Physics Flashcards
What is the TR range for T1 images?
400-750 ms (1.5T)
600 - 850 ms (3T)
Describe Faradays Law
Moving magnetic field passing through a coil of wire will induce current to flow in that wire.
What does TR control?
T1
What does TE control?
T2
⬇️ TR and ⬇️ TE =
T1 weighted image
⬆️ TR and ⬇️ TE =
PD weighted image
⬆️ TR and ⬆️ TE =
T2 weighted image
Describe TE (time of echo) Also known as 2 Tau
Time of echo - from 90 degree RF to center of echo
Why do small coils have higher signal-to-noise ratio?
Smaller coils see less NOISE. Signal is NOT increased but SNR goes up with less noise.
What are the two types of shims?
- Hard/passive shim
- Electronic/active shim
Which shim is permanently placed during install process to shim the magnetic field?
The hard/passive shim
What shim uses specialized electromagnets to correct the magnetic field?
Electronic/active shim (must be checked with each p.m.)
Describe (FID)
Free Induction Decay
(aka T2*)
Dephasing of protons in the
x/y plane. This is the first step of tissue relaxing.
Protons are FREE from the INDUCING energy of the RF, so they DECAY.
The T1 time of a tissue is the time it takes for ____ of the NMV to return to Bo.
63%
The T2 time of a tissue is the time it takes for the NMV to decay down to _____ of its original value.
37%
What is a Faraday cage?
RF shielding: Copper sheets that line all MRI rooms to keep RF in and outside RF out.
What is the Larmor Frequency?
How fast the Hydrogen atoms wobble (“precess”). Wobble is always 42.57 MHz!
What is the Larmor Equation?
Wo = yBo
Wobble/frequency = (gyromagnetic ratio)(magnet)
Ex:
Wo= (42.57) (3T) = 127.71 MHz
What is the difference between T1 and T2 relaxation?
T2 is a decay process - protons spread out into X/Y.
T1 is a re-growth process - protons return to Bo.
What are 2 types of T2 relaxation?
Spin/Spin (True T2) - recover at the next 90°
T2* - recover at 180° or gradient reversal.
Name 2 types of resolution.
- Spatial resolution - ability to see Small Structures.
- Temporal resolution- imaging quickly over Time
What is T1 aka?
T1 Recovery or Spin Lattice
What is T2 aka?
Spin Spin Relaxation, True T2, spin spin interaction
Who created the electromagnet?
Serbian inventor, Nikola Tesla
Define gauss
The measurement of the magnetic field strength in the periphery of the magnet scan room.
What are the primary contributors to noise in an image?
The patient and system electronics
What is the FDA SAR restriction?
Whole body limit of
4W/kg for 15 min of scanning per sequence.
Define Magnetophosphenes
Visual light flashes induced by gradient field strength.
What is the function of the gradient magnetic field?
To create image orientation and spatial resolution (detail)
What are the 3 gradients used in MRI?
Slice select gradient
Frequency encoding gradient
Phase encoding gradient
What is the function of the Slice Select gradient?
Determines:
Slice thickness
Scan plane
Orientation of pulse sequence
What is the function of the Frequency/Readout Encoding Gradient?
Encodes the long axis of anatomy.
Does not contribute to scan time but is on during sampling of echo.
What is the function of the Phase Encoding Gradient?
Shapes images detail
Scan time
Switches on/off to fill Kspace
Formula: Spin Echo Time
TR x Phase x NEX = ms
(Convert to minutes: Divide ms by 60,000 then take seconds and multiply by 60)
Two gradients applied at the same time during slice selection are used to _____.
Encode oblique slice planes
An isotopic voxel refers to a ____ shaped voxel.
Cubic
A gradient is defined as:
A small change in the magnetic field along a particular axis.
Where are the collected echos stored prior to being processed into an image via Fourier Transform?
Raw data file on the computer.
Raw Data ➡️ Fourier ➡️ Kspace
What effect does a narrow receiving bandwidth have on susceptibility (metal) imagine?
Increases susceptibility artifact. Never use narrow bandwidth on metal implants.
Define Ernst angle
The optimal flip angle that yields the maximum signal for a particular spin in the least amount of time.
As flip angle increases, what happens to SNR?
SNR increases to a point called the Ernst angle.
The general public is limited to magnetic field of ____ gauss.
5
Heart valves are considered acceptable to scan at ____.
1.5T
Sedated patients must always be monitored with ______
A pulse oximeter
Normal resting heart rate for adults:
60 to 90 beats per min
Tachycardia
Heart rate exceeding 100 beats/min
Bradycardia
Heart rate below 60 beats/min
Normal pulse rate:
60-100 beats/min
Pathogen
Any disease producing agent, esp a virus, bacterium or microorganism.
Etiology
The origin of a disease
Enteral
Oral, sublingual or rectal delivery
Parenteral
Any route other than mouth (PO) or rectal
Nosocomial infection
Infection acquired during hospital stay or receiving medical treatment
Fomite
Object that carries microorganisms between people via indirect contact
Vector borne infections
Microorganisms transferred via insect. Ex malaria
The source of infection where pathogens thrive in numbers sufficient to cause a threat is known as ______.
Reservoir
What is the TE range for T2?
70-120 ms
What is TR range for T2?
2000-6000 ms
What is TR range for PD?
1500-3000 ms
What is TE range for PD?
10-30 ms (same as T1!)
FOV % Matrix =
FOV % Matrix = pixel size
Where is the storage location of MR signal data?
K-space
Where are high amplitude signals stored in Kspace?
Center
Describe Half Fourier / Zero Fill
Kspace: Acquiring more than half of PHASE dimension kspace samples, then interpolating the data with 0s for the other half.
