Bolus MR/CT Flashcards
Define perfusion
The volume of blood delivered to the capillary bed of a block of tissue in a given period of time
What is the units for perfusion?
ml of blood per 100g of tissue per minute (or just ml/100g/min)
What does perfusion equal to?
Cerebral blood flow
How is perfusion distinct from bulk flow of blood?
Through arteries/veins
What is perfusion?
Flow at the capillary level, where exchange of nutrients between the blood and tissue occurs
Why measure perfusion?
Changes in blood supply are intimately linked with many disease processes?
- Stroke (loss of blood supply to regions of the brain)
- Tumours (angiogenesis)
- Vascular dementias
Fmri
- Increased perfusion reflects changes in neuronal activity
Why use MRI to measure perfusion?
- Non-invasive
- Good spatial resolution
- Can be used in conjuction with other MR techniques such as structural imaging, diffusion -weighted imaging, MT, 1H, 31P spectroscopy
- Wide availability of MR scanners
What is angiogenesis?
growth of new blood vessels, creation of more vessels because tumour is energy hungry
What can perfusion be used to look at?
functional activation because when neurons become active due to a functional or cognitive task, their energy demands go up and in order to satisfy increased energy demands, perfusion/blood flow also increases
- Increased perfusion reflects changes in neuronal activity
MR methods for perfusion imaging: exogenous contrast agents
Known as Dynamic Susceptibility Contrast (DSC) or Bolus Tracking MRI
Based on the injection of a bolus of paramagnetic tracer which affects the signal in an MR image as it passes through the tissue
Requires fast imaging to track the passage of the bolus accurately
Typical tracers: gadolinium (Gd), dysprosium (Dy)
How do you acquire T2/T2* images?
- As gadolinium passes through the vasculature, it stays within the blood vessels then causes a decrease in T2/T2*
- If we are acquiring a series of images with T2/T2* weighting – when the gadolinium is present – there is a large drop in signal intensity – the vasculature is full of gadolinium – reduces T2/T2* - much reduced signal intensity
What is the consequence of T2/T2*?
Signal loss in T2/T2* - weighted images
when agents stays intravascular
What is the mechanism for signal loss in DSC-MRI?
• When gadolinium is present within bloodstream and because gadolinium is paramagnetic – the gadolinium aligns with the magnetic field and it makes the magnetic field inside the blood vessels much higher than the field outside
• There is a high field in the blood vessels and there is much lower field outside – get a gradient effect
- The field going from high to low but has to do it in a continuous way, so you get a range of field surrounding the blood vessels – range of different resonance frequencies because Larmor frequency is directly proportional to local magnetic field
- Range of frequencies means that things become out of sync quickly
- Transverse signal decays much more quickly
What is the passage of a DSC bolus?
- Inject a bolus as quickly as possible (<10s)
- Typical DSC bolus: 0.1 –> 0.3 mmol/kg body weight
- Results in transient decrease in MR signal as bolus passes through the image voxxels
- In order to define the passage of the bolus accuratelt, need to aqcuire an image every few seconds
- Imaging method of choice: PI
- EPI can acquire ~10 slices per second, and so a time resolution of 1-2s allows coverage of a large volume of the brain
What is the summary parameters for the information available from DSC?
- BAT = Bolus Arrival Time
- FWHM = Full Width Half Max
- PA = Peak Area
- TTP = Time To Peak
- MP = Maximum Peak
What is FWHM?
width of the peak at the halfway point between baseline and maximum change – measure the width of the peak there
What does the width of the peak represent?
how quickly the gadolinium is going through
• The peak area is within enclosed area
What it TTP?
From 0 to maximum change
What is the maximum peak?
a measure of the height from baseline down to the minimum level it reaches – extent of the change – the maximum signal intensity change
How is summary parameters calculated?
Easily without the need for extensive post-processing of the MR data
What is the summary parameters related to?
Cerebral blood flow (CBF)
Cerebral blood volume (CBV)
Mean transmit time of blood through the tissue (MTT)
What is the summary parameters dependent on?
Experimental conditions such as tracer injection rate and image scaling
What is summary parameters used for?
Measure perfusion quantitatively, we need to relate it directly to the signal intensity change caused by the bolus passage
How does signal change?
as a function of time because as the gadolinium goes through the vasculature – it affects the T2
• T2 becomes a function of time – signal intensity is changing as gadolinium arrives
• Change T2 in a well-known way
What is relaxitivity proportional to?
Concentration of contrast agent
What is the DSC concentration curve dependent on?
rate which gadolinium is flowing through the voxel
• Also depend on how quickly the injection was done
For perfusion quantification, what do we need to distinguish between?
- High CBF & wide input bolus
- Low CBF & Narrow input bolus
gives same tissue concentration vs time curve!
What needs to be accounted for?
Contribution of input bolus
What are other information available from DSC-MRI?
1.Cerebral blood volume (CBV) is proportional to the normalised total amount of tracer in the voxel
2.Mean transit time (MTT) can be calculated using the central volume theorem:
MTT = CBV/ CBF
What are the problems of DSC-MRI techniques?
Main problem:
Delay and dispersion of bolus causes inaccurate estimation of AIF
CBF quantitation depends on knowledge of AIF
Leakage of contrast into the brain (BBB)
Technical problem of deconvolution: which method is most reliable?
Limited repeatibility due to maximum allowed dose
What does DSC involve?
Repetitive serial imaging through the tumour during passage of blood that has been labelled with either contrast material or with an endogenous magnetic tracer label
What can DSC be obtained by?
Bolus tracking technique that monitors the passage of contrast medium through a capillary bed
It is based upon the inhomogeneity of the magnetic field during the passage of a short bolus of contrast medium through the capillary bed
What has DSC been used for?
Evaluation of tumour angiogenesis in different regions of the body notably the brain, hepatic and breast malignancy
What does DSC rely on?
The susceptibility induced signal signal loss on T2* weighted sequences which results from a bolus of gadolinium-based contrast passing through a capillary bed
What are the most commonly calculated parameters for DSC?
- rCBV
- rCBF
- MTT
Why is T2* techniques more commonly employed for DSC?
The former requires higher doses of contrast
What is the process of DSC?
A bolus of gadolinium-containing contrast is injected intravenously and rapid repeated imaging of the tissue (most commonly brain) is performed during the first pass. This leads to a series of images with the signal in each voxel representing intrinsic tissue T2/T2* signal attenuated by susceptibility-induced signal loss proportional to the amount of contrast primarily in the microvasculature
Then a regions signal is interrogated over the time course of the perfusion sequence, and a signal intensity time curve is generated, from which various parameters can be calculated
These values can then be used to create colour maps of regional perfusion
