03) MRI: Image Characteristics

1.

The parameter that best describes three-dimensional resolution is:

A) Field of view

B) Matrix size

C) Pixel size

D) Voxel size

E) Signal-to-noise ratio

D) Voxel size

2.The voxel volume increases with:

A) Field of view

B) Matrix size

C) Increasing frequency-encoding gradient strength

D) Increasing slice-select gradient strength

A) Field of view

Feedback:

Voxel volume is given by:

Increasing the FOV increases the voxel volume. Increasing Matrix size results in a decrease in voxel volume as does increases in the frequency and slice select gradiant strengths.

3.

Pixel size is smaller when:

A) The gradient is applied for a shorter duration

B) The gradient amplitude is increased

C) The gradient amplitude is decreased

D) The gradient is applied for a longer duration

B) The gradient amplitude is increased

D) The gradient is applied for a longer duration

Feedback: Pixel size is inversely proportional to the product of the gradient amplitude and its duration. The higher the product, the better the spatial resolution will be: i.e. the smaller the pixel size. The product can be increased by increasing either amplitude or duration or both.

4.

Reducing the voxel size will result in all of the following EXCEPT

A) smaller pixel size

B) lower signal-to-noise ratio

C) lower spatial resolution

D) better ability to distinguish between two closely spaced objects on an image

C) lower spatial resolution

Feedback: Reducing the voxel size results in a higher spatial resolution since the smaller the voxels, the better is the ability to distinguish two closely spaced objects (which is the spatial resolution).

5.

Surface coils have a higher signal-to-noise ratio than whole body coils for all of the following reasons EXCEPT

A) Increased signal strength

B) reduced volume of coverage

C) Increased noise sensitivity

D) proximity to the volume of interest

C) Increased noise sensitivity

Feedback: The surface coil has a much reduced volume of coverage than whole body coils and the noise detected by a coil is directly proportional to the volume of coverage. So a small volume of coverage means a decrease in noise sensitivity.

6.

When the magnetic field strength increases from 1.5T to 3T, we observe

A) a quadrupling of the signal-to-noise ratio

B) a reduction in the precessional frequency

C) a modification of the proportions of spins in the parallel and anti-parallel state(s)

D) Increased receiver coil sensitivity

C) a modification of the proportions of spins in the parallel and anti-parallel state(s)

Feedback: The energy level splitting between parallel and antiparallel spins increases with field and this changes the proportion of spins in the two spin states (a greater population difference at higher fields).

7.

When the number of signal averages is increased, it results in a linear increase in:

A) Acquisition time

B) Signal-to-noise ratio

C) Spatial resolution

D) Field of view

A) Acquisition time

8.

An increase in the receiver bandwidth will result in all of the following EXCEPT:

A) Requires a stronger readout gradient

B) Increases the signal-to-noise ratio

C) Reduces chemical shift artifacts

D) Reduces the minimum possible TE

E) Reduces metallic artifacts

B) Increases the signal-to-noise ratio

Feedback: The noise is proportional to the square root of the receiver bandwidth, thus increasing as the receiver bandwidth is increased. The signal to noise thus decreases. Since the frequency content per pixel increases with receiver bandwidth, mismapping from chemical shift and/or metallic objects decreases.

9.

Increasing the sampling time results in a reduced signal-to-noise ratio.

A) True

B) False

B) False

Feedback: Increasing the sampling time of the MR echo corresponds to a narrow receiver bandwidth. A narrow receiver bandwidth has less noise and this results in a increased signal to noise ratio.

10.

Slice cross-talk can occur in all of the following situations EXCEPT:

A) Slice separation distances are too small

B) Slice profile is not rectangular

C) Oblique slices intersect

D) Slice select amplitude is too high

D) Slice select amplitude is too high

Feedback: Slice select gradient strength determines slice thickness and the higher the slice select gradient, the thinner the slices selected. This does not lead to slice cross talk. In all other choices, adjacent slices can overlap and lead to cross-talk.

11.

Using partial k-space filling techniques reduces both SNR and acquisition time.

A) True

B) False

A) True

Feedback: Partial k-space filling synthesizes part of the k space using the the symmetry properties of the k-space data. Since only part of the data is acquired, this reduces the acquisition time. However, since the noise in the synthesized data is the same as in the acquired data, this reduces the SNR.

12.

For a given square matrix size, increasing the FOV from 25 cm to 50 cm results in:

A) A doubling of the voxel volume

B) A doubling of the signal-to-noise ratio

C) An increase in truncation artifacts

D) A reduction in spatial resolution

D) A reduction in spatial resolution

Feedback: Since the FOV increases by a factor of 2 in both the readout and phase-encode directions, this results in voxel volume and SNR increasing by a factor of 4. Since the voxel volume increases, the spatial resolution decreases.

13.

For a given FOV size, reducing the matrix size from 512 x 512 to 256 x 256 results in:

A) A doubling of the voxel volume

B) A reduction in signal-to-noise ratio

C) A reduction in total acquisition time

D) Improved spatial resolution

C) A reduction in total acquisition time

Feedback: The time taken to acquire 256 phase encode lines as opposed to 512 phase encode lines is cut by a factor of 2. The voxel volume increases by a factor of 4, SNR increases and spatial resolution decreases (as voxel volume increases)

14.

For a given square matrix size and FOV size, increasing the number of signal averages from 2 to 8 results in:

A) A doubling of the voxel volume

B) A doubling of the signal-to-noise ratio

C) A reduction in total acquisition time

D) A reduction in spatial resolution

B) A doubling of the signal-to-noise ratio

15.For a given square matrix size and FOV size, increasing the slice thickness from 2 mm to 4 mm results in:

A) Reduced voxel volume

B) A doubling of the signal-to-noise ratio

C) A reduction in the total acquisition time

D) Improved spatial resolution

B) A doubling of the signal-to-noise ratio

Feedback: Increasing the slice thickness by a factor of 2, increases the voxel volume by a factor of 2. The signal is proportional to voxel volume and thus increases by a factor of 2 which in turn leads to a doubling of SNR.In terms of the other choices, increasing slice thickness increases voxel volume, does not impact total acquisition time, and decreases spatial resolution.

16.

In a spin-echo sequence, T1-weighting is obtained with:

A) Long TE, Short TR

B) Long TE, Short TR

C) Short TE, Short TR

D) Short TE, Long TR

C) Short TE, Short TR

Feedback: Short TE results in a lowered sensitivity to T2 differences while a short TR results in an increased sensitivity to T1 differences, thus leading to a T1 weighted image.

17.

A T2-weighted spin echo sequence is obtained with:

A) Long TE, long TR

B) Long TE, short TR

C) Short TE, short TR

D) Short TE, long TR

A) Long TE, long TR

Feedback: Long TR results in a lowered sensitivity to T1 differences while a long TE results in an increased sensitivity to T2 differences, thus leading to a T2 weighted image.

18.

A PD-weighted spin echo sequence is obtained with:

A) Long TE, long TR

B) Long TE, short TR

C) Short TE, short TR

D) Short TE, long TR

D) Short TE, long TR

Feedback: Short TE results in a lowered sensitivity to T2 differences while a long TR results in an decreased sensitivity to T1 differences, thus leading to a proton density (PD) weighted image.

19.

Gradient echo sequences are more sensitive to magnetic susceptibility effects due to the presence of a 180° refocusing RF pulse

A) True

B) False

B) False

Feedback: Gradient echo sequences are more sensitive to magnetic susceptibility effects but they do not have a 180° refocusing RF pulse. In fact they are sensitive to susceptibility effects because of the absence of the 180° refocusing RF pulse and the use of gradients for refocusing.

20.

When gradient echo sequences are used, the longer the TE, the more T2* - weighted the images are.

A) True

B) False

A) True

Feedback: The signal in a gradient echo image is proportional to exp(-TE/T2*), so the longer the TE is, the greater is the sensitivity to T2* differences, leading to a more T2* weighted sequence.