providing mri technique by further knowledge in different organs and systems
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MRI technique enable the student to understand different MRI protocols
Upper &lower extremity
Chest & breast
MRA = Magnetic resonance Angiography , amazed technique which display or show the details of blood vessels ( lumens ), such as circle of wills in brain or any other blood vessel in the body.
A magnet is a material or object that produces a magnetic field. For details about specific types of magnets see:
→ Permanent magnet
→ Resistive magnet
→ Superconductive magnet
An electromagnet whose strong magnetic field (typically at least 0.5 T) is generated using superconductive coils. The conductive wires of the coils are made of a cryogenically cooled Niobium Titanium alloy. Liquid helium is used as the cryogen or liquid nitrogen for pre-cooling.
The most important component of the MRI scanner is the magnet.
A permanent magnet is sometimes referred to as a vertical magnet.
These magnets are constructed of two magnets (one at each pole).
The patient lies on a scanning table between these two plates.
Advantages of these systems are:
1) Relatively low cost, 2) No electricity or cryogenic liquids are needed to maintain the magnetic field, 3) Their more open design may help alleviate some patient anxiety.
Disadvantages are :
1) Low field strength typically 0.02 T .
2) It has a high weight = HEAVY WEIGHT
3) Poor diagnosis
Resistive magnets are constructed from a coil of wire. The more turns to
the coil, and the more current in the coil, the higher the magnetic field.
These types of magnets are most often designed to produce a horizontal
field due to their solenoid design.
The main advantages of these types of magnets are:
1) No liquid cryogen, 2) The ability to “turn off” the magnetic field, 3)
Relatively small fringe field.
The main Disadvantages are :
1) Low magnetic field strength .
2) Always producing heat , and this thing is huge problem for imaging .
3) Not enough for imaging.
Superconducting magnets are the most common. They are made from
coils of wire (as are resistive magnets) and thus produce a horizontal
field. They use liquid helium to keep the magnet wire at 4 degrees Kelvin
where there is no resistance. The current flows through the wire without
having to be connected to an external power source.
The main advantages are :
The ability of superconducting magnets is to attain field strengths of up
to 3 Tesla for clinical imagers, and up to 10 Tesla or more, & good resolution & most common used.
Disadvantages are :
1- take more time to produce the image .
2- always making noise with scanning .
3-Not suitable for some care patients.
Coils that create weak additional magnetic fields in various spatial directions. Used to correct in homogeneity in the main magnetic field.
Coils used to generate magnetic gradient fields. Gradient coils are operated in pairs in the magnet, at the same current, however, of opposite polarities.
One of the coils increases the static magnetic field by a certain amount, the opposite coil reduces it by the same amount. This changes the magnetic field overall. The change is the linear gradient. According to the coordinate axes, there are x, y, and z gradient coils.
Gradient coils polarization
RF coils are the "antenna" of the MRI system that broadcasts 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).
Different tissue types have different transverse magnetizations. Where the signal is strong, the image shows bright pixels; weaker signals result in darker pixels
What determines the signal strength?
Clearly to a large degree from the proton density in the respective voxel: the greater the number of protons contributing to the magnetization, the stronger the signal.
But even more important for medical diagnostics is the effect of the two relaxation constants T1 and T2 on the image contrast.
The other additional sequences are :
Saturation , EPI , diffusion and perfusion.. ..
FSE used to make SE much faster , they do have a number of additional 180° RF pulses. Even though those RF pulses can be used to create separate images with different TE times (such as PD and T2) in one excitation
SE TE 20
FSE TE 20
conventional SE or FSE sequence
1-STIR [short TI inversion recovery]
2-FLAIR [fluid attenuation inversion recovery]
It usually takes longer to acquire .
B-FLAIR[fluid attenuation inversion recovery]
Are also part of IR sequence family with widespread use, The main purpose of T2 FLAIR sequences is to null or suppress the cerebrospinal fluid (CSF) .
Suppressing CSF enables us a much better visualization of adjacent white matter (WM) tissues with possible lesions. Therefore, it is one of very essential sequences of any routine brain imaging.
Fat = bright
Water = hypo intense
Water weighted sequence
Water = bright
Fat = relatively hypo intense
Good for identifying pathology
T1 (hypo intense)
T2 (hyper intense)
FLAIR (hyper intense)
FLAIR SCANS ARE T2 SCANS WITH THE FREE WATER SIGNAL NULLED.
is simply due to the fact that we acquire the whole k-space needed to form an MR image in a long ETL in a very fast way.
GRE sequences can be divided However, into:-
7) Echo Planar Imaging (EPI) Sequences
Selection of right MR imaging parameters almost always results in better image quality and prevents the typical imaging artifacts seen on daily scanning. Therefore, it is very important to understand the working mechanism of those parameters for the best decision making.In general, we can divide MR parameters into three main groups:
1-Field of View (FOV) (24x24) (24x20)cm
Effect of FOV on spatial resolution and SNR. Note improved visualization of the lens in the 12cm image (B) when compared to the 24cm FOV image (A), achieved at the expense of SNR.
2- Matrix size:(256x256) (512x512)
the higher imaging matrix results in noisier image, it has better resolution.
3- Spatial Resolution
Spatial resolution determines how "sharp" the image looks. Spatial resolution is defined by the size of the imaging voxels.
Since voxels are three dimensional rectangular solids, the resolution is frequently different in the three different directions.
The size of the voxel and therefore the resolution depends on matrix size, the field-of-view (FOV), and the slice thickness.
Resolution = FOV/matrix in mm.
4-Slice Thickness ( TH ):
Slice Thickness ( TH )
is defined in mm and determines the depth of your voxel on slice encoding direction. the thicker the slice is, the higher the SNR becomes. However, the thicker the slice, the lower the resolution becomes.
Sample images acquired with slice thickness of 3 mm ( a ) and 15 mm ( b ) are shown on the same volunteer, so the thicker the slice, the lower the resolution becomes.
5- Slice Spacing or Gap :
can be defined in mm or in % of the [TH] depending on MR manufacturers.
6- Number of acquisition ( NSA , number of excitations[ NEX ]):
Sample images acquired with a NEX of 1 ( a ) and NEX of 4 ( b ) are shown on the same volunteer.
7- (BW) Bandwidth [Hertz]
is a measure of frequency range, the range between the highest and lowest frequency allowed in the signal. For analog signals, which can be mathematically viewed as a function of time, band
higher bandwidth gives :-
1- a much sharper image, reduces ringing artifacts
2- reduce the total scan time .
Total BW= Frequency matrix / Ts (in seconds).
8- Scan time
1- TR 2-TE 3-T1 4-PD
6-T2*:It is characterized by loss of transverse magnetization at a rate greater than T2,caused by magnetic field in homogeneity occurs in all magnets.
7- Inversion Time [TI]
8- Echo Train Length ( ETL ) or Turbo Factor :
defined as the number of refocusing 180° RF pulses after the initial excitation RF pulse.
9-Flip Angle ( FA ):
3-MR Imaging Options:
In MR imaging, there are several imaging options used to enhance or alter the image contrast and to reduce the MR artifacts.Options such as fat saturation(FS), cardiac gating(CG), and flow compensation (FC) can be combined with certain pulse sequences depending on how they were designed and set up.
The definition of artifacts is : Abnormal appearance in the MRI images which may obscure some small lesions.
A chemical shift artifact is caused by the difference in Frequency of fat and water.
The artifact manifests itself as a miss registration between the fat and water pixels in an image .
The effect being that fat and water spins in the same voxel are encoded as being located in different voxels.
Chemical shift artifact can be reduced by performing imaging at low magnetic field strength, by increasing receiver bandwidth, or by decreasing voxel size.
The artifacts tend to be more prominent on T2-weighted than on T1-weighted images.
Fat suppression methods often eliminate visible artifacts
Example: Two groups of non-parallel slices in the same sequence, e.g., L4-5 and L5-S1.
MRI SAFETY & APPLICATIONS
Any electronic device
USEFUL FOR DETECTION OF:
Cervical dorsal lumbar