8: Special Procedures Flashcards
MRA: Magnetic Resonance Angiography
Gradient echo based imaging technique used to visualize blood filled structures such as arteries and veins by reducing signal of stationary tissue
Images the flow of blood and creates contrast between surrounding tissue and blood vessels
Three types of MRA:
- Time of flight
- Phase contrast
- Contrast enhanced
Time of flight MRA (TOF)
an application of gradient echo sequences placed perpendicular to flow of blood.
Phase contrast MRA (PCA)
the use of phase information from high velocity blood flow with complete suppression of stagnant tissue
relies on the velocity of precessing proton along the gradient direction
Contrast Enhanced (CE-MRA)
imaging after the injection of gadolinium. T1 images are then taken resulting in T1 shortening in contrast enhanced tissue
TOF relies on:
Flow phenomenon
Flow Phenomenon:
technique that visualizes precessing protons entering into an imaging plane
During TOF (Time of Flight MRA): Precessing protons appear (bright or dark) and background anatomy becomes (black or white)
bright, black
How can you limit venous flow during TOF MRA?
saturation bands are placed outside FOV to de-phase venous glow from being sampled.
TOF can be achieved by:
2D slice or 3D volumetric placement perpendicular to the flow of blood.
2D TOF is ideal for:
longer vascular anatomical structures
3D TOF is ideal for:
compacted, detailed areas of vascularity
Any TOF MRA ABOVE the heart will need a:
superior sat band
Any TOF MRA BELOW the heart will need a:
inferior sat band
Any TOF MRV ABOVE the heart will need a:
inferior sat band
Any TOF MRV BELOW the heart will need a:
superior sat band
PCA differs from TOF by:
by applying two sequential phase encoding pulses to both stationary tissue and moving protons
In PCA, tissue that is stationary will experience:
both applications thus canceling signal presence
VENC:
Velocity encoding: the expected flow of blood being sampled in a specific ROI.
In PCA, moving blood will:
will return signal creating contrast between blood and stationary tissue
4 Types of blood flow:
- Laminar
- Spiral
- Turbulent
- Vortex
Laminar flow:
nonturbulent streamline of flow
Spiral flow:
movement in one direction in a spiral fashion
Turbulent flow:
laminar flow obstructed by a pathological presence resulting in fast redirection and randomized flow patterns. (pathology presence)
Vortex flow:
laminar flow obstructed by a pathological presence resulting in a regurgitated flow pattern (like a U)
Diffusion Weighted Images (DWI)
measures Brownian motion (b-values) of water molecules within a voxel/pixel.
Diffusion Weighted Images (DWI) is used for Clinical diagnosis of: (3)
- Confirmation of an acute or chronic stroke within the first 48 hours of onset
- Multiple Sclerosis
- Concussions
Diffusion as a Cellular Level:
measuring the rate of molecule movement (or diffusivity) in and out of the cellular space, we can identify if the histology is healthy or not.
Facilitated Diffusion
demonstrates the increase movement of water within a particular voxel, usually implies high diffusivity rates or lack of cellular obstruction.
Tissue with higher amount of cellular space creates more space for movement. dark
Restricted Diffusion:
Black or White?
describes abnormally decreased diffusivity within tissue compared to normal tissue movement. Pathology such as a stroke or mass will result in limited space within the voxel begin scanned due to cell swelling. this results in less movement of water within a voxel compared to healthy tissue in another voxel. (swelling, stroke, mass) white
B-Values
manually chosen parameter that controls gradient strength and time
Adjustment of the b-values is (directly or indirectly) related to the appearance of the images diffusion weighting
directly
Optimal routine b-value
B=1000
Increase in b-value = CSF enhancement and SNR
increased b-value = - CSF - SNR
Decrease in b-value = CSF enhancement
and SNR
decrease in b-value = + CSF + SNR
ADC Mapping:
Usually a combination of B=0 and B=1000 values
After post-processing, clinicians are about to evaluate and confirm diffusion of water by measuring specific ROI’s within tissue thus confirming diagnosis
Ischemic stroke on DWI
elevated signal: implies low diffusivity of water molecules suggesting presence of obstruction
Ischemic stroke ADC
decreased signal: sodium concentrated cells begin to swell reducing diffusivity, the presence of larger cells creates obstruction resulting in dark signal and true restricted diffusion.
Perfusion:
the process of blood being delivered to other tissues in the body
Perfusion MRI:
the ability to differentiate grading of metastatic and vascular diseases
3 Techniques of Perfusion MRI:
- Dynamic Susceptibility Contrast (DSC)
- Dynamic Contrast Enhanced (DCE)
- Arterial Spin Labelling (ASL)
Dynamic Susceptibility Contrast (DSC)
Evaluates T2* signal loss after gadolinium injection in the capillary vessels. Parameters such as rCBV, rCBF and MTT are analyzed
Dynamic Contrast Enhanced (DCE)
Evaluates T1 shortening parameters after bolus gadolinium interaction
Arterial Spin Labeling (ASL)
Identifies cerebral blood flow spins by magnetically labeling. ASL is the only perfusion study that does not utilize contrast
Which is the only perfusion study that does not utilize contrast?
Arterial Spin Labeling (ASL)
MR Spectroscopy
imaging technique to evaluate the histology (chemical makeup) of abnormal tissue allowing prediction of severity for prognosis
Chemicals evaluated (5) in MR Spectroscopy
- Myo-inositol
- Choline
- Creatine
- NAA
- Lactate
Dynamic Imaging
Beginning with a full MR image (contrast and resolution data) dynamic studies follow with continual CONTRAST ONLY image acquisition detailing the contrast changes on a specific body part in a short scan time
allows to “wash in/wash out” effect of contrast in a particular area of tissue
MR Prostate MR Pituitary
Fluoroscopic triggering: CINE
contrast imaging method which allows techs to view “fluoro-like” live gradient images to view the in-flow of contrast in a vessel of interest
Timing Bolus Method
contrast technique were small amounts (1-3 cc) of contrast is injected with rapid gradient pulses until contrast is present in area of interest.
The time it takes from injection to contrast presence is called TIMING BOLUS
Automatic (Bolus) Triggering
when a trigger marker is placed over the region of interest, this detects the increase in signal intensity from contrast and begins to automatically scan.
What parameter will yield in better visualization of arterial walls in a phase contrast MRA?
VENC
