Topic 26: MRI pulse sequence + flow imaging Flashcards
(37 cards)
Describe RF coils
- Consist of loops of wire
- When current through = magnetic field at 90° to B0
- Configuration of RF transmitter + receiver + coil = directly affect quality of MR signal
Give types of coil in MRI
1) Transmit/receive
2) Surface coils
3) Phased array coils
4) Volume coils
Describe transmit coils
- Energy transmitted at frequency of H
- Short + intense RF = RF pulse
MAIN COIL: - Body coil = within bore of magnet itself
- Head coil
Describe receive coils
- RF coils in transverse plane = generate voltage within when moving MF cuts across loops of wire
- Voltage = MR signal sampled to form image
- Induce MR signal = transverse magnetization occur perpendicular to receiver coils
What is the use for surface coil?
- Improve SNR
Describe surface coils
- Used to improve SNR
- Nearer coil to structure = greater SNR
- Small + shaped
- SNR enhanced using local coils = greater spatial resolution of smaller structure
- Local coils = body coil transmit RF + receive MR signal
Give surface coil applications
1) Musculosketetal = detect fractures + evaluate soft tissue + diagnose arthritis
2) Neuroimaging = visualize superficial near skull/spine
3) Breast imaging = high-res images of tissue
4) Cardiac imaging = cardiac anatomy + myocardial function + vascular anatomy
5) Abdominal + pelvic imaging = liver + kidney + pancreas + pelvis
Describe volume coils
- Surround whole body/specific region
- SNR = image less than surface/phased coils
- E.g. body + head coils
- Main coil = body coil = magnet bore
Give volume coil applications
1) Whole body imaging = uniform FR excitation + reception for comprehensive imaging
2) Abdominal imaging = liver + kidney + pancreas + spleen + GI tract
3) Pelvic imaging = bladder + uterus + ovaries + prostate + pelvic bone
4) Thoracic imaging = lungs + heart + mediastinum + thoracic spine
5) Angiography = larger vessels arterial + venous imaging
6) Large field of view study = whole spine/brain
Describe phased array coils
- Multiple coils + receivers = individual signals combined = 1 image
- Improves SNR + increases coverage
- Advantage of small surface acoils combined + large FOV = inceased anatomy coverage
- Increase longitudinal coverage
- Improve uniformity across whole volume
Give the phased array coil applications
1) Musculoskeletal = joints + bones + muscles + ligaments = sensitivity + spatial resolution
2) Neuroimaging = vascular structure
3) Cardiac = myocardial function
4) Abdominal + pelvic = diagnosis of abdomonal pathologies
5) Dynamic imaging = blood flow + cardiac motion + joint movement = high temporal resolution
Describe pulse sequence
- Series of RF pulses + gradients applications + intervening time periods
- Controls way system applied RF pulse + gradients
- Selecting intervening time periods = image weighting is controlled
Describe conventional spin-echo pulse sequences
- Used to produce T1/T2/proton density weighted images
- Most basic pulse sequences in MRI
- 90° excitation pulse + 180° rephasing pulse + echo
Define spin echo
- Regenerated signal = echo
- Regenerated by RF pulse = spin
- Rephasing NMV eliminates effect of MF inhomogeneties
What is the use for CSE?
- Single spin echo pulse = single 180° RF pulse applied after excitation pulse = single spin echo
- Typical sequence = produce T1 weighted images
- Dual echo sequence = 2 180° RF pulse applied = 2 spin echoes = 2 images per slice location = 1 proton density + T2 weighted
Describe fast spin echo
- Faster version of spin echo
- Multiple 180° rephasing pulses = each 1 spin echo
- Typical = 2-30 180° RF pulse applied every TR
- Number of 180° RF pulses + resultant echoes = echo train length = turbo factor
- Echo spacing = spacing between each echo
- Use = brains + spines + joints + extremities + pelvis
Describe the turbo factor
- Short = decreases effective TE = increases T1 weighting + increases scan time
- Long = increases effective TE = increases T2 weighting + reduces scan time
- Image blurring increases with turbo factor
Describe inversion recovery
- Spin echo sequence = begins at 180° inverting pulse
- Time interval = TI = main factor controling weighting in IR sequences
Describe IR images
- Heavily T1 weighted
- Large contrast between fat + water
Describe TI
- TI values = suppression of signal from tissue
- TI needed to null signal from tissue = 0.69 x T1 relaxation time of that tissue
- IR sequences divided based on TI values
Give the TI values
1) Short = 80-150 ms
2) Medium = 300-1200 ms
3) Long =1500-2500 ms
Describe STIR
- Short TI inversion recovery = use shory TI
- TI of this magnitude = 90° excitation pulse at time NMV of fat passing through transverse plane
- No longitudinal fat component
- 90° excitation pulse = no transverse component of fat = no signal
- Fat suppressed image created
Describe FLAIR
- Fluid attenuated inversion recovery = long TI
- Done to null signal from CSF = same as STIR sequence
- CSF = long T1 recovery = TI longer to corresponsd to null
Describe inversion recovery as a sequence
- Very versatile
- Use = CNS + musculoskeletal system
- FLAIR = increases visibility of periventricular lesions + cervical/thoracic cord lesions
- STIR = null signal from normal bone marrow = increase visibility of bone lesions
