MRI PREFI
Echo Planar Imaging (EPI)
- Advanced Fast Techniques - Developed by Dr. Mansfield in the 1980s, and is described as an ultrafast gradient echo imaging technique. - described as generating sub-second imaging using multiple gradient reversals that followed one set of RF pulses.
REFOCUSING
- After encoding, the next step is an application of a 180 degree pulse to refocus the signal or echo.
GdDTPA (gadolinium diethylenetriaminepentaacetic acid dimeglumine
- First commercial enhanced contrast enhancing agent - also known as gado-penetrate dimeglumine - commercially known as Magnevist
Shallow Flip Angle (FA)
- Instead of utilizing a 90 degree RF pulse, the GE sequence utilizes a shallow flip angle.
Gradient Reversal
- The basic concept of this technique is the areas in the external magnetic field after the application of the gradient coils that were considered to be low gradient fields will be reversed and will become high gradient fields.
Bioeffects of RF Electromagnetic Fields
- The physical dimensions and configuration of the tissue in relation to the incident wavelength are important factors that determine the relative amount and pattern of energy that is absorbed during exposure to the RF radiation.
Multi-Echo Spin Sequence
- This modification basically adds multiple echo extensions by an addition of 180 degree RF pulses and read-out gradients - only limited by the additional time required for the extra RF and gradient pulse necessary to generate the successive echoes and as a result, may limit the number of slices available.
ETL
- This occurs because of the abrupt differences in signal intensity from one TR to another - Increasing ETL will lead to an increase of possible blurring artifacts that may jeopardize edge detail.
Effects on Cell and Nerve Function
- This phenomenon is attributed to paramagnetic anisotropy retained by the heme of the hemoglobin S that is polymerized by deoxygenation.
FLIP ANGLE (FA)
- This scan parameter is chosen when performing Field Echo Sequences to satisfy a unique echo appearance of the resultant image - are related to the number of spins directed to the transverse plane
The Gradient Echo Sequence
- Was developed in the mid 1980s - This dramatically reduced the required MRI scan time but as a result has a lower signal to noise ratio compared to spin echo sequences. - The gradient echo sequence varies with the spin echo sequences in two fundamental characteristics
Chemical Shift Imaging
- a phenomena in MRI caused by a consequence of a precessional frequency difference of hydrogen in fat and in water.
SHORT TE
- allows minimal loss of transverse signal due to T2 relaxation - reduces T2 effects
SHORT TR
- allows tissues with short T1 value to fully recover along the longitudinal direction and tissues with a long T1 value only to partially recover - increases the T1 effects
Cardiac Implants
- associated risks with MRI related to the possible movement of the device, damage to the reed switch, changes to the mode of operation, electromagnetic interference and induction of current in the lead wires.
Phase Cancellation
- based on the mechanism of chemical shift origin and is where we utilize very specific echo times in gradient echo to cancel the certain phases in order to suppress fat from water in where both reside in the same voxel.
Spin Echo Sequence
- basic sequence of events in a spin echo sequence is a 90-degree RF pulse excitation, spatial encoding, a 180 degree RF pulse and a signal read-out - sequence is also used in earlier discussions of MRI
ECHO TRAIN SPACING (ETS)
- determines the separation in milliseconds of the echoes in the echo train. - Increasing ETS increases the contrast mix, thereby increasing the dependence on T2 contrast
ENCODING
- distributes the rotation of the spins relative to their spatial location along the gradient axis.
Iron Oxides
- function as a contrastagentscreatingalargedipolarmagneticfieldgradient that is experienced by water molecules which diffuse close to the particles
FDA
- governs the licensing and right to operate of these diagnostic imaging devices
T1 VALUE
- indicates how long it takes a tissue to regain 63% of its original longitudinal magnetization after having been tipped 90 degrees by an RF pulse - it has longitudinal recovery. - would start at zero - the curve would be increasing
T2 VALUE
- indicates how long it takes for that issue to lose 63% of its transverse magnetization after it has been tipped 90° by a burst of RF energy - a transverse signal loss - starts at 100%. - the curve would be decreasing
FLAIR (Fluid Attenuated Inversion Recovery) Technique
- is a variation of the STIR Technique. - In this technique rather than using a short TI value to catch fat at the null point in longitudinal relaxation, in FLAIR we now use a very long TI to catch water at the null point.
Severe
- life-threatening, potentially fatal event typically requiring hospitalization if an outpatient. - Severe respiratory distress, responsiveness, arrhythmia, convulsion, cardiopulmonary arrest, progressive angioedema, marked hypotension
S/P
- medical abbreviation which means status post - term used in medicine to refer to a treatment, diagnosis, or just an event that a patient has experienced previously
Mild
- nonmedical intervention with exception of possible antihistamine administration - Rash, itch, cough, hives, sneezing, nasal stuffiness, mild eye swelling, mild facial swelling, vomiting, nausea, perspiration, warmth, anxiety, flushing, altered taste
POSITIVE CM ENHANCEMENT
- occurs when the tissue of interest appears brighter in an image after administration of a contrast enhancing agent.
Fast Spin Echo
- particularly similar to conventional spin echo.
Inversion Recover Sequence
- provides heavily T1 weighted images, is often effective in defining small lesions or the internal structure of lesions where detectability may involve only subtle differences in T1 value
NUMBER OF ACQUISITIONS (NEX)
- referred to as Number of Excitations or Averages. - represents the number of times we collect data for a given phase encoding value and average the information to represent one image.
Extracellular
- refers to the fluid that is not contained in cells (outside the cells)
Signal read-out
- refers to the receiver coils starts to receive the signals.
Moderate
- requiring immediate medical treatment or transfer to emergency department - Dyspnea, bronchospasm, symptomatic tachycardia, symptomatic bradycardia, mild laryngeal edema, hypotension
3DFT Imaging
- start with a basic spin echo pulse sequence and easily convert it into a volume image (3D) sequence by adding another gradient function.
Multi-Slice Spin Echo Sequence
- the dead time between the end echo collection and the next 90 degree RF pulse is utilized for excitation and acquisition of a data line of another slice. - This sequence variation is very trivial and it is particularly due to the fact that adjacent slices are not excited simultaneously
SIGNAL READOUT
- the final step of the pulse echo sequence - where Data Acquisition and Frequency Encoding of the signal occurs.
GAP
- the gap between slices may also affect spatial resolution. - By increasing the Gap between slices, there is a decreased occurrence of the Crosstalk artifact or cross-contamination.
STIR
- the simplest form of fat suppression and is derived from basic inversion recovery sequence. - This results in images with high LESION contrast and conspicuity.
ECHO TIME (TE)
- the time from the original RF pulse to the peak of its reemitted echo. - TE controls the amount of spin dephasing (T2 Effects) allowed to occur before the signal is collected.
VOXELS
- these are three dimensional units from which signal is obtained during the reconstruction process - directly affect the spatial resolution of an image by representing the intersections of hundreds of data lines during spatial encoding.
Magnetization Transfer Contrast (MT)
- tissue contrast can be manipulated such that the measurements are not solely dependent on T1, T2 and Proton density. - commonly observed or applied on ToF (Time of Flight) sequences.
Spectral Pre-Saturation Technique
- use variations in the way the RF pulse is applied and when it is applied.
Water-Fat Suppression Technique
- used in mid to low field MRI, where the precessional difference between fat and water are very slight. - is also called "Sandwich" spin Echo Sequence:
NEGATIVE CM ENHANCEMENT
- when the target tissue darkens in an image following administration of a contrast enhancing agent.
SPIN ECHO IMAGING
- where a 180° RF burst is introduced to the magnetization vector while in free induction decay - The result would be a strong measurable echo of the initial 90° RF pulse
Paramagnetic materials
are used as contrast media in MR imaging to shorten T1 relaxation time, thus influencing the perceived contrast of the resultant image
1. Hydrogen Proton Density 2. Field Homogeneity 3. Gap 4. Voxel Volume 5. Slice Thickness 6. TR 7. TE 8. NEX 9. Matrix Size 10. FOV 11. Artifact Reduction Techniques
11 FACTORS AFFECTING SNR
1. Iron Oxides 2. Antibody Targeted Agents 3. Hyperpolarized Noble Gases 4. Gadolinium Based Contrast Media
4 TYPES OF CONTRAST MEDIA USED IN MRI
1. Voxels 2. Pixels 3. Slice Thickness 4. Gap 5. Matrix 6. Fast Scan Techniques 7. ETL 8. ETS
8 FACTORS AFFECTING SPATIAL RESOLUTION:
- No 90 degree RF pulse means faster longitudinal relaxation so a shorter TR (faster scan) can be used. - No 180 degree RF pulse means fewer RF pulses and allows the use of a short TR with less RF power deposition to the patient. - Short TR provides a quick T2* scan through may be limited in number of slices.
Advantages of GE Imaging
ECHO TIME (TE):
Increasing TE, decreases SNR by allowing more time for tissue vectors to lose phase coherence.
High Magnetization
Bright signal
1. Phase Cancellation and 2. Spectral Presaturation
Chemical Shift Imaging has two primary forms:
REPETITION TIME (TR):
Increasing TR improves SNR by increasing the time that longitudinal magnetization is allowed to recover for all tissues.
Low Magnetization
Dark signal
- T2* contrast rather than True T2 - More work for the gradients and noisier for the patient o The most noisiest knocking sounds of the machine comes for the gradient coils. - Relatively low SNR but high contrast - Unpredictable contrast was initially a problem, but has become something of an advantage now that it's used to give increased sensitivity to hemorrhage.
Disadvantages of GE Imaging
· Pulse Sequence · TR · TE · ETL · ETE · TI · Flip Angle · Relaxation Rates · Hydrogen Density · Flow · Contrast Media
FACTORS AFFECTING CONTRAST
1. Contrast of the image 2. Spatial resolution 3. SNR- signal to noise ratio
FACTORS AFFECTING MRI IMAGE QUALITY
1) STIR 2) Chemical Shift Imaging 3) Phase Cancellation
Fat Suppression Techniques
1) There is no application of 90 Degree RF excitation pulse 2) There is no application of 180 Degree RF refocusing pulse
Fundamental Characteristics of GE Sequence:
EMP
High frequency electromagnetic pulse to shut-off the electricity
1) Fast Spin Echo 2) 3DFT 3) Fat Suppression Techniques 4) Chemical Shifting Imaging 5) Advanced Fast Imaging Techniques 6) Functional Imaging Techniques 7) Magnetization Transfer Contrast
MRI PULSE SEQUENCE (ADVANCED MR PULSE SEQUENCE)
Bioeffects of Time-varying Magnetic Field
Mechanism of Interaction = Induced Current
Bioeffects of Static Magnetic Field
Mechanism of Interaction = Polarization
Bioeffects of Oscillating Magnetic Field
Mechanism of Interaction = Thermal Heating
screening and safety
Most important part prior to any MRI procedure
1. Cardiac Implants 2. Cochlear Implants
NON-MRI COMPATIBLE IMPLANTS
PROTON (SPIN) DENSITY
Produces tissue contrast based on a difference in the concentration of hydrogen protons within the tissues
1. Spin Density Weighted Image 2. T1 weighted 3. T2 weighted
THREE IMAGING WEIGHTS
1. Multi-Echo Spin Echo Sequence 2. Multi-slice Spin Echo Sequence 3. Inversion Recover Sequence o STIR Technique o FLAIR Technique
Spin Echo Sequence Modifications
1) The Spin Echo Sequence 2) The Gradient Echo Sequence
Standard MR Pulse Sequences
1. Mild 2. Moderate 3. Severe
TYPES OF ANAPHYLACTIC REACTION STATISTICS
1. Minimum slice thickness 2. Minimum TE 3. Gradient reversal scans much less sensitive to magnet inhomogeneity and bulk susceptibility artifacts 4. Large numbers of slices can be scanned with short or long TR 5. Raw data set is inherently well suited
Technical Advantages of 3DFT Imaging
o Phase Cancellation o Water-Fat Separation Techniques o Spectral Presaturation Techniques
Techniques in Chemical Shift Imaging:
RELAXATION RATES
The basis of both T1 and T2 relaxation times is thermal motion or the random collision of atomic particles within living tissues.
1. spin density 2. T1 3. T2 4. flow 5. chemical shift
The five main tissue factors that affect the MR appearance are:
Effects on Cardiovascular Function
The increase in T-Wave Amplitude is directly proportional to the intensity of the static magnetic field
PIXELS
are the individual units that collectively comprise the dimensions of the matrix. They are two dimensional units (Length x Width).
Cochlear Implants
Uses a high field strength cobalt-samarium magnet in conjunction with an external magnet to align and retain a radiofrequency transmitter coil on the patient's head
According to Shellock and Kanal
With respect to the ferromagnetic qualities of metallic implants, materials and devices, investigations have generally demonstrated that an MR procedure may be performed safely in a patient with a metallic object if it is nonferromagnetic or if it is attracted minimally by the static magnetic field
SHORT TE
allows minimal loss of transverse signal due to T2 relaxation
LONG TR
allows tissue to reach full longitudinal magnetization
Magnetic fields
are measured in gauss (G), tesla (T) or kilogauss (G), with 1 T equal to 10,000 g
Proton Density/Spin Density
basis is Hydrogen Concentration
T1 and T2
basis is Magnetization
Hyperpolarized Noble Gas
came from the fact that one significant area of the body that contains few protons can and can only be imaged as a region lacking in signal are the airways.
Magnevist
commercially available since 1981 and was first used in humans in 1984
SPATIAL RESOLUTION
defined as the sharpness of an image or its ability to identify small objects and to define clearly edges and boundaries between tissues.
INVERSION TIME (TI)
defined as the time between inverting the 180 degree RF pulse and the 90 degree excitation pulse.
VOXEL VOLUME
determined by slice thickness and pixel size or resolution.
CONTRAST
difference in relative brightness between pixels which is the result of signal intensity received from each voxel during the procedure
T1 weighted
if there is longitudinal relaxation, therefore there is longitudinal magnetization.
T2 weighted
if there is transverse relaxation, therefore there is transverse magnetization
Antibody Targeted Agents
initial report concerning the use of paramagnetically labeled monoclonal antibodies identified with several important realities associated with the implementation of this technique.
STIR (Short TAU inversion recovery) Technique
is a special IR sequence used for tissue signal suppression, most specifically Fat Suppression
Perfusion Imaging
is based on the rapid bolus of contrast media
SPIO (Super-Paramagnetic Iron Oxide)
is to reduce the T2 of Tissues in which it accumulates, causing lower signal intensities on T2- Weighted or T2* - Weighted images post contrast.
T1
its fluid in the brain is darker
T2
its fluid in the brain is whiter
T2 relaxation
losing 63% of original value or loses 63% of transverse magnetization
Gold standard for orthopedic implants
made of titanium (MRI compatible implants), however, this will cause an artifact
MRI image quality
measure of the diagnostic accuracy and appearance of the image
MATRIX SIZE
refers to the number of rows and columns of pixel in an image
T1 relaxation
regaining 63% of original value or regain 63% of longitudinal magnetization
RF ABSORPTION
rise in body temperature is most often reported with exposure to oscillating (RF) fields and is caused primarily from magnetic field induction instead of electric fields,
PULSE SEQUENCE
selected is chosen to intensify tissue contrast based on diagnostic needs. Each pulse sequence has a unique application and benefit for a desired result.
ARTIFACT REDUCTION TECHNIQUES
several artifact reduction techniques and these techniques effectively increase SNR.
Slice Encoding Gradient
simply indicates or points out to the phase encoding gradient to encode and process data of a third dimension (volume or voxel)
SIGNAL TO NOISE RATIO (SNR)
term used to describe the relative contributions to a detected signal made by the true signal and by random superimposed signal (noise).
HYDROGEN DENSITY
the concentration of resonating hydrogen atoms in a given anatomical region. The MR signal in a given tissue is determined in large parts by the hydrogen concentration.
HYDROGEN PROTON DENSITY
the primary source of signal within our imaging volume.
FIELD HOMEGENEITY
the product of the field strength of the system and the measured field homogeneity over a clinically useful imaging volume.
EFFECTIVE ECHO TIME (ETE)
the relative echo time visualized on the image of a fast spin echo sequence
Type II (Antibody Targeted Agents) and Type III (Hyperpolarized Noble Gases)
these are still experimental contrast agents
GAP
used in a particular pulse sequence, cross contamination of slices may occur with a corresponding decrease in SNR
Diffusion Imaging
where MR images the Brownian motion, or random translational motion of molecules in fluid, which is responsible for molecular diffusion.
EXCITATION
where a 90 degree RF pulse is applied in order to flip the spins from longitudinal to the transverse plane
multi-spin echo sequence
where in 1 TR we are able to produce multiple echoes
SLICE THICKNESS
· A change in the slice thickness will affect spatial resolution by increasing or decreasing the volume imaged in 2D FT Imaging techniques.
IMAGE WEIGHTING
· All spin echo images have some amount of spin density, T1 and T2 image contrast
TR = Time of Repetition
· Amount of time that elapses between 90° RF pulses on a given slice. · Generally set in the range of 350 to 3000ms.
T2 WEIGHTED IMAGING
· Desired tissue contrast is produced by looking at the variations in transverse relaxation rates. · This is done by using a LONG TR AND LONG TE sequence
T1 WEIGHTED IMAGING
· Desired tissue contrast is produced by looking at variations in longitudinal relaxation rates · This is done by SHORT TR - SHORT TE sequence
T1 AND T2 VALUES
· Driving force behind the initial diagnostic success of Spin Echo MRI imaging.
SLICE THICKNESS
· Increasing the slice thickness increases the signal volume per image, resulting in a higher SNR ratio.
TE of Time to Echo
· Is the amount of time between 90° RF pulse and the measurement of the signal echo. · Generally set in the range of 10 to 120ms.
CONTRAST MEDIA
· MR compatible contrast agents are commonly used to enhance diagnostic proficiency.
SPIN DENSITY WEIGHTED IMAGING
· Tissues should be fully re-aligned along the main magnetic field for these concentration variations to be seen · This can be done by using a LONG TR- SHORT TE sequence
FLOW
· refers to the hydrogen in motion during the time of acquisition. Examples of flow include blood, CSF, bile and other body fluids.
FIELD OF VIEW
· relationship between matrix and FOV has a definite effect on SNR by determining the size of the individual pixels within the matrix.
REPETITION TIME (TR)
· the time of interval necessary for longitudinal magnetization to occur after RF perturbation (a fancy term for RF disturbance, the event in where the machine emits an RF at Larmor frequency which would result in the excitation of the atoms)
ECHO TRAIN LENGTH (ETL)
· the total number of echoes collected during one TR in a fast scan imaging sequence. Its contribution to image contrast is based on how many echoes are being used to generate one image and where they are collected during the sequence.