MRI Flashcards
What is the Larmor Equation?
Precession Frequency= Gyromagnetic ratio X Field Strength
How do protons line up when you enter a MRI?
In the Longitudinal direction
What is T1?
- Recovery of Longitudinal relaxation (Spin-Lattice)
- Involves the exchange of thermal energy
- Things are T1 relaxed when they reach 63% of final value
- Greater the Field strength, the longer the T1
How does T1 relaxation changes with magnetic field?
-Higher magnetic field= Longer T1 relaxtion time bc the protons are precessing faster (larmor equation)
What is T2?
- Decay of Transverse relaxation (Spin-Spin)
- T2 relaxed when decays 63%
- Decay occurs due to inhomogenieties of external field and internal fields
What is T1 and T2 bright?
- T1 short= Bright
- T2 Long= Bright
What is T2 Star?
- Due to external field inhomogenities
- Always FASTER than T2
What is repetition time (TR)?
-Time between two RF pulses
When do you give 180 RF Pulse?
-1/2 TE
What is Short and Long TR and TE for Spin echo?
- Short TR: 250-700 ms
- Long TR: >2000 ms
- Short TE: 10-25
- Long TE: >60 ms
What is Short and Long TR and TE for Gradient echo
- Short TR <50 ms
- Long TR >100 ms
- Short TE 1-5 ms
- Long TE >10 ms
What does PD do?
- Long TR and Short TE (Excellent SNR)
- Minimizees the difference in contrast between T1 and T2
What is the Fourier Transform?
Converts from the Time Domain to the Frequency Domain
What is K Space?
- A fourier Plane with MR Signal is stored
- To convert to image= 1/2 Fourier Tranforms
- Center of K space has contrast information
- Peripheral K Space has spatial resolution information
What determines the duration of Study?
TR X Number of Phase encoding Steps X Number of Excitations
What is Frequency Encoding gradient applied?
Same time as read out
What is Slice Selection Gradient (SSG)?
- Selects the area of interest and determines the view
- Perpendicular to the desired slice plane
How to do you create a Spin Echo Sequence
- Select SSG and apply 90 RF Pulse
- Apply phase and frquency encoding
- Apply 180 RF pulse and SSG
- At Echo, apply the frequency encoding
How do you select for a thinner slice?
Thinner Slicer= Thin Transducer Bandwidth = Large Slice Selection Gradient
What determines the duration of a 3 D Study?
TR X Number of Phase encoding Steps X Number of Excitations X Slice Thickness
What determines study time for Fast Spin Echo Sequence
1/Echo Train Length
How does TR affect number of slices?
Longer TR= Can Select more Slices
What is Phase Matrix?
- Number of Phase Encoding steps (lines you need to fill)
- 1 Line= 1 TR
What is Number of Excitations?
- Number of times you sample the data
- More= Better image quality but more scan time
How do you Increase SNR?
- Higher Magnet
- Increase TR
- Decrease TE
- Large Pixel Size (Large FOV and Small Matrix)
- Increase slice thickness
- Increase Transmit Bandwidth
- Decrease receiver bandwidth
- Decrease (Small) SSG
- Small RF Coil
- Increase Number of Excitations
How do you improve spatial resolution?
- Small FOV
- Large Matrix
- Thinner Slices
- Thinner Transmit bandwidth
- Steeper SSG
What is Fast Spin Echo?
- Applying Multiple 180 RF pulses per 90 RF pulse (get multiple echos)
- Echo train length= Number echos per RF Pulse
- Acquisition Time= 1/ETL
- T2 Blurring= Loss of transverse magnetization with each echo
- Longer T2 FAT signal due to J coupling
What is inversion recovery?
- Give 180 Pulse first, then 90 RF at the null point, and then another 180
- TI= Inversion time between first 180 RF and 90 RF
- STIR is less susceptible to artifact and you CANNOT use GAD with it
- Increased scan time due to increase TR from addition of TI
What is GRE?
- Uses flip angle less than 90 and No 180 (makes it at T2*)
- Short TR/TE with fast recovery and shorter scan
- Lower Specific absorption Rate (SAR)= less heating
What is Echo PLanar Imaging?
- Fastest way to acquire sequence (DWI)
- One 90 RF and one 180 RF pulse, then turning Phase and frequncy encoding on and off very frequently= fills K space very fast
- very susceptible to Susceptibility artifact
How do you fat sat?
- Give a prepartory RF pulse narrow pulse at the resonance of fat
- Add spoiler gradient that dephases the protons primed by RF pulse (no long able to produce signal)
- Procedd with normal scan
What is In and Out of Phase?
- Drop out of microscopic fat signal on out of phase
- 1.5 T: Out @ 2.2, In @ 4.4, out @ 6.6
- 3T: Out @ 1.1, In @ 2,2, Out @3.3
- Set set of in/out matters for Liver hemochromatosis
What is Type 1 Chemical Shift?
- Bright on one side and Dark on the other
- Occurs on SE and GRE
- Occurs in the Frequency encoding direction
- Increase with Higher Magnetic field strength
- Decrease with increased gradient strength and WIDER readout bandwidth
What is Type 2 Chemical Shift?
- India ink artifact that occurs on Fat/Water interaface
- Fix by switching to SE sequences
What is Aliasing and how do you fix it
- Undersampling
- Switch the Phase and Frequency encoding direction (more time)
- Increase FOV (Loss of spatial resolution)
- Saturation bands and Surface Coils
How to you fix Type 1 chemical shift artifact?
- Switch Phase and Frequency encoding direction
- Increase RECEIVER BANDWIDTH
- Use STIR or fat suppression
How do you fix Type 2 Chemical Shift Artifact?
- Shorter TE
- Use Spin echo instead of GRE
How do you fix partial volume averaging?
- Smaller Pixels
- More Slices
What is Truncation/Gibbs Artifact?
- Ripples in data when transferring from the K Space through inverse fourier transform
- Seen at high contrast interfaces (CSF and Cord)
- Seen in Phase and Frequency encoding but more common in Phase.
- Better with Larger Matrix, Decreased pixel, and decreased bandwidth.
How do you fix motion Artifact?
- Breath Holding
- Respiratory Gating (increases acquisition time)
- ROPE (Respiratory Ordered Phase Encoding)
- Breathing Navigator: Echo from the Diaphragm that helps to time images
- Switch phase and frequency encoding direction
- BLADER/PROPELLER: Over sample the center of the K space so you have redundant material
- Apply fat sat band to the abdomen
What is magic angle artifact?
- loss of signal at 55 degrees on short TE sequences (T1 and GRE/PD)
- resolves with T2 and higher magnet strength
What is cross talk and how do you fix it?
- Overlap from non ideal slice profiles
- Improve with interleave (odd then even) and increasing gap between sections
- Does not occur with 3 D
What is zipper artifact?
- Stray RF Signal
- Improve by fixing RF shielding, closing door, taknig electronics out of room
What is inhomogenous fat sat due to?
- Fat prescesses at different frequencies so may not be suppressed
- Switch to STIR
- Shim the magnet (Passive done at installation, Active done wth a coil after aech sequence)
What is Susceptibility?
- Ability to be magnetized by an external field
- More severe with GRE than SE
- Fix by switching to SE, reduce field strength, thinner slices, Align longitudinal axis of metal with the axis of the main field.
- Worse with longer scan times (In versus out of phase0
What are Eddy Currents?
- Due to rapid turning on and off the gradient
- Seen on DWI as smearing at the edges
What is the dielectric effect?
- Tissues have inherent dielectric constant that results in reduction of wavelength by some inverse constant which result in eddy current.
- Worse in Fat People, ascites and stronger magnet
- Ex: Dark signal over center of liver.
- Fix by applying dielectric pads or Parallel RF transmission with each coil sending an independent RG Pulse
What is crisscross/herringbone artfact?
-Error in Data processing or reconstruction
What is signal flair (breast)?
-Breast positioned to close to the coil
What is the 5 G line?
Implantable devices will be OK outside the line
What is the limit on Noise for MR?
140dB for MRI
What is neurostimulation?
- Induced Electric currents that lead to painful neural stimulation
- Due to High Receiver Bandwidth and Switching Gradients quickly
- Reduce reader bandwidth and Increase TR
What is Specific Absorption Rate (SAR)?
-Ability to cook a patient with a RF Pulse
=Field Strength^2 X Alpha ^2 X Duty Cycle
-Field Strength of Magnet
-Alpha is the Flip Angle
-Duty cycle is the inverse of TR
FDA Limits: 4W/kg over 15 min or 3W/kg over 30 min
What are Weekly QC for MRI scanners?
AC FGH STV
- Artifact Analysis
- Center Frequency
- Film Quality Control
- Geometric Accuracy
- High and Low contrast resolution
- Set up and scanning
- Table Positioning
- Visual Checklist
What are Annual QC for MRI scanners?
MD SSR
- Magnetic Field Imhomogenities
- Display monitor Checks
- Slice Thickness Accuracy
- Slice Position Accuracy
- Radiofrequency Coil Check
What are the 4 MRI Zones?
1: General Public
2: Waiting room, where you get screened
3. Restricted, control rooom
4. MRI scanner Room
What is a spoiled Gradient Echo?
- Gradient used to get rid of transverse magnetization
- Shorter TR (basically T1)
