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One of the main requirements for MRI is a strong magnet, what is it's purpose?
is to align the net magnetization of the patient's protons and to establish the proton's resonant frequency
what are the requirements of a strong magnet for it to be used?
- a large enough opening to comfortably fit a patient
- must generate a magnetic field that is extremely uniform and should contain only negligible variations in the field within the imaging volume where the patient lies
these variations in the magnetic field are called what?
magnetic field inhomogeneities
what does the term homogeneity mean?
it refers to the uniformity of the main magnetic field
what is the pulse sequence controller responsible for?
- for the timing and correct performance of each system component once the sequence hast been started
- dictates when and how much gradient power is needed to vary the magnetic field in order to spatially encode the MR echoes
- also dictates when the RF energy must be transmitted and for what duration of time
how does the pulse sequence controller control how much RF energy is sent to the patient?
- it sends a signal to the RF power amplifier which must be converted into an analog or continuous waveform. That conversion is performed by the digital-to-analog converter (DAC). By controlling the duration of a specific RF pulse, the controller determines how much RF energy is transmitted into the patient.
- Recall that the amount of RF energy that is transmitted into the patient determines the angle that the net magnetization tilts into the transverse plane
What does the digital-to-analog converter (DAC) do?
responsible for converting the digital instructions from the pulse sequence controller into a continuous analog waveform, which is then passed through the RF power amplifier
Where is the analog RF energy transmitted to and from?
transmitted through RF coils, or antennae, into the patient. RF coils are also responsible for detecting the tiny currents, or echoes, which result from the disturbance created by external RF energy and subsequent relaxation experienced by the patients' net magnetization of protons
How do the echoes emitted from the patient reach the console?
- the echoes emitted from the patient's protons are very small signals which must be passed through a pre-amplifier in order to greatly increase, or amplify, their strength. The amplified echoes, which are analog signals, are first sampled via the sample-and-hold component (S/H) and then digitized by the analog-to-digital (ADC) into a digital form which is recognized by the computer. The amplified and digitized echoes are then electronically filtered. After passing through this receiver section of the system the accumulated echoes are temporarily stored ina raw data file where, as k-space data, they are calculated into an image by the Fourier Transform. This calculation occurs in the array processor.
- The calculated image is transferred to the storage device, which is linked to the host computer, where it can be accessed by the operator at the system console
What are the two categories of magnets used to generate strong magnetic field necessary for MRI?
- permanent magnets
what are the two categories of electromagnets?
- resistive magnets
- superconducting magnets
what are the most common types of magnets used in MRI?
how are permanent magnets constructed?
from hundreds of permanently magnetized ceramic bricks, which when stacked together such that their magnetic fields face in the same direction, add their magnetic forces to generate the main magnetic field required to align a patient's net magnetization of hydrogen protons
what are the advantages of permanent magnets?
- low power consumption = lower costs to maintain
- stable magnetic field
- low magnetic fringe field
what are the disadvantages of permanent magnets?
- cannot turn off field
- very heavy
- limited to low magnetic field strength
what are electromagnets made from?
from coils of wire through which a cuirrent is passed
what are resistive magnets made of?
- they consist of an air core or iron core wrapped with a coil of long wire.
- may be designed to have a vertical or horizontal magnetic field, and they typically attain maximum magnetic field strengths which are somewhat lower than superconducting magnets
what are the advantages of resistive magnets?
- can quickly turn off magnetic field
- low magnetic fringe field
what are the disadvantages of resistive magnets?
- high power consumption
- water cooling is required
- limited to lower magnetic field strengths
what are superconducting magnets made of?
- are also electromagnets, but with special components and operating conditions. The coil of long wire used to carry the current for magnetic field generation is made of a blend of elements, which when kept extremely cold, have negligible electrical resistance. The absence of resistance is very important, because it allows a very high current to flow in the wire, which generates a very strong magnetic field. This property, therefore, is called superconductivity. The superconductive property of the wire also allows the magnet power source to be connected only once to establish the magnetic field. Then, provided that the magnet windings are kept sufficiently cold, the field will persist after the power supply is disconnected since no additional power consumption is necessary to maintain the strong magnetic field. Therefore, the absence of resistance from the magnet's superconductive windings keeps the nergy consumption costs low.
- However, the overall operational costs are not low with a superconducting magnet because of the requirement to keep the coil of superconductive wire very cold
The wire of a superconducting magnet must be maintained very cold. What temperature does it need to be maintained at? How is this achieved?
- absolute zero or zero degrees kelvin
- it is achieved by immersing the windings in a cryogen, typically liquid helium at about -269 degrees celcius or 4 degrees kelvin
what are the advantages of a superconducting magnet?
- magnetic field can be turned off
- low power consumption
- high magnetic fields possible
- stable magnetic field with high homogeneity
what are the disadvantages of a superconductive magnet?
- large service requirements
- cryogen maintenance requirements
- large magnetic fringe field
What is shimming?
shimming is the process by which magnetic field inhomogeneities are greatly reduced in order to make the field more uniform.
inhomogeneities may be the result of what?
- ferromagnetic materials in the
- vicinity of the magnet, or may be due to unavoidable imperfections in the magnet itself
in order to achieve clinically acceptable image quality, what needs to be done?
- the patient's anatomy within the region of interest must reside in a homogenous magnetic field, otherwise, the patient's net magnetization of protons would precess at many different frequencies prior to the initiation of a gradient, and result in very rapid dephasing of the MR echo
- Shimming can help
what are the two ways shimming can be accomplished>
passively and actively
what is passive shimming?
- which is performed when the MR system is installed, uses plates of metal which are firmly attached to precise locations on the surface of a cylinder within the bore of the magnet
- the plates of metal will compensate for distortions within the existing magnetic field
- a probe is placed at numerous points in the bore of the magnet to measure the magnetic field. The measurements are processed by a software program which indicates the location and quantity of metal plates required to correct for variations within the magnetic field and ensure maximum homogeneity
what is active shimming?
- is usually initiated by the technologist to improve the homogeneity of the magnetic field in a specified region of interest. It is similar to passive shimming except that current passing through wire coils replaces the plates of metal
- a magnet with active shimming contains a cylinder with many coils of wire on its surface. Current passed through a loop of wire will generate a perpendicular magnetic field. Therefore, by passing a specified amount of current in a given direction through each shim coil, an addition or subtraction of field strength can be made in order to compensate for inhomogeneities in the magnetic field, thus making it more uniform
what are gradient coils responsible for?
- selects a slice
- frequency and phase encodes
- creates a linear change in the magnetic field
what are gradient coils made of?
are thick bands of conductive material wrapped around a cylinder that fits inside of the shim cylinder. There are three sets of coil pairs wrapped onto the cylinder's surface. There are also three power supplies called gradient amplifiers that drive electrical current through the gradient coils
What happens when current is applied to any of the gradient coils?
the linear change in magnetic field strength does not occur immediately. A small, discrete amount of time is required to vary, or ramp up, the field from its resting value to the specified gradient value, Similarly, a small, discrete amount of time is required to allow the field to return or ramp down to its resting value once the application of current to the gradient foils ceased
What is one way a gradient's behavior can be expressed over time?
is by a trapezoidal waveform in which the trapezoid's sides represent the time required to ramp up to the desired gradient amplitude and the time required to ramp down to the level of the main magnetic field. The ramp up and ramp down times are also expressed as the gradient rise time and gradient fall times, respectively.
What is another important description of gradient behavior?
slew rate- combines the gradient rise time and gradient amplitude into a single parameter which is expressed in the units of millitesla per meter per second
what is duty cycle?
a related parameter to slew rate which is the percentage of time a gradient is applied at a specific amplitude
the gradient rise time, the slew rate, the duty cycle and the maximum amplitude are all indicators for what?
the efficiency of an MR system's gradients and the system's ability to perform gradient-intensive, rapid scanning techniques
What is an RF coil?
- a loop of wire which acts as an antenna
- responsible for transmitting the RF energy which disturbs the protons in a particular location of anatomy within the pts body
- also responsible for detecting the very tiny signals emitted from the body in the form of MR echoes
what is Faraday's law of induction?
any magnetic change which occurs near a neighboring conductive material, will cause a voltage to be induced across that conductive material
how is the MR echo transmitted through through the RF coil?
the MR echo creates a magnetic flux caused by the precessing net magnetization that originates in the pt (in response to an RF pulse). This precessing net magnetization generates oscillating magnetic waves which create a magnetic flux that interacts with the nearby RF coil made of eletrically conductive material. Therefore the MR echo induces a voltage in the RF coil (Faraday's law of induction). This voltage is measured and eventually stored as rawdata until it is processed by the Fourier Transform into an image. The MR echo generated by the NM is the mr signal. The RF coil is the reciever coil. The voltage induced in the receiver coil is proportional to the strength of the MR signal. Therefore, as the MR echo becomes weaker during the relaxation processes, it induces a progressively lower voltage in the receiver coil over the time course of the relaxation processes
Can RF coils be used for both transmitting the RF energy into the pt and receiving the tiny voltages induced by the MR echoes emitted from the pt?
- in some, cases one coil will act as the transmitter coil, and a second coil will act as the receiver coil. When two coils are used, they must be decoupled, or electrically isolated from one another, so that energy is applied to only one coil at a time, for safety reasons
What are the prerequisites for transmit coils?
permanently installed within the bore of the magnet and is often used for body imaging, transmitter coils should be large enough and have a geometry which will produce a uniform field of RF energy. Usually have a volumetric design, or should completely surround the volume into which RF energy is to be transmitted.
Why is the transmitter bandwidth important during the transmission of RF energy?
because the transmitter bandwidth corresponds to the range of precessional frequencies disturbed by the RF energy, and in conjunction with the application of the slice select gradient, plays an important role in selecting the slice thickness
What are the prerequisites for receiver coils?
- size and shape should be matched to the anatomy being imaged.
- in general, the sensitivity of the coil falls off rapidly as the distance from the coil is increased. This is particularly true for smaller coils
Why is the receiver bandwidth important during the reception of the MR signal?
Recall that the MR signal contains a range, or bandwidth, of precessional frequencies as a result of applying the frequency encoding gradient while the MR echo is generated. The receiver bandwidth corresponds to the range of frequencies sampled within the MR signal. The receiver bandwidth affects image characteristics such as signal-to-noise ratio and severity of chemical shift artifact
what happens when receiver coil size and shape is not matched up to the anatomy being imaged?
the sensitivity of the coil falls of rapidly as the distance from the coil is increased
volumetric coils are better for which part of the body?
head and abdomen because they are round and fills the coil better which creates a better SNR
surface coils have the most signal and the weakest signal where?
- the most signal on the tissues near the body's surface against the coil
- least signal in deep structures away from surface of coil
whats the advantage of array coils?
- one or more of these small receiver coils can be used depending on the region of interest for a given exam. can be switched on and off
- can also allow for a time-saving method for parallel imaging
what is the fringe field?
magnetic field outside of the magnet
what are the two types of magnetic shielding for the fringe field?
- passive and active
- passive shielding restricts the main magnetic field by encasing the magnet in iron
- active shielding requires an additional set of coils on the exterior surface of the magnet. when current is passed through these active shielding coils, an opposing magnetic field is generated.
What are the reasons for RF shielding?
- to keep RF energy inside the magnet room when transmitting
- to keep unwanted radio signals from entering the magnet room while receiving