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.