RF SYSTEM Flashcards
- The electromagnetic components of the RF system may be provided by outside
suppliers and assembled by the MRI manufacturer. These components are attached
to the RF coils, which are made with vary designs.
RF system
– make the magnetic field homogeneous (they maintain the mri should receive
balanced magnetic fields) in Xray they act as filter
Shim coils
– detect the returning radio signals
Receiver coils
– are the antenna of MRI system that broadcast the RF signal to the patient and/or
receives the return signal RF coils can be receive only, in which case the body coil is used
as a transmitter, or transmit and receive (transceiver)
RF coils
5 RF BODY COILS: SUPAHEBIFA
Surface
Paired saddle
Helmhortz pair
Bird cage
Faraday cage
– are the simply a loop of wire either circular or rectangular, that is
placed over the region of interest. The depth of the image of a surface coil is
generally limited or about one radius. Commonly used in spines, shoulders, tmj’s and
other relatively small body parts
Surface coils
– provide better homogeneity of the RF in the are of interest and
are used as volume coils, unlike surface coils. Also used of the X and Y gradient
coils. Commonly used for imaging of the knee
Paired saddle coil
– consist of two circular coils parallel to each other. They are
use as gradient coils in MRI scanners. They also used occasionally RF coils for
pelvis imaging and spine imaging (dedicated for spina and pelvic imaging, bigger
anatomy or bony structure)
Helmhortz pair coil
– provides the best RF homogeneity of all the RF coils. It has the
appearance of a bird cage, hence, its name. This coil is commonly used as a
transceiver coil for imaging of the head. This type of coil is also used occasionally for
imaging of the extremities such as the knees. (For the brain)
Bird cage coil
(to prevent radio frequency interference from outside)
- Specification: to provide radio frequency shielding, uses aluminium, or in certain
conditions galvanized steel, to form the shielded faraday cage
-
Faraday Cage
is used in MRI to describe the relative contributions to a detected
signal of the true signal and random superimposed signals (background noise)
signal to noise ratio
SNR formula
Mean signal/Standard Deviation of Background noise
If SNR INCREASE these 4 factors will DECRASE
T1, TE, MATRIX SIZE, RF BANDWIDTH
If SNR WILL INCREASE THESE 7 FACTORS WILL INCREASE ALSO
Bo, T2, TR, NEX, Pixel Size, Slice Thickness, FOV
If all parameters are constant…the smaller the sensitive volume of a coil, the ___ the noise from the adjacent
structures of the selected slice plane which it can detect, and the better the signal to
noise ratio will be
Lower
A ___or better, a ___coil has a higher signal to noise ratio than a body coil
local coil,surface
– frequencies associated either with RF-excitation (transmitter bandwidth,
tBW) or signal reception (receiver bandwith, rBW) or the highest or lowest signal
RF bandwidth
is the range of frequencies accepted by
the receiver to sample the MR signal. rBW has a direct relationship to the signal to noise
ration (SNR = 1/sqrt (rBW)
receiver (or acquisition) bandwidth (rBW)
– RF excitation pulse required for slice selection in a pulse
sequence. The slice thickness is proportional to the bandwidth of the RF pulse (and
inversely proportional to the applied gradient strength) lowering the pulse bandwidth can
reduce the slight thickness
transmit bandwidth
Slice thickness in MRI is determined: BS
The band width of the RF (longer BW, lower ST)
Steepness of the slope of the gradient coil (strength)
– (mri uses techniques) to acquire images with different tissue contrast
mechanisms. Pulse sequences are a set of specific instructions programmed into the
computer with an expectation as to how the images should appear
Pulse sequence
)
- Diffusion movement of molecules due to random motion. If an incident occurs less
than 72 hours prior to the scan, swelling due to edema occurs.
- Used for CVA and CNS
- The combination of actual diffusion values and T2 signal.
DWI (diffusion weighted imaging
- Diffusion movement of molecules due to random motion. If an incident occurs less
than 72 hours prior to the scan, swelling due to edema occurs. - Used for _____
- The combination of actual diffusion values and T2 signal.
DWI (diffusion weighted imaging)
CVA and CNS
DWI is most applicable when the tissue of interest is dominated by isotropic water movement
e.g., grey matter in the cerebral cortex and major brain nuclei, or in the body-where the
diffusion rate appears to be the same when measured along any axis. However, DWI also
remains sensitive to T1 and T2 relaxation
was introduced to take
into account the fact that the diffusion process is complex in biological tissues and reflects
several different mechanisms
ADC concept
– are images representing the actual diffusion
values of the tissue without T2 effects
- They are therefore much more useful, and objective measures of diffusion values can
be obtained; however, they are much less pretty to look at.
- They appear basically as grayscale or inverted DWI images
Apparent diffusion coefficient maps (ADC)
– most widely used. starts with 90 pulse and ends with 180 degrees pulse.
Spin Echo
– a gradient to refocus the spins. Gradient echoes are more susceptible to
blood flows. They utilize the flip angles lesser than 90 degrees in addition to TR and TE.
- Are an alternative technique to spin echo sequences
- Utilization of gradient fields to generate transverse magnetization
- Flip angles of less than 90 degrees
- Aka reverse polarity of gradient
Gradient echo
– fast gradient imaging technique. For turbulent blood
flow.
Fast gradient echo (FGRE)
– is quicker because it covers a wide range of tissue, however it
does increase the specific absorbed rate. FSE starts with a 90 degree pule and
followed with a series of 180-degree pulse.
Fast spin echo
