) is the output.
The quadrature detector typically contains two doubly balanced mixers, two filters, two amplifiers, and a 90o phase shifter.
There are two inputs and two outputs on the device. Frequency n and no
are put in and the
MX and MY components of the transverse magnetization come out.
There are some potential problems which can occur with this device which will cause artifacts in the image. These will be addressed in
Chapter 11.
Although MRI does not use ionizing radiation to produce images there are still some important safety considerations which one should be familiar with. These concern the use of strong magnetic fields, radio frequency energy, time varying magnetic fields, cryogenic liquids, and magnetic field gradients.
Magnetic fields from large bore magnets can literally pick up and pull large ferromagnetic items into the bore of the magnet. Caution must be taken to keep ALL ferromagnetic items away from the magnet for two main reasons. The first reason is they can injure or kill an individual in the magnet. The second reason is they can seriously damage the magnet and imaging coils. The force exerted on a large metal object, such as a mop wringer can damage the concentric cryogenic dewars within a magnet. The kinetic energy of such an object being sucked into a magnet can smash an RF imaging coil.
Despite numerous safety warnings issued by the manufacturers, I have heard numerous stories of ferromagnetic objects being pulled into imaging magnets. The most common one is similar to this story. A metal pail on wheels was filled with water and had a mop wringer in it. The pail was located approximately 10 feet from the bore of a 1.5 T magnet. The magnet pulled it across the floor and lifted it up off the ground three feet into the magnet. The wringer caused serious damage to the magnet in that the cryogen boil off rate increased and the magnetic field homogeneity decreased. The head coil located in the bore of the magnet was destroyed. The most frightening story was of a law enforcement officer being allowed to go near a magnet with a loaded firearm. The handgun was pulled out of its holster, and into the magnet. The force of the impact with the magnet caused the gun to discharge. Luckily, no one was injured in this incident. In addition to the damage to the MRI and the bullet lodged in the scan room wall, the gun was magnetized. Mechanical objects, in general, do not function properly when magnetized. Please, respect the physical laws of nature that cause ferromagnetic objects to be attracted by magnets!
Similar forces are at work on ferromagnetic metal implants or foreign matter in those being images. These forces can pull on these objects cutting and compressing healthy tissue. For these reasons individuals with foreign metal objects such as shrapnel or older ferromagnetic implants are not imaged. There are additional concerns regarding the effect of magnetic fields on electronic circuitry, specifically pacemakers. An individual with a pacemaker walking through a strong magnetic field can induce currents in the pacemaker circuitry which will cause it to fail and possibly cause death. Magnetic fields will also erase credit cards and magnetic storage media.
The United States Food and Drug Administration (USFDA) safety guidelines state that field strengths not exceeding 2.0 Tesla may be routinely used. People with pacemakers must not be exposed to magnetic fields greater than 5 gauss. A 50 Gauss magnetic field will erase magnetic storage media.
The radio frequency energy from an imaging sequence can cause heating of the tissues of the body. The USFDA recommends that the exposure to RF energy be limited. The specific absorption rate (SAR) is the limiting measure.
The recommended SAR limitations depend on the anatomy being imaged. The SAR for the whole body must be less than 0.4 W/kg . It must be less than 3.2 W/kg averaged over the head. Any pulse sequence must not rise the temperature by more than 1o Celsius and no greater than 38o C in the head, 39o C in the trunk, and 40o C in the extremities.
Some RF coils, such as surface coils, have failure modes which can cause burns to the patient.
The animation window contains a picture of an RF burn to the elbow of a man's arm.
The patient's arm was against the wall of a body coil being operated in a transmit mode with a surface coil as the receiver.
A malfunction in the body coil caused the third degree RF burn.
The burn first appeared as a simple blister and progressed to a charring that had to be surgically removed.
The surgeon excised a volume approximately 3 cm in diameter and 2.5 cm deep.
Therefore, if you are operating an imager and your patient or volunteer tells you he or she is experiencing a burning sensation, stop the scan.
Additionally, care should be taken to keep RF imaging coils in proper operating order.
The USFDA recommendations for the rate of change of magnetic field state that the dB/dt for the system must be less than that required to produce peripheral nerve stimulation.
Imaging gradients do produce high acoustic noise levels. The American OSHA limits the peak acoustic noise to 200 pascals or 140 dB references to 20 micropascals. Here are some examples of the sounds made by the turning on and off of the magnetic field gradients in various imaging sequences.
| Sequence | TR (ms) | TE (ms) | Slices | Sound |
|---|---|---|---|---|
| Spin-Echo | 500 | 35 | 1 | |
| 200 | 1 | |||
| 15 | 10 | |||
| Echo-Planar | 120 | 54 | 10 | |
| Gradient Echo | 16.7 | 4 | 19 |
An MRI phantom is an anthropogenic object that can be imaged to test the performance of the magnetic resonance imaging system. Phantoms are used instead of a standard human because it is much easier to locate a phantom standard at each of the many MRI systems in the world then it is to send the standard human from site to site to be imaged. Phantoms are composed of materials that have a magnetic resonance signal. Many materials have been used as the signal bearing substance in MRI phantoms.
Some of these are aqueous paramagnetic solutions; pure gels of gelatin, agar, polyvinyl alcohol, silicone, polyacrylamide, or agarose; organic dopped gells; paramagnetically dopped jells; and reverse micelle solutions.
Water is most frequently used as the signal bearing substance in an MRI phantom. It is usually necessary to adjust the spin-lattice (T1) and spin-spin (T2) relaxation times of aqueous solutions so images may be acquired in reasonable time periods (i.e. short TR). Paramagnetic metal ions are typically used to adjuct the relaxation times of the water hydrogens. The approximate functional form of the T1 and T2 values of aqueous solutions of various paramagnetic species at 1.5 T are listed below.
| Aqueous Nickel |
|---|
|
T1(s) = 1/(632 [Ni (mole/L)] +0.337) T2(s) = 1/(691 [Ni (mole/L)] + 1.133) |
| Nickel in 10 wt % gelatin |
|
T1(s) = 1/(732 [Ni (mole/L)] +0.817) T2(s) = 1/(892 [Ni (mole/L)] + 4.635) |
| Aqueous Oxygen |
| T1(s) = 1/(0.013465 [O2 (mg/L)] + 0.232357) |
| Aqueous Manganese |
|
T1(s) = 1/(5722 [Mn (mole/L)] +0.0846) T2(s) = 1/(60386 [Mn (mole/L)] + 3.644) |
There are two basic types of MRI phantoms: resolution and RF homogeneity. As the names imply, one is used to test resolution and the other RF homogeneity.
Resolution Phantoms
A resolution phantom can be used to test several spatial properties of an imager. These spatial properties include in-plane resolution, slice thickness, linearity, and the signal-to-noise ratio as a function of position. Resolution phantoms are typically constructed from plastic. Portions of the inside of the phantom are removed to create a test pattern. The phantom is filled with an aqueous solution. When imaged, the image displays the signal from the water in the removed portions of the plastic. Some resolution phantoms also have signal standards with known T1, T2, and
r values that allow the phantom to be used to test contrast-to-noise ratios.
Here is an example of a resolution phantom.
A 24 cm field-of-view image of an axial slice through this phantom displays the following features.
The series of identical size squares are used to test the linearity. In-plane resolution is determined by a group of thin signal-bearing regions. Three signal standards contain a liquid with a known T1, T2, and r values.
The slice thickness (Thk) metric is a wedge shaped cut-away in the plastic. The width of the image of this wedge increases as the slice thickness is increases. The following schematic diagrams of the phantom imaged with a thin
,
and thick
slicethickness will help you see how this shape an help measure slicethickness.
Here are images of the resolution phantom imaged with a
3 ,
5 ,
and 10 mm
slice thickness. Note the change in the slice width metric.
RF Homogeneity Phantoms
Homogeneity phantoms are used to test the spatial uniformity of the transmit and receive radio frequency magnetic fields. The transmit RF field (B1T) is the B1 field is that used to rotate magnetization. The receive RF field (B1R) is the sensitivity of the RF coil to signals from precessing spin packets. The ideal situation for most transmit/receive coils is a spatially uniform B1T to assure uniform rotation of the spins, and a spatially uniform B1R to assure uniform sensitivity across the imaged object. Here is a picture of a 27 cm diameter homogeneity phantom.
A series of spheres can be used to measure the homogeneity over a larger volume. Here is an array of homogeneity pantoms which can be used to measure the homogeneity of B1R field from a surface coil used for imaging te spine.
Several images from an RF homogeneity phantom must be used to calculate B1T and B1R. Please click on the detail icon to receive more information on these calculations.
Copyright © 1996-99 J.P. Hornak.
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