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what is laminar flow?
blood flow that has a parabolic flow profile, in which the velocity of protons in the center of the vessel is greater than the velocity of the protons moving adjacent to the vessel walls
what is turbulent flow?
when the blood is not moving in smooth streamlines, and is actually moving in more than one direction. It is often found in tortuous sections of vessels and distal to bifurcations and stenoses
what is flow void?
when the net magnetization of the flowing protons is phase shifted, the signal generated by NM of protons is mismapped and placed elsewhere along the phase encoding direction of the image. When this occurs, the vessel on the resulting image often appears dark, as though it has no signal.
what is inflow enhancement?
application of flow compensation which replaces the dark signal void with a bright signal. This ensures that the phase shift of moving protons is eliminated and therefore placed in the appropriate location
why is blood so much brighter than the surrounding stationary tissue?
because protons within the blood that have recently entered the slice have not yet received any disturbance by the RF energy compared to the surrounding stationary tissues. These inflowing protons enter the slice with their full longitudinal magnetization. Since the blood is moving and has not received partial saturation because it has not been influenced by the RF energy, it has a much larger magnetization than the stationary surround tissues which have become partially saturated by the RF energy
what type of sequence is used for time of flight MRAs?
2D or 3D gradient echo sequence
what happens during a time of flight MRA sequence?
a set of base images, maximizing the inflow effect, must be acquired. Gradient moment nulling or flow comp must be applied in order to rephase the signal from moving protons
what happens once the base images are collected?
they must be processed to genearate a projection image for full visualizatioin of the vasculature which is called maximum intensity projection (MIP)
what is Maximum intensity projection (MIP)?
its responsible for projecting the brightest pixels from an anatomical stack of 2D or 3D base images, onto a plane, to generate an image of the projected view of the vessels of interest
what is a 2D time of flight MRA
is accomplished with a 2D gradient echo pulse sequence that suppresses that stationary tissue while maximizing the blood inflow effect. The RF energy selectively disturbs the protons in one slice at a time. Gradient moment nulling is applied. Gets mipped
what is 3D time of flight MRA?
accomplished with a 3D gradient echo pulse sequence. Gradient moment nulling must be applied. The RF energy disturbs the protons in an entire volume of anatomy and the volume of anatomical data is divided into slices via an additional phase encoding process. The signal of flowing blood for this exam tends to lose its signal more because it gets saturated alot due to the long distance it has to travel with the amount of volume that is chosen with a 3D sequence.
which blood flow is brighter, in a 2D TOF or 3D TOF?
2D ToF. Because the blood only travels a thinner slice compared to a 3D Tof where a blood travels through a thicker volume. When blood travels through a thicker volume it has more of a change of getting saturated with RF pulses than blood traveling through a thinner slice in a 2D where it does not get saturated as much
3D time of flight angiography is generally reserved for which type of studies or anatomy?
- vessels with faster flow, for tortuous vessels where fine resolution is required and for vessels where a limited distance of coverage will suffice
- circle of willis and the limited section of the carotid arteries at the bifurcation
If you still want to use a 3D time of flight sequence but still concerned that the flow may be a bit too slow to traverse the length of the volume before becoming saturated, what type of sequence can you run?
- 3D multi-volume MRA
- multiple, smaller 3D volumes
- once all of the regions are imaged, they need to be combined and Mipped
what type of artifact do you get with a 3D multi-volume MRA?
- venetian blind artifact
- lines seen in the projection images at the intersection of the small volumes
what is spatial saturation with MRA?
addition of saturation pulse to the MRA pulse sequence to eliminate the inflowing blood from the irrelevant vessels to limit the study to either the arteries or the veins
what is phase contrast angiography?
- dependent upon the phase shift in the MR echo that is induced by the flowing blood as it moves through equal and opposite gradients
- sensitive to a specific flow velocity and the velocity of flow varies greatly from one vessel to another.
- Velocity-encoding value refers to the amplitude of the gradients and specifies the flow velocity which will generate the maximum signal intensity in the images
what's one drawback of phase contrast angiography?
- more sensitive to motion than time of flight angiography
- increased scan time
what are the benefits of contrast enhanced MRA?
- superb contrast between vessel and stationary tissue
- the ability to collect the few necessary slices in a single breath hold (important for abd studies)
- true depiction of the vessel lumen in regions of turbulent flow
what is the b -value or b-factor for diffusion sequences?
- is dependent upon the amplitude, duration and spacing of the diffusion gradients. when a b value of 0 is selected, the diffusion gradients are off and the result is a conventional T2 image. as the b-factor is increased, the sequence becomes more sensitive to lower diffusion coefficients
- b-values are in range of 500 s/mm^2 to 1200
what is an ADC image?
is a qualitative representation of the apparent diffusion coefficient. ADC images are desirable because they are solely based on changes in the ADC, whereas DW images are based on both changes in the ADC and T2-weighting. Thus, sometimes on DW images long T2 tissues can mimic regions of reduced diffusion called T2 shine through. ADC images do not suffer from T2 shine through
about how long does it take before the apparent diffusion coefficient of the damaged tissues return to about the same as the surrounding tissue?
- 1 week to 10 days
- the ADC of damaged tissues actually increases with a longer time in between
- this allows us to differentiate between old and new stroke
how does perfusion scan in mri work?
- inject contrast about 5cc/sec
- a multi-shot 3D EPI sequence is used. Will get about 25-30 slices in 2 secs
- images are postprocessed to evaluate the volume occupied by the vascular system per gram of tissue, the amount of flow per gram of tissue, the time of arrival, the time to peak, and the mean transit time.
- Will look at the regions that have a delayed flow which points to regions that are at risk for stroke,
how does functional mri work?
looks at the increase in oxygen in the blood due to movement of a finger, visual perception, language or memory
how does spectroscopy work?