MAGNETIC RESONANCE ANGIOGRAPHY (MRA).pptx

MR. ROHIT BANSAL
ASSISTANT PROFESSOR (RADIOPHYSICS)
MAMC, AGROHA
MAGNETIC RESONANCE ANGIOGRAPHY
(MRA)

 MR Angiography is a group of technique to image blood
vessels and surrounding anatomy with different signal
intensities.
 It is used to generate images of the arteries (and less
commonly veins) in order to evaluate then for stenosis
(abnormal growing), occlusions, aneurysms, or other
abnormalities.
 MRA is often used to evaluate the arteries of the neck and
brain, the thoracic and abdominal aorta, the renal arteries,
and the legs.

INDICATIONS
 Aneurysm in aorta, chest, abdomen, or in other arteries.
 Atherosclerotic (Plaque) disease.
 AVM (arterio-venous malformation).
 Visualize blood flow.
 To guide interventional radiologists, making repair to diseased blood vessels.
 Detect injuries to one or more vessels.
 Evaluate arteries bleeding or tumour prior to surgery.
 To identify dissection or splitting in the aorta.
 To show the extant and severity of coronary artery disease.
 To examine pulmonary arteries in the lung.
 To detect pulmonary embolism.
 To detect congenital abnormalities.
 To evaluate stenosis and obstruction in vessels.

PATIENT PREPARATION
 Ask patient to wear hospital gown and remove all metallic and
radio-opaque objects.
 Guidelines about eating and drinking before an MRI exam vary at
different facilities.
 Ask patient about any serious health problem and history of any
allergic reaction.

FLOW PHENOMENA
 Laminar flow is flow that is at different but consistent velocities across a vessel.
The flow at the centre of the lumen of the vessel is faster than at the vessel wall,
where resistance slows down the flow. However, the velocity difference across the
vessel is constant.
 Turbulent flow is flow at different velocities that fluctuates randomly. The velocity
difference across the vessel changes erratically. Vortex flow is flow that is initially
laminar but then passes through a stricture or stenosis in the vessel. Flow in the
centre of the lumen has a high velocity but, near the walls, the flow spirals.
 Stagnant flow is where the velocity of flow slows to a point of stagnation. The
signal intensity of stagnant flow depends on its T1, T2 and proton density
characteristics. It behaves like stationary tissue.
 Flow mechanisms are often termed as follows:
1. First order motion laminar flow
2. Second order motion acceleration
3. Third order motion jerk

BLACK BLOOD IMAGING
 This technique uses conventional T1 weighted spin echo sequences
with pre-saturation pulses, to visualize flowing vessels that appear
black in contrast to surrounding structures.
 Saturation eliminates phase ghosting and provides intra-luminal
signal void to distinguish between patent and obstructed vessels.
 It can be used to evaluate vascular patency especially in the neck,
brain, chest and abdomen.
 As flowing blood appears black, persistent signal within vessel lumen
after the application of saturation pulses indicates either stagnant
flow within the vessel, or vascular occlusion.

BRIGHT BLOOD IMAGING
 This technique uses gradient moment nulling, usually in
conjunction with coherent gradient echo sequences, to make
flowing blood bright.
 Gradient moment rephasing complements flow by making vessels
containing slow flowing spins appear bright, so enhancing the
signal from blood and CSF.
 Gradient moment rephasing is widely used in the chest and
abdomen, brain, extremities and for the myelographic effect of
CSF in T2 weighted images of the spine.
 As GMN (Gradient moment nulling) requires a longer minimum TE
due to the use of additional gradients, a reduction in the number of
slices available often results.

TIME OF FLIGHT ANGIOGRAPHY (TOF-MRA)

MECHANISM
 Time of flight MRA (TOF-MRA) produces vascular contrast by manipulating
longitudinal magnetization of stationary spins.
 It uses a gradient echo pulse sequence in combination with GMN to enhance signal
in flowing vessels.
 The TR is kept well below the T1 time of the stationary tissues so that T1 recovery is
prevented. This saturates the stationary spins, whilst the in-flow effect from fully
magnetized flowing fresh spins produces high vascular signal.
 However, if the TR is too short, the flowing spins may be suppressed along with the
stationary spins that reduce vascular contrast.
 To evaluate signals from arterial flow, saturation pulses are applied in the direction
of venous flow. For example, to evaluate the carotid arteries in the neck, apply
saturation pulses superior to the imaging volume to saturate the signal from
inflowing venous blood.
 TOF-MRA is only sensitive to flow that comes into the FOV. Any flow that traverses
the FOV can be saturated along with the stationary tissue.

2D VS. 3D TOF-MRA
 TOF-MRA is acquired in either 2D (slice by slice) or 3D (volume)
acquisition modes.
 In general, 3D volume imaging offers high SNR and thin contiguous
slices for good resolution.
 However, as TOF-MRA is sensitive to flow coming into the FOV or
the imaging volume, spins in vessels with slow flow can be
saturated in volume imaging.
 For this reason, 3D TOF should be used in areas of high velocity
flow (intra-cranial applications) and 2D TOF in areas of slower
velocity flow (carotids, peripheral vascular and the venous systems).
 In 3D techniques, there is a higher risk of saturating signals from
spins within the volume.

CLINICAL USES
 The carotid bifurcation, the peripheral circulation and
cortical venous mapping can be imaged with 2D TOF-
MRA.

PARAMETERS
TR 45 ms
Minimum allowable TE
Flip angles approximately 60°
The TR and flip angle saturate stationary nuclei but moving spins
coming into the slice remain fresh, so image contrast is maximized.
The short TE reduces phase ghosting.
Gradient moment rephasing, in conjunction with saturation pulses
to suppress signals from areas of undesired flow, are used to
enhance vascular contrast.

GENERAL TOF ADVANTAGES
 Sensitive to T1 effects (short T1 tissues are bright. Contrast may
be given for additional enhancement).
 Reasonable imaging times (approximately 5 min depending on
parameters).
 Sensitive to slow flow.
 Reduced sensitivity to intra-voxel dephasing.

GENERAL TOF DISADVANTAGES
 Sensitive to T1 effects (short T1 tissues are bright so that
haemorrhagic lesions may mimic vessels).
 Saturation of in-plane flow (any flow within the FOV or volume of
tissue can be saturated along with background tissue).
 Enhancement is limited to either flow entering the FOV or very
high velocity flow.

MECHANISM
 Phase contrast MRA utilizes velocity changes, and hence phase
shifts in moving spins, to provide image contrast in flowing vessels.
 Phase shifts are generated in the pulse sequence by phase
encoding the velocity of flow with the use of a bipolar (two lobes, one
negative one positive) gradient.
 Phase shift is introduced selectively for moving spins with the use of
magnetic field gradients.
 This technique is known as phase contrast magnetic resonance
angiography (PC-MRA).
 PC-MRA is sensitive to flow within, as well as that coming into the
FOV.

 Immediately after the RF excitation pulse spins are in phase, a gradient is applied
to both stationary and flowing spins. Although phase shifts occur in both stationary
and flowing spins, these shifts occur at different rates.
 During initial application of the first bipolar gradient, there is a shift of phases of
stationary spins and flowing spins.
 After the second part of the application of the first bipolar gradient, the stationary
spin returns to their initial phase, but those of moving spins acquire some phase.
 The bipolar gradient is then applied with opposite polarity so that the same
variants occur, but in the opposite direction.
 PC-MRA then subtracts the two acquisitions so that the signals from stationary
spins are subtracted out leaving only the signals from flowing spins. The
combination of PC-MRA acquisitions results in what are known as magnitude and
phase images.
 The unsubtracted combinations of flow sensitized image data are known as
magnitude images.
 The subtracted combinations are called phase images.

 The bipolar gradients induce phase shifts along their axes.
 By applying bipolar gradients in all three axes the sequence is
sensitized to flow in all three directions X, Y and Z. These are known
as flow encoding axes.
 The sequence is also sensitized to flow velocity using a velocity
encoding technique (VENC) that compensates for projected 40
Phase contrast MRA (PC-MRA) flow velocity within vessels by
controlling the amplitude or strength of the bipolar gradient.
 If the VENC selected is lower than the velocity within the vessel,
aliasing can occur.
 This results in low signal intensity in the centre of the vessel, but
better delineation of vessel wall itself.
 With high VENC settings, intra-luminal signal is improved, but
vessel wall delineation is compromised.

2D VS. 3D PC-MRA
 2D techniques provide acceptable imaging times and flow direction
information.
 2D acquisitions, however, cannot be reformatted and viewed in
other imaging planes.
 3D offers SNR and spatial resolution superior to 2D imaging
strategies, and the ability to reformat in a number of imaging
planes retrospectively.
 The trade-off however is that in 3D PC-MRA, imaging time
increases with the TR, NEX, the number of phase encoding steps,
the number of slices and the number of flows encoding axes
selected.
 For this reason, scan times are sometimes long.

CLINICAL USES
 PC-MRA can be used effectively in the evaluation of
arteriovenous malformations, aneurysms, venous occlusions,
congenital abnormalities and traumatic intra-cranial vascular
injuries.

PARAMETER
3D VOLUME ACQUISITIONS
 28 slices volume, 1 mm slice thickness
 Flip angle 20° (60 slice volume flip angle reduced to 15°)
 TR less than or equal to 25 ms
 VENC 40–60 cm/s
 Flow encoding in all directions

2D TECHNIQUES ACQUISITIONS
 Cranial
• TR 18–20 ms
• Flip angle 20°
• Slices thickness 20–60 mm
• VENCs :20–30 cm/s for venous flow 40–60 cm/s for higher velocity with some
aliasing 60–80 cm/s to determine velocity and flow direction.
 Carotid
• Flip angles 20° to 30°.
• TR 20 ms.
• VENCs :40–60 cm/s for better morphology with aliasing 60–80 cm/s for
quantitative velocity and directional information.

CONTRAST MECHANISM
 Gadolinium is a T1 shortening agent that enhances blood if given
in sufficient quantities into the bloodstream.
 If used in conjunction with a T1 weighted sequence, blood
appears bright and is well seen in contrast to surrounding non-
enhancing tissues.
 A conventional or fast incoherent gradient echo sequence should
therefore be used.

ADMINISTRATION
This is administered intravenously, usually via the ante-cubital
fossa by hand or mechanical injection.
Doses must be sufficiently high to give adequate vessel
delineation. 40 to 60 ml (about 0.3 mmol/kg) of gadolinium is
required.

IMAGE TIMING
 To obtain an arterial-phase image in which arteries are bright and veins are dark, it
is essential that the central K space data (i.e., the low spatial frequency data) are
acquired while gadolinium concentration in the arteries is high but relatively lower
in the veins.
 The time it takes contrast to travel from the ante-cubital fossa to the area of
interest depends on:
• The distance of the area from the injection site;
• The type of vessel (e.g., artery or vein);
• The velocity of flow;
• The speed of injection;
• The length of the acquisition.

 For long acquisitions lasting more than 100 s, use sequential ordering of K space,
so that the centre of K space is collected during the middle of the acquisition.
Sequential ordering results in fewer artefacts. Begin injecting the gadolinium just
after initiating imaging. Finish the injection just after the midpoint of the acquisition,
being careful to maintain the maximum injection rate for the approximately 10–30 s
prior to the middle of the acquisition. This will ensure a maximum arterial gadolinium
during the middle of the acquisition when central K-space data are collected.
 For short acquisitions less than 45 s contrast agent bolus timing is more critical
and challenging. There are several approaches to determining the optimal bolus
timing for these fast scans. For a typical breath-hold scan duration of 35–45 s in a
reasonably healthy patient with an IV site in the ante-cubital vein, a delay of
approximately 10– 12 s is appropriate. Therefore, begin the injection, and then 10 s
later start imaging while the patient suspends breathing. More reliable and precise
techniques are also available. These include:
• using a test bolus to measure the contrast travel time precisely;
• using an automatic pulse sequence that monitors signal in the aorta and then initiates
imaging after contrast is detected arriving in the aorta;
• imaging so rapidly that bolus timing is unimportant.

MAGNETIC RESONANCE SPECTROSCOPY (MRS)

 MRI images depict differences in water density in various brain tissues. The
hydrogen nuclei in water molecules affected by MRI’s magnetic fields constitute of
the brain’s soft tissue.
 MRI does not image the remaining 20% of brain material, including all
macromolecules (DNA, RNA, most proteins, and phospholipids); cell membrane;
organelles, such as mitochondria; and glial cells.
 Magnetic Resonance Spectroscopy (MRS), also known as Nuclear Magnetic
Resonance (NMR) Spectroscopy, is an MRI method that varies the
radiofrequency used for aligning hydrogen protons to allow imaging of the
concentrations of that remaining brain material.
 For example, MRS can image N-acetyl aspartate (NAA), a brain metabolite found
in both neurons and glial cells, and creatine, and acid that helps supply cells with
energy and is present in much higher concentration in neurons than in glia.
 Thus, MRS imaging can distinguish brain cells from other substances and
neurons from glia.

 While magnetic resonance imaging (MRI) identifies the anatomical
location of a tumour, MR spectroscopy compares the chemical
composition of normal brain tissue with abnormal brain tissue.
 MRS can detect brain-cell loss in degenerative diseases such as
Alzheimer's, loss of myelin in demyelinating diseases such as
multiple sclerosis, and persisting abnormalities in brain
metabolism in disorders such as concussion.
 MRS can also image molecules that participate in transmitting
information between neurons. One is choline, the precursor
molecule for acetylcholine; another is glutamate, the major
excitatory neurotransmitter molecule in the brain.
 MRS can image many other brain molecules as well, to provide
new avenues for investigating brain development, function, and
disease.

HOW DOES MR SPECTROSCOPY WORK?
 MR spectroscopy is conducted on the same machine as conventional MRI. The MRI scan uses a
powerful magnet (more than 1.5 T), radio waves, and a computer to create detailed images.
Spectroscopy is a series of tests that are added to the MRI scan of your brain or spine to measure
the chemical metabolism of a suspected tumour.
 MR spectroscopy analyses molecules such as hydrogen ions or protons. Proton spectroscopy is
more commonly used. There are several different metabolites, or products of metabolism, that can
be measured to differentiate between tumour types:
1. Amino acids
2. Lipid
3. Lactate
4. Alanine
5. N-acetyl aspartate
6. Choline
7. Creatine
8. Myoinositol

 The frequency of these metabolites is measured in units called parts per
million (ppm) and plotted on a graph as peaks of varying height. By
measuring each metabolite's ppm and comparing it to normal brain
tissue, the neuroradiologist can determine the type of tissue present.
 A radiology technologist performs the test in the MRI suite in a hospital's
radiology department or an outpatient imaging centre.
 The patient lies on a moveable bed with their head cradled on a headrest
and their arms at your sides. An antenna device called a "coil" is placed
over or around the area of the body to be imaged. It is specialized to
produce the clearest picture of the area it is placed over. When the
patient is comfortably positioned, the table slowly moves into the
magnetic field. As the exam proceeds, the patient hears a muffled
"thumping" sound for several minutes at a time. This is the sound of the
pictures being taken. The patient may be given an injection an IV
contrast dye (gadolinium) into their arm or through to enhance the
images.

 MRI often uses the larger available signal to produce images, whereas MRS very
frequently only acquire signal from a single localized region, referred to as “voxel”.
 Because water molecules contain hydrogen and the relative concentration of water
to metabolite is about 10000:1, the water signal is often suppressed or the
metabolites peaks will not be discernible in the spectra. This is achieved by adding
water suppression pulse sequence.
 For the spectrum acquisition, different techniques may be used including single or
multi voxel imaging:
1. Single Voxel Spectroscopy:
 In single voxel spectroscopy, the signal is obtained from a voxel previously
selected.
 This technique results in a high-quality spectrum, a short scan time and good field
inhomogeneity.
 It is used to obtain an accurate quantification of metabolites.

 Multi Voxel Spectroscopy:
 The main objective of multi voxel spectroscopy is to obtain
simultaneously many voxels and a spatial distribution of
metabolites within a single sequence.
 It is used to determinate spatial heterogeneity.
 Limitation of MRS:
 The major limitation of MRS is its low available signal due to the
low concentration of metabolites as compared to water.

Peaks
 lipids: resonates at 1.3 ppm
 lactate: resonates at 1.33 ppm
 alanine: resonates at 1.48 ppm
 N-acetylaspartate (NAA): resonates at 2.0 ppm
 glutamine/glutamate: resonates at 2.2-2.4 ppm
 GABA: resonates at 2.2-2.4 ppm
 2-hydroxyglutarate: resonates at 2.25 ppm 6
 citrate: resonates 2.6 ppm
 creatine: resonates at 3.0 ppm
 choline: resonates at 3.2 ppm
 myo-inositol: resonates at 3.5 ppm
 water resonates at 4.7 ppm

PATHOLOGY
 Glioma
MRS can help increase our ability to predict grade. As the grade increases, NAA and
creatine decrease and choline, lipids and lactate increase.
In the setting of gliomas, choline will be elevated beyond the margins of contrast
enhancement in keeping with cellular infiltration.
 Non-glial tumors
May be difficult but in general non-glial tumors will have little, if any, NAA peak.
 Radiation effects
Distinguishing radiation change and tumor recurrence can be problematic. In recurrent
tumor choline will be elevated, whereas in radiation change, NAA, choline and creatine
will all be low.
 Ischemia and infarction
Lactate will increase as the brain switches to anaerobic metabolism. When infarction
takes place then lipids are released and peaks appear.

 Infection
As in all processes which destroy normal brain tissue, NAA is absent. Within bacterial
abscess cavities, lactate, alanine, cytosolic acid and acetate are elevated/present.
Of note choline is low or absent in toxoplasmosis, whereas it is elevated in lymphoma,
helping to distinguish the two.
 White matter diseases
progressive multifocal leukoencephalopathy (PML) may demonstrate elevated myo-
inositol
Canavan disease characteristically demonstrates elevated NAA
 Hepatic encephalopathy
Markedly reduced myo-inositol, and to a lesser degree choline. Glutamine is increased.
 Mitochondrial disorders
Leigh syndrome: elevated choline, reduced NAA and occasionally elevated lactate

MAGNETIC RESONANCE ANGIOGRAPHY (MRA).pptx

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