What is Perfusion?

A special technique for evaluation of microscopic

blood flow in capillaries and venules.

Perfusion imaging is much faster than Diffusion.

Both are used for the evaluation of stroke.

Perfusion has other applications.

Perfusion shows ischemic penumbra (healthy tissue

that surrounds ischemic tissue) and Diffusion does

not.

What is Perfusion? (cont.)

Two Categories:

•Monitoring tissue signal changes using an

exogenous (injectable) MR relaxation

contrast agent. (Gadolinium)

•Monitoring tissue signal changes using an

endogenous contrast agent, which is an

inherent MR tissue contrast mechanism.

(Deoxyhemoglobin)

MR Perfusion Mapping

Visualizing signal changes during the

vascular transit of an injected MR contrast

agent bolus through the use of high speed

T2*-sensitive MR imaging.

The T2* FID-EPI is the most common

imaging sequence used in Perfusion

imaging.

MR Perfusion Timing (tracking)

What is the typical transmit time of a bolus

through the human brain?

15-20 seconds.

Therefore, the greatest challenge of

exogenous perfusion is acquiring the image

at the first pass of the bolus.

Timing is everything!

Peak Time

Factors that determine peak time:

•Heart Rate.

•Form of the bolus (speed).

•Concentration of contrast (amount and

osmolality).

Bolus Tracking Perfusion

This is used to differentiate between

normal and abnormal tissue.

Important factors used:

•Transit Time

•Blood Flow

•Blood Volume

Cardiac Perfusion

Parallel Imaging

Multichannel Coil Technology

(basics)

Radiological “Wish List” for MR (and perhaps,

other modalities as well):

Higher spatial resolution

Decreased acquisition time

Higher signal to noise ratio (SNR)

More images per patient for more diagnostic

information

Minimize SAR (problem with high-field MR)

Some factors to help meet these needs:

1) Protocol/Pulse-sequence optimization

2) Faster image reconstruction hardware

– but also –

3) Single-element coil: increasing SNR requires increased acquisition time

4) Receiving coil element size: decrease increased SNR per volume, but smaller tissue volume

5) Tissue proximity: decrease increased SNR But: Single-element coil + one receive channel = slow dataflow.

Solution:

Number of RF receive channels: increase decreased acquisition time

Multichannel Coil (cont.)

Multichannel Coil (cont.)

Circularly-polarized (CP) coil led to a ~ 40% increase in SNR (two-element coil).

Also called a “quadrature coil”.

Recent development: Multichannel technology Coil uses multiple elements (“loops”) in phased array with overlapping anatomical coverage

Each element acquires MR signals from the entire region.

Highest signal in closest proximity to the element.

Small element size higher received signal / higher

overall signal

RF hardware uses multiple channels for receiving signal from multiple elements.

Multichannel Coil (cont.)

Putting the coil and RF system together…

Signal from each element is (ideally) transferred

through its own high-bandwidth RF channel.

Reconstruction corrects element-to-element

signal variations before forming the final image.

Advanced reconstruction and storage hardware

necessary to process the rapid inflow of

information.

Multichannel Coil (cont.)

The following slide shows element

arrangement for an 8-element head coil, and

images acquired from each element, together

with the final single combined image.

Note that images from each surface element

show greater sensitivity near the element.

Multichannel Coil (cont.)

Commercially available, eight-channel coil for brain imaging. (MRI Devices Corporation, Waukesha, WI).

Multichannel Coil (cont.)

Clinical benefit of multichannel technology:

Higher SNR achieved with multichannel technology allows

greater flexibility in sequence parameter selection.

If SNR is higher than needed, we can afford to lose a little SNR

to gain:

An increase in spatial resolution

A reduction in acquisition time (e.g., minimize motion-

induced artifacts, increase number of images per exam).

Multichannel Coil (cont.)

Advancements in multi-element/multichannel

technology (to 32 elements and beyond) will

continue to play a role in the development of

imaging techniques with higher spatial resolution,

faster scan times, and increased diagnostic quality.

Multichannel Coil (cont.)

18

Advancements in multi-element/multichannel coils:

New 96-channel head coil (Wald, MGH)

High-field imaging with 8-channel coil

Parallel Imaging

No image from a single surface coil element is optimally sensitive over the

whole area. However, an image reconstructed from all coil elements leads

to an increased SNR over a standard acquisition, because each region of

the image is reasonably sampled by more than one element.

If SNR is higher than needed, one can use the technique of parallel imaging

to increase acquisition speed.

How?

We can decrease sampling of data by each element receiver.

Also, reduced sampling less RF excitations per unit time lower SAR.

Decrease sampling of data = decreased k-space sampling

Parallel Imaging

Rather than fill all of k-space, parallel imaging acquires a

fraction of k-space to save time. Because the anatomy is

sampled by multiple coil elements, we can reconstruct the

missing information.

Less samples leads to decreased SNR.

Parallel Imaging

How fast can we go?

If we have M coil elements covering the FOV, we can skip up to M-1 lines

for each line in k-space we sample. The number of lines “skipped”:

acceleration factor (R). This can be fractional as well:

# of phase-encodes to cover k-space

R = ––––––––––––––––––––––––– # of phase-encodes used in acquisition

Names for acceleration factors: iPAT factor (Siemens)

SENSE factor (Philips)

ASSET factor (GE)

Parallel Imaging

Increasing acceleration leads to decreasing SNR. However, the benefits

may be greater than saving time as well.

For EPI images, which are greatly affected by susceptibility differences,

parallel imaging can improve geometric distortion and/or image voids.

Why?

Because the gradients are switching so quickly for an EPI image, one can

accrue errors that lead to distortion. These are alleviated using parallel

imaging, where the sequence requires less lines in k-space to be read out.

Parallel Imaging Example of Parallel Acceleration on the GE 3T:

R=1 R=2.0 R=2.8 R=3.2 R=4.0

SNR vs. Acceleration

Short-axis cardiac images – 32-channel coil – 1.5 T magnet

Reeder SB et al. MRM 54:748, 2005

Reconstructing an Image

Step 1:

The MR signal is detected by RF coils.

Step 2:

The resulting data set is digitized and arranged into mathematical construct called “k-space”.

Step 3:

Subsequent processing of this data set – the Fast Fourier Transformation (FFT) – yields the final MR image.

SMASH

SMASH (SiMultaneous Acquisition of Spatial Harmonics) is “k-space based” because the reconstruction algorithm operates on partial k-spaces (one from each coil), before image generation by the FFT.

1. Two (or more) k-space acquisitions with two (or more) coils. Each coil fills one k-space with a reduced number of lines (e.g., for an acceleration factor of 2,only every 2nd line is acquired).

2. “Artificial” lines are calculated to fill the gaps in k-space (matrix inversion with information from the coil sensitivity profiles). This is achieved via the SMASH reconstruction algorithm.

SMASH

Parallel MR Imaging with iPAT

More than Just Common SENSE

Daniel S.Grosu,MD,MBA

Siemens Medical Solutions USA,Inc.

Parallel Imaging (k-space Example)

The following slide shows fast spin echo T2 weighted sagittal

scans of the lumbar spine, without (A) and with (B) parallel

imaging.

In (B), every second Fourier line has been skipped

(acceleration factor of 2). Scan time is thus reduced by a

factor of two (comparing B to A).

SENSE

SENSE (SENSitivity Encoding) [2] is “image based” because the reconstruction algorithm operates on partial images (from each coil) that have been generated by the FFT.

1. The first step is identical to the first step in SMASH.

2. Each k-space (with a reduced number of lines) is subjected to a conventional FFT at this stage.

3. This results in two (or more) aliased images with rectangular FoVs.

SENSE-based techniques do not work well with “pre-aliased” images. If the original field of view (before using parallel acquisition) is smaller than the object and is already aliased, a wraparound artifact will be present.

SENSE

Parallel MR Imaging with iPAT

More than Just Common SENSE

Daniel S.Grosu,MD,MBA

Siemens Medical Solutions USA,Inc.

Parallel Imaging

(Image Based Reconstruction)

The following slide shows fast spin echo T2-

weighted sagittal scan of the lumbar spine , without

(A) and with (B) parallel imaging.

In (B) every second Fourier line (parallel imaging

with an IPAT factor of 2). Thus the scan time for (B)

is half that of (A). Note that there are residual wrap

around artifacts (arrow, B), a major drawback to the

use of image-based reconstruction technique when

anatomy is larger than FOV.

Acronyms used in parallel imaging

SENSE: SENSitivity Encoding (Phillips)

ASSET: Array Spatial & Sensitivity Encoding Technique (GE)

RAPID: Rapid acquisition through a parallel imaging design (Hitachi)

iPAT: integrated PArallel Technology (Siemens)

1. GRAPPA: GeneRAlized Autocalibrating Partially Parallel Algorithm (Siemens) SMASH based technique.

2. mSENSE; Modified SENSE (Siemens) SENSE based technique.

Parallel Imaging (Drawbacks)

K-space based reconstruction: The ability to construct effective sensitivities from the spatial sensitivities for each coil element depends on the sensitivity profile. This, in turn, depends on the coil element design; therefore, coil design is more critical with this technique.

Image-based reconstruction: If an aliasing artifact would be present in the chosen FOV for a non-parallel image sequence, then this aliasing will cause reconstruction problems if parallel imaging is attempted.

1) The Physics of Clinical MR, for Neuroradiology, Taught

Through Images

AUTHORS: VAL M. RUNGE1 MD, WOLFGANG R. NITZ2 PHD,

STUART H. SCHMEETS2 BS, RT, WILLIAM H. FAULKNER, JR.3 BS, RT, NILESH K. DESAI1 MD

References:

2) The Physics of Clinical MR, Focusing on the Abdomen,

Taught Through Images

The Difference Between

MRI and fMRI

Conventional MRI Functional MRI

from Culham J: fMRI for Dummies

What is fMRI? (module: sup. read. #6)

Functional MRI is based on the

increase in blood flow to the local

vasculature that accompanies neural

activity in the brain.

Microvascular MR signal

Microvascular MR signal on T2 and T2*

weighted images is strongly influenced by the

oxygenation state of the blood.

70% of the brain's blood lies within the

microvascular capillaries and venules.

Glucose is the necessary nutrient for electrical

activity in the brain.

Brain activation is the demand for nutrients by

the brain to maintain neural metabolism.

Oxyhaemoglobin & Deoxyhaemoglobin

Hemoglobin is a molecule that contains iron and

transports oxygen in the vascular system as

oxygen binds directly to iron.

Oxyhemoglobin. Oxygen is bound to

hemoglobin and the magnetic properties of iron

are suppressed (diamagnetic).

Deoxyhemoglobin. Oxygen is not bound to

hemoglobin and the magnetic properties of iron

are more magnetic (paramagnetic).

Hemoglobin

Source: http://wsrv.clas.virginia.edu/~rjh9u/hemoglob.html, Jorge Jovicich

Hemoglogin (Hgb): - four globin chains

- each globin chain contains a heme group

- at center of each heme group is an iron atom (Fe)

- each heme group can attach an oxygen atom (O2)

- oxy-Hgb (four O2) is diamagnetic no B effects

- deoxy-Hgb is paramagnetic if [deoxy-Hgb] local B

Ferromagnetic strong susceptibility

Paramagnetic weak susceptibility

Diamagnetic “no” susceptibility

Iron – ferromagnetic

Oxyhemoglobin – diamagnetic (electrons from oxygen shields iron)

Deoxyhemoglobin – paramagnetic

Oxygenated blood volume ↑, leads to ↓ in local susceptibility and local magnetic inhomogeneity.

Deoxygenated blood volume ↑, leads to ↑ in local susceptibility and local magnetic inhomogeneity.

Blood Interaction with a Magnet

Scanning Techniques and Parameters

•Interleaved Echo-Planer Imaging (EPI) is

the most common scanning technique for

fMRI.

•Gradient-Recalled Echo (GRE)

•Spin-Echo (SE). These sequences require

longer TE’s (~100ms) to maximize blood

susceptibility contrast. Lowers image

signal and SNR (trade-off).

Scanning Techniques and Parameters (cont.)

Data acquisition:

•A spiral acquisition of k-space is the

fastest.

•If using a linear acquisition an interleaved

slice acquisition is preferred to reduce

cross-talk from adjacent slices.

Spatial Resolution in fMRI

What affects spatial resolution in

fMRI?

•SNR

•Pixel size (matrix)

•Partial-volume effects

•FOV

•Slice Thickness

SNR in fMRI

What affects increase SNR in fMRI?

•Larger magnetic field strengths.

•Decrease TE.

•RF coils.

•FOV

•NEX/NSA

BOLD

Blood Oxygenation Level Dependent

= The ratio of deoxyhemoglobin to

oxyhemoglobin in blood.

• T2* is dependent on the presence of blood

deoxygenation

• T2* effect is larger by factors of 3 to 10 and

is the dominant and most widely-studied

mechanism employed in fMRI.

BOLD (cont.)

In short, the response to a local increase in

metabolic rate is increased delivery of blood

(oxygenated) to the activated region. Such a

change in hemodynamics produces small

alterations in T1, T2 or T2*, which can be

visualized as a change in MR image intensity

(approx. 1-10%).

Oxygenated blood is the source of contrast in

bold imaging.

Terminology

• CBV: Cerebral Blood Volume

• CBF: Cerebral Blood Flow

• HBr: Deoxy- Hemoglobin

• HRF: Hemodynamic Response Function

BOLD (cont.)

Another way to look at it:

Neural activity Signalling Vascular response

Vascular tone (reactivity)

Autoregulation

Metabolic signalling

BOLD signal

gli

a

arteriole

venule

B0 field

Synaptic signalling

Blood flow,

oxygenation

and volume

Remember?

Ferromagnetic strong susceptibility

Paramagnetic weak susceptibility

Diamagnetic “no” susceptibility

Iron – ferromagnetic

Oxyhemoglobin – diamagnetic (electrons from oxygen shields iron)

Deoxyhemoglobin – paramagnetic

Blood Oxygen Level Dependent

Imaging (BOLD) -Theory

Protons near paramagnetic tissue (i.e., more deoxygenated blood) experience a quicker dephasing after a spin excitation. Increase in T2* rate reduction in local signal.

So, for pulse sequences sensitive to T2* contrast, deoxyhemoglobin appears dark, and oxyhemoglobin appears bright.

Right Hand Motor Task

Left Hand Motor Task

Auditory Task

Task presentation systems

Visual presentation

Slide projector

LCD panel & overhead projector & rear screen

projection

Large screen LCD MRI projection systems

Electronic goggles

MRI compatible corrective lenses

fMRI Projection System

Lie Detector using fMRI?

http://blog.wired.com/wiredscience/2009/03/n

oliemri.html

Lie Detector using fMRI?

“Laboratory studies using fMRI, which measures blood-

oxygen levels in the brain, have suggested that when someone

lies, the brain sends more blood to the ventrolateral area of

the prefrontal cortex. In a very small number of studies,

researchers have identified lying in study subjects with

accuracy ranging from 76 percent to over 90 percent. But

some scientists and lawyers like Greely doubt that those

results will prove replicable outside the lab setting, and others

say it just isn’t ready yet.”

Questions?