02) MRI: Image Formation

1.

For the image below acquired using a 2D sequence, assume that you wish to image a smaller field of view than what is shown. Along which direction should phase encoding occur to minimize the chance of aliasing?

A) S-I

B) A-P

C) L-R

2.

For the image below acquired using a 2D sequence, assume that you wish to image a smaller field of view than what is shown. Along which direction should phase encoding occur to minimize the chance of aliasing?

A) S-I

B) A-P

C) L-R

B) A-P

3.

For the image below acquired using a 2D sequence, assume that you wish to image a smaller field of view than what is shown. Along which direction should phase encoding occur to minimize the chance of aliasing?

A) S-I

B) A-P

C) L-R

4.

During frequency encoding, the phases of all moments in a slice are the same.

A) True

B) False

5.

During image acquisition, data in a small region in k-space is corrupted by noise, increasing the value dramatically. Which image most likely would be reconstructed under these circumstances?

Image (b) from R. Ballinger, http://www.mritutor.org/mritutor/index.htm , 1994-2009

Image (c) from J. Zhuo & R. P. Gullapalli, "MR Artifacts, Safety, and Quality Control", Radiographics, 2006; 26: 275-97.

A) A

B) B

C) C

6.

Using an ordinary 2D MRI sequence to image one slice of tissue, what is acquired during each interval TR?

A) Data that fills one row in k-space

B) Data that constructs one row of pixels in an image

C) Data that constructs one image

D) Data that comprises one spatial frequency

A) Data that fills one row in k-space

7.

For this image, in what direction is the frequency encoding gradient oriented?

A) S-I

B) A-P

C) L-R

8.

The gradient systems aren’t working properly, producing a gradient that allows the field to vary unevenly (see picture below).

If everything else works properly during slice selection, which picture below shows the most likely spatial distribution of excited tissue (within green areas)?

A) A

B) B

C) C

B) B

9.

The scanner has a computer problem. After running a full imaging sequence, the computer for your scanner makes a mistake during image reconstruction. All of the data in k-space except for one location is set to zero. In this case, which of the following images is the most likely outcome after reconstruction?

A) A

B) B

C) C

D) D

A) A

10.

The gradients are malfunctioning. During slice selection, everything proceeds as planned, except the slice-select gradient does not turn on. What is the most likely outcome?

A) Nyquist ghosting increases in the image.

B) Nuclei of a different kind (e.g., potassium) are excited

C) A large portion of nuclei throughout the scanner are excited

D) Nuclei precess at a different frequency than usual.

C) A large portion of nuclei throughout the scanner are excited

11.

The MRI RF system is malfunctioning. During slice selection, the RF pulse transmits without a bandwidth (i.e., the excitation frequency is exactly 63.87 MHz). What is the most likely outcome?

A) No nuclei are excited anywhere in the scanner

B) A very thin slice of nuclei are excited

C) A very thick slab of nuclei are excited

D) Phase is coherent throughout all tissue

B) A very thin slice of nuclei are excited

12.

The gradients are broken again. Before readout, the frequency-encode gradient accidently turns off. What is the most likely outcome?

A) All nuclei in a slice are phase incoherent

B) All nuclei in a slice are phase coherent

C) Another slice is excited

D) Only one pair of ensembles will be observed during readout.

13.

Nuclear magnetic moments that are not phase coherent when they precess produce a strong signal in a coil.

A) True

B) False

14.

The purpose of phase and frequency encoding is:

A) To excite a slice of tissue

B) To detect a voltage in a coil.

C) To specify the thickness of a slice

D) To “label” nuclei uniquely according to their position.

15.

Motion artifacts occur predominantly along which direction?

A) Phase encoding

B) Frequency encoding

A) Phase encoding

16.

Zipper artifacts occur predominantly along which direction?

A) Phase encoding

B) Frequency encoding

A) Phase encoding

17.

Chemical shift artifacts occur predominantly along which direction?

A) Phase encoding

B) Frequency encoding

18.

What component of image formation is adjusted differently after each time TR?

A) Phase encoding

B) Readout

C) Fourier transform

D) Frequency encoding

A) Phase encoding

19.

Gradients affect the resonant frequency of nuclei according to the Larmor Equation, which states that:

A) Hydrogen nuclei precess at a unique frequency

B) The gyromagnetic ratio depends on gradient strength

C) The resonant frequency of a nucleus depends on the magnetic field in that region.

20.

If the direction of the slice-select gradient is different between two imaging sequences, which of the following is altered between the two sets of images?

A) The slice thickness

B) The slice orientation

C) The slice resolution

D) The slice contrast

B) The slice orientation

21.

What is different between 2D and 3D imaging sequences?

A) The number of phase-encoding directions

B) The number of frequency-encoding directions

C) The number of slice-encoding directions

D) All of the above

E) A and C above

22.

If nuclear moments are all in phase coherence before phase encoding, they are not in phase coherence after phase encoding. This phenomenon occurs because the precession frequency depends on location when a magnetic field gradient is on.

A) True

B) False

A) True