MRI Imaging

What are the components of the MR system?

1. A large magnet to generate the magnetic field 2. Shim Coils to make the magnetic field as homogeneous as possible 3. RF Coil to transmit a radio signal into the body part being imaged. 4. A receiver coil to detect the returning radio signals 5. Gradient coils to provide spatial localization of the signals 6. A computer to reconstruct the radio signals into the final image

What is are the costs of increasing spatial resolution ?

1. Decreased signal-to-noise (s/n) ratio 2. Increased Scan Time

How can you increase the signal within an image?

1. Increase TR, FOV, slice thickness, or NEX 2. Decrease TE and matrix size

What parameters determine spatial resolution (SR)

1. Matrix size dir. 2. FOV (Field of view) indir. 3. Slice thickness (indir)

What Parameters control Magnetic Resonance?

1. Proton Density 2. Longitudinal Relaxation time (T1) 3. Transverse Relaxation Time (T2)

What Parameters determine the signal intensity on the MR image?

1. Proton density 2. T1 Relaxation time 3. T2 relaxation time 4. Flow

What are the three gradients used in MRI?

1. Slice Selection gradient (Z) Superior to Inferior 2. Phase encoding gradient (Y) Anterior to Posterior 3. Frequency encoding gradient (X) Right to Left

What are the most common pulse sequences?

1. T1 weighted spin echo sequences 2. T2 weighted spin echo sequences

What parameters impact Scan Time (ST)

1. TR (Repetition Time) dir. 2. Matrix Size dir. 3. NEX (# of excitations) dir.

What is K Space?

2D or 3D Fourier transform of the MR image measured Phase - lines in K space Frequency - points along each line Its complex values are sampled during an MR measurement, in a premeditated scheme controlled by a pulse sequence, i.e. an accurately timed sequence of radiofrequency and gradient pulses. often refers to the temporary image space, usually a matrix, in which data from digitized MR signals are stored during data acquisition. k-space holds raw data before reconstruction.

What is a SAT technique?

A SAT (saturation) technique uses extra RF pulses to eliminate artifacts from moving tissues outside the imaging volume.

What is an Adiabatic Pulse?

A frequency swept pulse B1. Excites the spectral frequencies simultaneously.

What is selective excitation, and how is achieved?

A magnetic field gradient is applied . RF pulse is applied that matches a location.

What is Phase in MRI signals

A position along a precessional path

What Scan Parameters impact Signal-to-noise

All of them

How else might a T2 weighted sequence be employed?

As a dual echo sequence. The first or shorter ehco (TE<30msec) is proton density weighted or a mixture of T1 and T2

What are the general scan parameters for T2-weighted spin-echo and FLAIR brain images?

Axial Plane, 256x256 matrix, 1 NEX, 22 CM FOV and 5 mm slice thickness, Scan Time < 4 minutes

How does one reduce ghost artifacts resulting from CSF flow

By using gradient moment rephasing or flow compensation techniques

What do Center lines in K-Space give you?

Contrast

How are MR signals sampled?

During "readout" at TE, points are stored in K-space until enough points are sampled to create an MR image.

When should SAT be used

For T1 weighted imaging of the sella, internal auditory canals and the spine.

When should gradient moment rephasing and flow compensation techniques be used ?

For T2-weighted spin-echo and gradient-echo acquisitions only. They should not be used for T1-weighted imaging because they increase the signal from CSF

Fluid Attenuated Inversion Recovery Sequence (FLAIR)

Has replaced the PD image. These images are T2-weighted with the CSF signal suppressed.

What are the benefits of a dual echo sequence?

Helpful for evaluating periventricular pathology (Such as multiple sclerosis)

When should a noncontrast T1-weighted image be used?

If preliminary scans suggest hemorrhage, lipoma, or demroid.

The no phase rap option is most effective when ?

In the anterior posterior direction for sagital and axial scans.

What determines the number of slices that can be obtained with one pulse sequence?

Increasing TE or shortening TR decreases the number of slices that can be obtained with one pulse sequence.

What does the magnetic field do to the Proton

It aligns the spins

What does flow do to a signal

It attenuates the signal due to rapidly flowing arterial blood

What does the application of the RF pulse do?

It excites the protons spins, and they precess at their Larmor frequency

When is contrast enhancement especially helpful ?

It is especially helpful for extra-axial tumors because they tend to be isointense to brain on plain scans. Identifies areas of BBB breakdown associated with intra-axial lesions Essential for detecting leptomeningeal inflammatory and neoplastic processes. Obtained routinely in patients with symptoms of pituitary adenoma Acoustic neuroma (sensorineural hearing loss)

What planes can MRI view

It is multiplanar and can be optimized

What is the sampling rate?

It is the rate of the read out. This rate should be twice the frequency. The sampling interval should be inverse of 2*v

What is a pulse sequence ?

It is the specific number, strength and timing of the RF and gradient pulses.

What does Cardiac gating do?

It reduces the artifacts from CSF pulsations, which results in superior object contrast and resolving power in the temporal lobes, basal ganglia and brain stem

What do phase and frequency signal information provide?

Location

How do you determine which pulse sequence was used or the weighting of the image?

Look at the cerebrospinal fluid (CSF). If it's bright (high signal) then it must be T2-weighted. If it is dark (low signal), it is T1-weighted.

Does TE affect Scan Time (ST)

No, but it does determine the maximum number of slices in multi-slice mode

Do flow compensation techniques contribute to specific absorption rate. (SAR) ?

No, but the extra gradient pulses lengthen the minimum TE, and gradient heating may limit the number of slices, the minimum FOV and slice thickness.

How does one obtain a thicker slice of image?

One increases the range of Slice selection radio frequencies transmitted.

A Steep slice selection gradient (High amplitude)

Produces thin slices

What do edge lines in K Space give you?

Resolution

What do the signal intensities on the T1, T2 and proton density-weighted images relate to ?

Specific Tissue characteristics e.g. changing chemistry or physical structure

IF a patient is suspected of intracranial disease which type of screen should be used?

T2-weighted spin-echo and FLAIR images.

What is Echo time ?

TE is the time between the initial 90 degree RF pulse and the echo.

What is Repetition time ?

TR is the time between consecutive 90 degree RF pulses

What are the most important parameters of the pulse sequence?

The Repetition time (TR) The Echo time (TE)

On MR images of the brain what are the primary determinants of signal intensity and contrast?

The T1 and T2 relaxation times.

What does amplitude signal information provide?

The color

What is Proton denisty

The concentration of protons in the tissue in the form of water and macromolecules

What are the costs of a SAT technique ?

The extra RF pulses cost SAR and take time, lengthening the minimum TR or decreasing the maximum number of slices in a multi-slice mode.

Which is the most direct way to increase signal and what is the cost?

The most direct way to increase the signal is to increase the NEX. The cost is that a 2X increase in NEX, doubles the scan time but only increases the signal by sqrt(2).

What is Frequency in MRI signals

The rate of precession

How are the MR signals created?

They are created by a 90 and 180 degree pulse

How are Gadolinium based contrast agents used and how do they work?

They are para-magnetic and have demonstrated excellent biologic tolerance. Injected IV at dose of 0.1 mmol/kg They no not cross the BBB unless it is disrupted by disease. Contrast diffuses into interstitial space and shortens the T1 relaxation time of the tissue, increasing signal intensity on T1 weighted images. Scans should be acquired between 3-30 minutes post injection

What can you learn from Flow sensitive pulse sequences and MR angiography ?

This yields data about blood flow and displays vascular anatomy.

What are RF gradient pulses used for?

To determine coordinate data

How can one vary image contrast?

Use Different Pulse sequences and change the imaging parameters

Features of the T2 weighted sequence ?

Uses long TR(TR > 2000msec) and long TE (TE<80msec)

Features of the T1 weighted sequence ?

Uses short TR(TR < 1000msec) and short TE (TE<30msec)

How can the contrast on an MR image be changed?

by changing the pulse sequence parameters.

What do the T1 and T2 relaxation times represent?

these times define the way that the protons revert back to their resting states after the initial RF pulse.