ADVANCED MR IMAGING MRI QUIZ

What is diffusion MRI?

MRI that images tissue microstructure using water diffusion.

What can diffusion MRI help show?

Connections between brain regions.

What does DWI voxel intensity reflect?

A measurement of water diffusion rate at that location.

Why is DWI useful in acute stroke?

It is more sensitive to early stroke changes than T1 or T2 measurements.

Where is isotropic diffusion common?

Cerebral cortical gray matter and major brain nuclei.

What is isotropic diffusion?

Water moves equally in every direction.

What is anisotropic diffusion?

Water moves preferentially in a particular direction.

How does isotropic diffusion appear on an FA map?

Dark.

How does anisotropic diffusion appear on an FA map?

Bright; color encodes diffusion orientation.

What does FA stand for?

Fractional anisotropy.

What does an FA map display?

Anisotropic diffusion relative to average overall diffusion.

What are ADC values?

Apparent diffusion coefficient values describing diffusion within a voxel.

What was the first successful clinical DWI application?

Imaging the adult brain following stroke.

How do stroke-injured areas appear on an ADC map in this document?

Darker than healthy tissue.

When is brain DWI especially important?

When a CVA is suspected.

What is diffusion in tissue?

Thermally induced motion of water molecules.

What determines diffusion weighting?

Gradient strength, gradient duration, and time between gradient pulses.

What is Brownian motion?

Molecular motion; DWI measures the rate of water diffusion.

What is the b-value?

A selectable DWI parameter controlling diffusion-weighting strength.

What happens when b-value increases?

Diffusion weighting and signal loss increase.

How many acquisitions are needed to calculate ADC?

At least two with different diffusion weightings.

What does low ADC correspond to?

High DWI signal and restricted diffusion.

What does high ADC correspond to on DWI?

Low signal intensity.

How may DWI be acquired?

Single-shot spin-echo EPI with diffusion gradients on each side of the 180° pulse.

What does the first diffusion gradient do?

Dephases spins.

What does the second diffusion gradient do if there is no net motion?

Rephases spins.

How can parallel imaging help DWI?

It can reduce magnetic susceptibility artifacts.

What determines DWI contrast?

Restricted molecular mobility, Brownian-motion sensitivity, and diffusion direction.

What are common DWI problems?

Susceptibility artifacts with EPI, bulk patient motion, and low SNR.

What is the final result of a perfusion study?

Calculated images showing various flow characteristics.

What does perfusion imaging help establish?

The vascularity of a process.

What tissue is used as an internal perfusion reference?

Normal-appearing white matter (NAWM).

What is the perfusion measurement unit?

Relative cerebral blood volume (rCBV).

What is rCBV proportional to?

Neovascularity or capillary density.

What are uses of perfusion imaging?

Tumor characterization, glioma grading, biopsy guidance, and distinguishing recurrence from radiation necrosis.

What perfusion protocol is described?

Ultra-fast GRE with IV gadolinium bolus at 4 cc/sec.

Where are perfusion exponential time curves generated?

On a separate workstation.

How are specific rCBV values calculated?

Using multiple ROIs on exponential time curves.

How is NAWM used in perfusion analysis?

As the control for comparison with pathology or edema.

What does DSC stand for?

Dynamic Susceptibility Contrast.

What type of imaging is DSC?

T2* GRE perfusion imaging used in brain perfusion.

What does DCE stand for?

Dynamic Contrast Enhancement.

What type of imaging is DCE?

Basic T1 perfusion imaging.

Which DCE applications are listed?

Pituitary and prostate dynamic sequences.

How does DCE timing compare with DSC?

DCE has longer acquisition times and is more dynamic.

What is functional MRI?

Rapid imaging while a patient rests or performs a task.

What does fMRI measure?

Blood-flow and oxygenation changes associated with brain activity.

What happens to local deoxyhemoglobin in the fMRI response described?

It increases and triggers increased blood flow to maintain nearby oxygen.

What does fMRI allow?

Real-time evaluation of cerebral activity.

What supports rapid fMRI and movie-like output?

Special hardware and advanced software.

What is MR spectroscopy (MRS)?

A noninvasive method for obtaining biochemical information about tissue.

What is the final result of MRS?

A spectrum of detected metabolites.

What does peak height represent on an MR spectrum?

The amount of the substance detected.

What does peak position represent?

The metabolite's frequency or chemical shift.

What determines a proton's chemical shift?

Its chemical environment and neighboring protons.

Which metabolites are listed in a normal brain 1H spectrum?

Choline, lactate, creatine, and NAA.

What is the major NAA peak?

2.02 ppm.

What can decreased NAA indicate?

Loss or damage to neuronal tissue.

What does normal NAA indicate?

Neuronal and axonal integrity.

What is the major choline peak?

3.2 ppm.

What is choline associated with?

Membrane turnover and increased cell division.

What can increased choline suggest?

Cell production, membrane breakdown, demyelination, or malignant tumor.

What is the major creatine peak?

3.0 ppm.

What does creatine reflect?

Brain energy metabolism.

What can gradual creatine loss indicate?

Tissue or major cell death.

What may increased creatine indicate?

A response to cranial-cerebral trauma.

What may absence of creatine indicate?

A rare congenital disease.

Where are major lipid peaks?

0.9–1.5 ppm.

What can increased lipids indicate?

Necrosis.

Why can lipid spectra be contaminated?

Lipids in scalp fat and tissue between scalp and skull can contribute.

How does lactate appear on MRS?

A doublet at approximately 1.33 ppm.

Why is lactate normally not visible?

Its concentration is usually below the MRS detection limit.

What does a visible lactate peak indicate?

Glycolysis in an oxygen-deficient environment.

What causes of elevated lactate are listed?

Ischemia, hypoxia, mitochondrial disorders, and some tumors.

What is the major myo-inositol peak?

3.56 ppm.

Which conditions are associated with increased myo-inositol in the document?

Alzheimer's, dementia, and HIV.

Where are glutamate and glutamine peaks?

2.2–2.4 ppm.

What conditions are associated with elevated glutamine and glutamate?

Hyperammonemia and hepatic encephalopathy.

What MRS changes are shown on the tumor side in the example?

Decreased NAA and increased choline.

What may decreased NAA with increased choline indicate?

Neoplasm or mesial temporal sclerosis.

What is multi-voxel MRS used for?

Detecting multiple ROIs in a larger volume of interest.

At 1 T, what is the water-fat frequency difference?

Approximately 147 Hz.

At 1.5 T, what is the water-fat frequency difference?

Approximately 220 Hz.

How does chemical shift change as field strength increases?

The water-fat frequency difference increases.

What factors are needed to calculate water-fat pixel shift?

Bandwidth, field strength, and matrix.

In the 26.8 kHz bandwidth, 224 matrix example, what is Hz per pixel?

120 Hz per pixel.

In that example, what is the fat-water pixel shift?

Approximately 1.8 pixels along the frequency/readout axis.

In the 28.2 kHz, 256 matrix cervical example, what is the shift?

2 pixels.

How do you convert 26.8 kHz bandwidth to Hz?

Multiply by 1000 to get 26,800 Hz.

How do you calculate frequency difference per pixel?

Bandwidth in Hz divided by frequency matrix.

How do you calculate water-fat pixel shift?

Chemical-shift frequency difference divided by Hz per pixel.