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1. PERFUSION AND DIFFUSION
MRI
MAAJID MOHI UD DIN MALIK

2. PERFUSION
Perfusion is the passage of fluid through the circulatory system or lymphatic system to
an organ or a tissue, usually referring to the delivery of blood to a capillary bed in
tissue. Perfusion is measured as the rate at which blood is delivered to tissue, or volume
of blood per unit time (blood flow) per unit tissue mass. The SI unit is m3/(s·kg),
although for human organs perfusion is typically reported in ml/min/g.

3. The word is derived from the French verb "perfuser" meaning to "pour over or
through". All animal tissues require an adequate blood supply
for health and life. Poor perfusion (malperfusion), that is, ischemia, causes
health problems, as seen in cardiovascular disease, including coronary artery
disease, cerebrovascular disease, peripheral artery disease, and many other
conditions.
PERFUSION

4. DISCOVERY
In 1920, August Krogh was awarded the Nobel Prize in Physiology or
Medicine for his discovering the mechanism of regulation
of capillaries in skeletal muscle. Krogh was the first to describe the adaptation
of blood perfusion in muscle and other organs according to demands through
the opening and closing of arterioles and capillaries.

5. PERFUSION MRI
Perfusion MRI or perfusion-weighted imaging (PWI) is perfusion scanning by
the use of a particular MRI sequence. The acquired data are then post
processed to obtain perfusion maps with different parameters, such as BV
(blood volume), BF (blood flow), MTT (mean transit time) and TTP (time to
peak).

6. CLINICAL USE
In cerebral infarction, the penumbra has decreased perfusion. Another MRI
sequence, diffusion weighted MRI, estimates the amount of tissue that is
already necrotic, and the combination of those sequences can therefore be used
to estimate the amount of brain tissue that is salvageable
by thrombolysis and/or thrombectomy.

7. SEQUENCES
There are 3 main techniques for perfusion MRI:
Dynamic susceptibility contrast (DSC): Gadolinium contrast is
injected, and rapid repeated imaging (generally gradient-echo echo-
planar T2 weighted) quantifies susceptibility-induced signal loss.
Dynamic contrast enhanced (DCE): Measuring shortening of
the spin–lattice relaxation(T1) induced by a gadolinium contras bolus
Arterial spin labeling (ASL): Magnetic labeling of arterial blood
below the imaging slab, without the need of gadolinium contrast

8. DYNAMIC SUSCEPTIBILITY CONTRAST
In Dynamic susceptibility contrast MR imaging (DSC-MRI,
or simply DSC), Gadolinium contrast agent (Gd) is injected
(usually intravenously) and a time series of fast T2*-
weighted images is acquired.

9. DYNAMIC CONTRAST-ENHANCED IMAGING
Dynamic contrast-enhanced (DCE) imaging gives information about
physiological tissue characteristics. For example, it enables analysis of blood
vessels generated by a brain tumor. The contrast agent is blocked by the
regular blood–brain barrier but not in the blood vessels generated by the
tumor. The concentration of the contrast agent is measured as it passes from
the blood vessels to the extracellular space of the tissue (it does not pass the
membranes of cells) and as it goes back to the blood vessels.

10. The contrast agents used for DCE-MRI are often gadolinium based. Interaction with the
gadolinium (Gd) contrast agent causes the relaxation time of water protons to decrease, and
therefore images acquired after gadolinium injection display higher signal in T1-weighted
images indicating the present of the agent. It is important to note that, unlike some
techniques such as PET imaging, the contrast agent is not imaged directly, but by an indirect
effect on water protons.
DYNAMIC CONTRAST-ENHANCED IMAGING

11. ARTERIAL SPIN LABELING
Arterial spin labeling (ASL) has the advantage of not relying on an
injected contrast agent, instead inferring perfusion from a drop in signal
observed in the imaging slice arising from inflowing spins (outside the
imaging slice) having been selectively saturated.

12. DIFFUSION
Diffusion is the movement of a substance from an area of high concentration to
an area of low concentration.
Diffusion happens in liquids and gases because their particles move randomly from
place to place.
Diffusion is an important process for living things; it is how substances move in and
out of cells.

13. WHAT CAUSES DIFFUSION?
In gases and liquids, particles move randomly from place to place. The particles
collide with each other or with their container. This makes them change direction.
Eventually, the particles are spread through the whole container.
Diffusion happens on its own, without stirring, shaking or wafting.

14. WHY IS DIFFUSION USEFUL?
In living things, substances move in and out of cells by diffusion. For example:
Respiration produces waste carbon dioxide, causing the amount of carbon dioxide to
increase in the cell. Eventually, the carbon dioxide concentration in the cell is higher
than that in the surrounding blood. The carbon dioxide then diffuses out through the
cell membrane and into the blood.

15. Water diffuses into plants through their root hair cells. The water moves
from an area of high concentration (in the soil) to an area of lower
concentration (in the root hair cell). This is because root hair cells
are partially permeable. The diffusion of water like this, is called osmosis.
WHY IS DIFFUSION USEFUL?

16. DIFFUSION MRI
Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) is the use
of specific MRI sequences as well as software that generates images from the
resulting data that uses the diffusion of water molecules to generate contrast in
MR images.

17. It allows the mapping of the diffusion process of molecules, mainly water,
in biological tissues, in vivo and non-invasively. Molecular diffusion in tissues
is not free, but reflects interactions with many obstacles, such
as macromolecules, fibers, and membranes.
DIFFUSION MRI

18. Water molecule diffusion patterns can therefore reveal microscopic details about tissue
architecture, either normal or in a diseased state. A special kind of DWI, diffusion
tensor imaging (DTI), has been used extensively to map white matter tractography in
the brain.
DIFFUSION MRI

19. TECHNICAL EVOLUTION OF DIFFUSION WEIGHTED
IMAGING
The goal of all imaging procedures is generation of an image contrast with a good
spatial resolution. Initial evolution of diagnostic imaging focused on tissue density
function for signal contrast generation. In 1970s, the work of Lauterbur PC, Mansfield
P and Ernst R, modern clinical MRI came into the field of medicine. MRI provided an
excellent contrast resolution not only from tissue (proton) density, but also from tissue
relaxation properties.

20. After initial focus on T1 and T2 relaxation properties researchers explored other
methods to generate contrast exploiting other properties of water molecules. Diffusion
weighted imaging (DWI) was a result of such efforts by researchers like Stejskal,
Tanner and Le Bihan.
TECHNICAL EVOLUTION OF DIFFUSION WEIGHTED
IMAGING

21. In 1984, before MRI contrast became available, Denis Le Bihan, tried to differentiate
liver tumors from angiomass. He hypothesized that a molecular diffusion measurement
would result in low values for solid tumors, because of restriction of molecular
movement. Based on the pioneering work of Stejskal and Tanner in the 1960s, he
thought that diffusion encoding could be accomplished using specific magnetic
gradient pulses.
TECHNICAL EVOLUTION OF DIFFUSION WEIGHTED
IMAGING

22. It was a challenging task to integrate the diffusion encoding gradients in to the
conventional sequences and initial experience in the liver with a 0.5T scanner was very
disappointing. Firstly diffusion MRI was a very slow method and it was very sensitive
to motion artifacts due to respiration.
TECHNICAL EVOLUTION OF DIFFUSION WEIGHTED
IMAGING

23. It was not until the availability of Echo-Planar Imaging (EPI) in the early 1990s, that
DWI could become a reality in the field of clinical imaging. EPI based diffusion
sequences were fast and solved the problems of motion artifacts. Early work by
Moseley et al and Warach et al established DWI as a cornerstone for early detection of
acute stroke.
TECHNICAL EVOLUTION OF DIFFUSION WEIGHTED
IMAGING

24. In a DWI sequence diffusion sensitization gradients are applied on either side of the
180° refocusing pulse. The parameter “b value” decides the diffusion weighting and is
expressed in s/mm2. It is proportional to the square of the amplitude and duration of
the gradient applied. Diffusion is qualitatively evaluated on trace images and
quantitatively by the parameter called apparent diffusion coefficient (ADC). Tissues
with restricted diffusion are bright on the trace image and hypointense on the ADC
map.
TECHNICAL EVOLUTION OF DIFFUSION WEIGHTED
IMAGING

25. CLINICAL APPLICATIONS OF DWI
Acute brain ischemia:
Ever since its inception acute brain ischemia has been the most
successful application of DWI . Diffusion MRI today is the imaging
modality of choice for stroke patients. The use of DWI in combination
with perfusion MRI, which outlines salvageable areas of ischemia and
MR angiography, provides a useful guide for stroke management.

26. Acute infarct. Axial FLAIR image (A) shows geographic hyper intensity involving right parieto-
occipital region and basal ganglia. Diffusion weighted imaging shows restricted diffusion with
high signal image (B) and low signal intensity on apparent diffusion coefficient map (C).

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