INTRODUCTION.
*MRI – non-invasive study of flow. CT & angiography – exogenous iodinated contrast media.
*Flow : intrinsic physiologic contrast media for MR.
*Doppler : non-invasive, but MR has advantages in Field Of View & tissue characterization
*Carr & Purcell 1954 : influence of the flow velocity on MR signal
*Hahn 1960 :Measurement of the velocity based on the signal phase.
*1980 : First flow imaging techniques based on RF tagging.
*Imaging techniques to map the blood velocity based on phased measurement = velocity phased encoding
GENERAL PRINCIPLES
*Flow influences the MR signal
*When we send 90 degree pulse, all protons influenced by the radio wave.
*After we turn the RF pulse off, we get a signal.
*But in the vessels, the blood always flow. The blood may have left the examined slice.
*So there is no signal coming out of vessel; it appears black in the picture
*The phenomenon is called flow void phenomenon.
GENERAL PRINCIPLES
METHODS :
*Phased contrast
*Time of Flight (TOF)
*Black-blood MRA
*Contrast enhanced MRA
*PHASED CONTRAST MRA & TOF
#Phased contrast & TOF both are more commonly employed clinical MRA techniques.
#Both rely the fact that there are detectable changes in macroscopic spins within the atomic nuclei of blood as well as other body tissue.
#Phased contrast : the spin changes occur as blood moves along a magnetic gradient are used to differentiate blood from surrounding tissue
#TOF : a “wash in & wash out” measurement is taken of spins that have been exposed to an RF pulse.
#Flowing blood provides an MR signal different from that of surrounding tissue.
#Phased contrast MRA – an image of the blood vessels by measuring the motion induced phase shifts that occur as blood moves within magnetic field gradient. Stationary background tissue has zero phase shift.
#TOF – uses flow related enhancement that occur when fully magnetized blood moves into imaging plane. The fresh blood no RF excitations give strong signal compared with stationary tissue
*BLACK BLOOD MRA
#Black blood – low vascular signal. Wash out Effects – produce signal loss.
#Stationary soft tissue – strong signal
#Moving blood – little or no signal.
#The designation refers to the “enhanced” flow-void or signal loss
Black blood à blood appear dark
6 ways to get it :
1.If the blood is not motion compensated & flow is fast enough.
2.Saturation can be used to suppress the signal from the blood.
3.Invert the spins from blood
4.Flow sensitive fast imaging
5.Letting the blood flow out of a slice
6.T2* contrast agent – to dephased signal.
*CONTRAST ENHANCED MRA
#Performed after the injection of an MR contrast agent.
#Has the potential to provide high resolution images with fewer artifacts.
#Contrast agents make the blood brighter in phased contrast MRA & TOF MRA.
#Different agents can be used to make blood darker in Black Blood MRA.
#MR contrast agent projection angiogram can be obtained via 3D gradient echo either by substracting data sets before & after in injection
#Dose 0.1-0.2mmol/kg BW
#Gd DTPA do not penetrate blood brain barrier.
#Angiogram obtained by MIP (maximum intensity projection)
#ontrast MRA favours imaging of the venous system (MRV) because of the more constant flow velocity compared to arteries.
MRA –fundamental of imaging
#radient echoes – applying gradient field
#hase – a function of time, gradient strength & location of spin
#T2* dephasing – local field inhomogeneity à effective decay is quickened
#Chemical shift – local field variations causes a shift in the freq for different molecules
#Resolution
#Contrast
arearadiologi 