MRI Chapters 5-8

It is emitted immediately after the termination of the transmitted RF pulse and its electronics of the receiver must respond more quickly.

What makes the FID more difficult to detect than an SE?

90°-180°.... 90°-180°... etc.

Describe the RF pulse sequence used for SE imaging.

The timing of the energizing of the radio frequency transmitter and the gradient magnetic field coils.

What does the term sequence mean when applied to MRI?

Echo planar imaging ( EPI )

Of all the available MRI pulse sequences, which provides the fastest imaging?

When the repetition time (TR) is not sufficiently long to allow complete longitudinal relaxation of the spin to ensemble to equilibrium.

How is the RF pulse indicated for an STE echo pulse sequence?

See figure 5.2

What is the TE in SE imaging?

The time between the initial 90° RF pulse and the middle of the spin echo. This time is 2 x the time between the 90° RF pulse and the 180° RF pulse

Diagram the transmitted and received signals for a double echo, SE pulse sequence.

See figure 5.2 for a diagram of the transmitted and received signals for a double echo, spin echo pulse sequence.

What is the inversion time in an IR pulse sequence?

The time between the inverting 180° RF pulse and the 90° RF pulse.

What is the MRI signal called when it is obtained during an IR pulse sequence?

A spin echo. Although the inversion recovery pulse sequence is generally identified as 180°RF pulse that follows each 90° RF pulse, and that produces the spin echo.

What is the minimum number of pulses required to produce and STE?

Proton density (PD), longitudinal relaxation time (T1) and transverse relaxation time (T2)

Name the three principle MRI parameters characteristic of each tissue.

T2*, T2, T1

Arrange the three MRI relaxation times, T1, T2, and T2*, from shortest to longest for soft tissue.

The concentration of mobile hydrogen, proton density (PD).

Which MRI characteristic of tissue principally determines the intensity of the MR signal?

Magnetic field inhomogenity

What is the principal reason that the T2* is always shorter than T2?

63%, approximately 5 T1s.

How much longitudinal relaxation occurs during one T1, and approximately how many relaxation times are needed for complete relaxation to reach equilibrium?

Frequency is the rate at which something revolves or spins. Phase is the direction that multiple spins exhibit at any given instant.

Distinguish between frequency and pulse.

The molecular species in which the hydrogen atom is embedded.

T1 relaxation is spin-lattice relaxation. To what does lattice refer?

The rate at which the signal relaxes bc of T2*. One needs transverse relaxation in a perfectly homogenous magnetic field for the envelope of the signal represent T2 relaxation.

What does the envelope of an FID represent?

Tissues with short T1 appear bright, tissues with long T 1 appear dark, but high proton density enhances the appearance of both.

In a T1W image, which tissues appear bright and which dark?

Tissues with long T2 appear brighter than tissues with short T2, however, all tissues with higher proton density will appear brighter.

When a T2W image is presented, what tissues will appear bright?

The B0 field is too intense and inhomogenous.

What is the principle reason MRI relaxation time cannot be measured directly?

The saturation recovery pulse sequence with varying repetition time (TR). The inversion recovery pulse sequence with varying inversion time (TI).

What pulse sequences can be used to determine T1 relaxation time?

In the middle of the spin echo that was produced.

At what time after the 180° RF refocusing pulse is the MR signal most intense?

A spin echo

What is the MR signal used to make an inversion recovery image?

A FID of long duration whose envelope would describe the true T2 of the tissue.

If you conducted a spin echo pulse Seymour in an absolutely uniform B0 magnetic field, what would be the results.

Bc of all the net magnetization is along the x axis, which is coplanar to the external magnetic field, B0. Some net magnetization must be projected onto the system plane for a signal to be detectable.

During inversion recovery imaging, why is a signal not detected after the initial 180° RF pulse?

See figure 7.1

Diagram the difference in the transverse relaxation representing T2 and that representing T2*

A FID followed by three spin echoes, each of decreasing intensity and alternating polarity.

What would result from the following RF pulse sequence: 90°.... 180°.... 180°..... 180°...?

There is no signal.

Describe the signal obtained during inversion recovery imaging when the inversion time equals the time that longitudinal relaxation passes through the origin.

See figure 7.7 for an illustration of the vector diagram that represents tissue magnetization after a 90° RF pulse.

Draw the vector diagram that represents tissue magnetization after a 90° RF pulse

A new signal of intensity versus inverse time, hertz, the NMR spectrum.

The aft changes the MR signal from intensity versus time to what quantity?

Aliasing or wraparound artifact .

What artifact results when the MR signal is not adequately sampled?

K space

What is another name for spatial frequency domain?

At least 2 data points, samplings, within each cycle of the MR signal.

What is the minimum sampling rate of an MR signal that will ensure aliasing does not occur

Nyquist

Whose name is applied to sampling theory?

Any bone soft tissue interfaces, breast micro calcifications, and calcified lung nodules.

What property of the image will be affected if the space between the samples raw data lines is doubled?

Yes, the more data acquired, the higher the capacity of the computer necessary, and the longer it will take to reconstruct an image.

Identify three examples of tissue with high spatial frequency components.

The gradient coils, which generate the gradient magnetic fields.

Are there any other disadvantages to over sampling an MR signal?

A special form of the Fourier transform particularly adapted from the computed generation of solutions where the samples signal is truncated, allowing the computation to be completed more quickly.

Which component of the MRI system is principally responsible for locating the position of the MR signal in the body?

See figure 8.1 for a graphic representation of the Fourier transform of a square wave and a spin echo.

Graphically show how the FT of a square wave and an SE should appear.

Proton density (PD), T1 relaxation time, T2 relaxation time

What are the three principle magnetic resonance imaging (MRI) tissue parameters?

90°

In a basic one-pulse sequence, what is the flip angle typically used?

Inverse Fourier Transform

Which of the following is used to produce the final MRI image from the raw data?

T2* is considered the "observed" or "effective" T2

What is the primary difference between T2 and T2* relaxation?

Linearity

Which of the following is NOT a fundamental property of Fourier Transform?

Spin Echo

Which of the following pulse sequences is typically used to produce T2-weighted images?

Spin echo

Which pulse sequence is defined as the reappearance of an MR signal after the FUD has disappeared, resulting from the effective reversal of rephrasing of the nuclear spins?

180°... 90°...180°

In Inversion Recovery (IR) pulse sequence, what is the correct order of RF pulses?

A matrix storing raw data representing spatial frequency information.

In MRI, what does the term k-space refer to?

They decrease T1 Time

How do MRI contrast agents typically affect T1 relaxation time?

To increase imaging speed

What is the purpose of using a flip angle alpha <90° in a partial flip angle pulse sequence?

Short TR and Short TE

In T1-weighted imaging, what combination of TR and TE is generally used?

An artifact caused by inadequate signal sampling.

What is aliasing in the context of MRI?

Differentiating between gray and white matter in brain tissue

Which of the following is NOT a typical application of T2* weighted imaging?

To convert the MRI signal from the time domain to the frequency domain.

What is the primary function of the Fourier Transform in MRI?

The time it takes for 63% of the longitudinal magnetization to recover.

Which of the following best describes the T1 relaxation time?

Free Induction Decay (FID)

What is the primary MRI signal called?

T2 never exceeds T1

What is the relationship between T1 and T2 relaxation times in tissues?

Multiple spins all in the same position of a periodic cycle.

Which of the following correctly defines phase coherence in MRI?

Bright

In T1 weighted images, how do tissues with short T1 relaxation times generally appear?