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What is Faraday's law of induction?
- If a recieveror any conductive loop is placed in the area of a moving magnetic field, i.e. the magnetization precessing in the transverse plane, a voltage is induced in this recieving coil. db/dt=dv
- b=mag field t=time v=voltage
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These persons should be educated about the effects of the static magnetic field, especially in high field superconducting magnets.
The nursing staff and the code team, the housekeeping staff and fire department, the anesthesiology and respiratory therapy department. Everyone who comes in contact.
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Family members and ancillary personnel accompanying the patient into the scan room must________________________.
be screened as if they are having the procedure themselves.
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Contraindications for using gadolinium include:
None
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Precautions for the use of gadolinium include:
Pregnacy, Low GFR, Prior contrast reactions, Pt w/ hx of asthma or allergies.
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Passive magnetic sheilding can be achieved by ling the MR room with:
Steel plates
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The proceesional frequency of a H element at 1.0 T is
- 0.5T = 21.28
- 1.0T = 42.57
- 1.5t = 63.86
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Ohm's law
- Basic law of electricity
- V = IR
- V voltage
- I current
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Larmour Equation
- Wo = Bo x y
- Wo The processional (resonant) frequency
- Bo Stength of magnet
- y Gyromagnetic ratio (for H) 42.52MhZ/T
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Scan Time
TR x #of phase encodings x NEX X #of slice encodings
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2 types of shielding
- Passive = lining room with steel
- Actve = any additional super conducting coils
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Signal to noise ratio
- S/N is one measure of image quality. It is the ratio of the amount of signal that is useful to the image to the amount of noise inherent to the background (equipment and patient) noise is a random factor
- Long TR, Short TE, Good S/N, arge FOV
- Thick slices, Many NEX, currect coil, Spin echoe sequences
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Ways to increase SNR
- Use SE or FSE pulse sequences
- Long TR short TE
- Flip angle of 90 degrees
- Course matrix 256x128
- Large FOV
- Thick slices
- Lowest RBW
- Highest NEX/NSA
- Well tuned and correctly sized coil
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Slew Rate
- Strength of the gradient over distance
- Typical slew rate= 70mTm/s
- High speed gradients slew rate = 120mT/m/s
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Ways to increase spatial resolution
- Thin slices
- fine matrix 256x256
- smaller FOV
- Selecting rectangular FOV/ asymetric FOV
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Spin Echo timing parameters
The time between the first 90 degree pulse and the peak of the spin echo is called th TE. The time between the 90 degree and 180 degree RF pulses is exactly 1/2 TE is called the tau
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FDA limit of the static field
8 T for adults and children over 1 month
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Factors that effect SNR
- PD of area under examination
- Voxel, volume, matrix/ FOV
- Nex/ NSA
- Reciever band width
- Coil type
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2D non FSE Scan time T1 Spin Echo
- TR x Phase Maix X NEX( NSA)
- For a T1 spin echo sequence w/ 500 msec TR, 256 phase matrix and 2 NSA
- 500 x 256 x 2 =256,000 msec
- 256,000msec/1,000 sec= 256 sec
- 256sec/1000 60 (min)= 4:27
- for 2D FSE same formula divided by ETL
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Spatial resolution
- The ability to distinquish between 2 points as separate and distinct. It is controlled by the voxel size
- Small voxels= good spatial resolution
- amsll structures can be easily differentiated
- Large voxels= poor spatial resolution (individual signal intentsities are averaged together and not distint wiyhin a voxel) patial voluming
- Thinner slices = good spatal resoluion
- Larger FOV = poor spatial resolution
- Larger matrix = low SNR= More scan time
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Slice Select Gradient
Transmits RF to specific location to excite nulcei @ that location only wiht it's specific frequency
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Phase Encoding gradient
Usually switched on just before 180 degrees rephase pulse. Locating a signal according to its phase alters the phaase along the remaining axis of the image, usuallynalong the short axis of the anatomy
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Frequency Encoding Gradient
- Usually located along the long axis of the anatomy and switched on in the middle of rephasing/dephasing of signal.
- On during signal collection
- AKA readout gradient
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Spin Echo
- Defined as a signal that results from:
- rephrasing of a free induction decay (FID) T2*.
- Application of a 180 degree rephrasing RF pulse.
- Reversal of T2 effects (phase) in coherance
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Stir
- The signal from any tissue can be suppressed if thr TI selected is 69% of the revery time of that tissue.
- Long TR Short TE Long TI.
- Do for bone frctures, bone cancer these will show bright in MS and trauma also
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