MRI Image Basics

Calculate the pixel area for the following sequence: TR 450, TE 12, FOV 18cm, 224 x 256 matrix, 4 NEX. A. 0.56 mm2 B. 0.70 mm2 C. 0.80 mm2 D. 1.50 mm2

0.56 mm2

Calculate the pixel area for the following sequence: TR 4000, TE 120, FOV 28cm, 304 x 304 matrix, 2 NEX, ETL 12. A. 0.76 mm2 B. 0.85 mm2 C. 0.92 mm2 D. 1.84 mm2

0.85 mm2 FOV/matrix x FOV/matrix = pixel area. 280/304 x 280/304 = 0.85 mm2.

The pixel size of the images which result from a 3D acquisition using a 240mm FOV, a 272 x 272 matrix, 32 slices and a volume of 64 mm is: A. 0.56 x 0.56 mm B. 0.88 x 0.88 mm C. 1.12 x 1.12 mm D. 1.58 x 1.58 mm

0.88 x 0.88 mm FOV ÷ Matrix = pixel size..... 240 ÷ 272 = 0.88mm

The smallest object that can be resolved in an image acquired with a 20cm FOV and a 224 x 224 matrix is _________ mm. A. 0.54 B. 0.89 C. 1.12 D. 1.89

0.89200 mm ÷ 224 = 0.892mm

Choose the correct slice thickness to create an isotropic voxel for the following: TR 2000, TE 90, Matrix 256 x 256, FOV 32cm. A. 0.75 mm B. 1.25 mm C. 2.00 mm D. 2.50 mm

1.25 mm An isotropic voxel has equal measurements in all three dimensions (phase, frequency and slice direction) FOV/Matrix = 1.25mm

Calculate the voxel volume for the following pulse sequence parameters: TR 450, TE 20, FOV 24cm, 320 x 320 matrix, 3mm slice thickness. A. 0.75mm3 B. 1.69mm3 C. 1.75mm3 D. 2.25mm3

1.69mm3 To calculate voxel volume, first determine pixel area. FOV/matrix x FOV/matrix = pixel area. Pixel area x slice thickness = voxel volume. 240/320 x 240/320 = 0.5625 mm2. 0.5625 x 3 = 1.69mm3

If the NEX is increased from 2 to 6, the signal-to-noise ratio increase would be: A. 1.4 B. 1.7 C. 2 D. 4

1.7 When increasing NEX/NSA, SNR (signal-to-noise ratio) increases by the square root (√) of the % increase. For example, a sequence with 2 NEX, increased to 6 NEX (3times the total NEX) would have an increase in SNR of √3, or 1.73 times more SNR than the original sequence.

Assuming all other parameters are the same, an echo time of ___________ ms provides the best Signal-to-Noise Ratio. A. 15 B. 30 C. 80 D. 110

15

The scan time of a fast spin echo sequence utilizing a TR of 3500ms, a TE of 90ms, a 256x256 matrix, 1NSA, a 220mm FOV and an echo train length of 5 is ______ minutes, approximately. A. 2 B. 3 C. 8 D. 11

3 TR (3500) x Phase Matrix (256) x NSA (1) ÷ ETL (5) = 179200 ms, then divide by 1000 to get 179.2 seconds, or nearly 3 minutes.

In a fast spin echo sequence with a 28 cm FOV, 256 x 512 matrix, TR 2200, 90 TE, 5mm slice thickness, 8 ETL and 3 NEX, the scan time would be: A. 2 minutes 11 seconds B. 3 minutes 31 seconds C. 3 minutes 52 seconds D. 4 minutes 8 seconds

3 minutes 31 seconds Scan time formula: TR x Phase Matrix x NEX ÷ ETL = total in milliseconds, then ÷ 1000 to achieve scan time in seconds. Be careful if you divide by 60 to achieve the scan time in decimal format (This example computes to 211 seconds, if you divide by 60 you get 3.52 seconds, which is 3 minutes 31 seconds, not 3 minutes 52 seconds)

Calculate the voxel volume for the following pulse sequence: TR 2000, TE 85, FOV 28cm, Matrix 272 x 320, 5mm slice thickness, 3 NSA. A. 0.88 mm3 B. 1.03 mm3 C. 1.91mm3 D. 4.55 mm3

4.55 mm3

Which parameter will provide the better spatial resolution? A. 4mm slice thickness; 256 x 192 matrix B. 4mm slice thickness; 256 x 256 matrix C. 6mm slice thickness; 256 x 192 matrix D. 6mm slice thickness; 256 x 256 matrix

4mm slice thickness; 256 x 256 matrix Thinner slices yield improved spatial resolution, in addition to smaller pixel size (higher matrix)

Which of the following pulse sequence parameter combinations would yield an image with the most blurring? A. 400TR, 25TE, 256 x 256, 5mm slice thickness, 1NSA B. 2000TR, 30TE, 256 x 256, 5mm slice thickness, 6 ETL, 2NSA C. 4000TR, 60TE, 256 x 256, 5mm slice thickness, 12 ETL, 4NSA D. 6000TR, 80TE, 256 x 256, 5mm slice thickness, 16 ETL, 4NSA

6000TR, 80TE, 256 x 256, 5mm slice thickness, 16 ETL, 4NSA The longer the ETL (echo train length), the increased chance of blurring in the image.

Assuming all other parameters are the same, a ___________ ms TR provides the best signal-to-noise ratio. A. 30 B. 200 C. 500 D. 800

800The longer the TR, the better the SNR.

What would the scan time be in a 3D volume acquisition as follows: TR 32 ms, TE 4 ms, Flip Angle 15°, FOV 18cm, 2mm slice thickness, 96 slices, 192 x 256 matrix, 1 NEX? A. 4 min 28 sec B. 5 min 12 sec C. 8 min 42 sec D. 9 min 50 sec

9 min 50 sec

Acquiring a bit more than half the phase k-space samples, then interpolating the data with zeroes for the remaining half is a technique known as: A. Half Fourier B. Zero fill C. Fast spin echo D. A and/or B

A and/or B Acquiring a bit more than half the phase k-space samples, then interpolating the data with zeroes for the remaining half is a technique known as Half Fourier, or Zero Fill.

The overall signal-to-noise ratio will increase in 3D volume imaging when there is an increase in: A. Number of slices or partitions B. TE C. Phase Matrix D. Field of View E. A and/or D

A and/or D The overall signal-to-noise ratio (SNR) will increase in 3D volume imaging when there is an increase in either the Field of View (FOV) and/or an increase in the number of slices or partitions (chunks).

Reducing the slice thickness will result in: A. An increase in partial volume averaging B. A decrease in SNR C. An increase in SNR D. A & B only E. None of the above

A decrease in SNR Reducing the slice thickness will result in a decrease in SNR, but will decrease partial volume averaging.

A set of specifically timed instructions to the magnet is known as: A. A recipe B. A pulse sequence C. Gyromagnetic instructor D. Fourier transformation

A pulse sequence A pulse sequence is a set of specifically timed instructions to the magnet telling it how images should look with regards to the tissue being sampled.

Reducing the phase matrix from 256 to 192 will: A. Increase the signal-to-noise ratio B. Decrease imaging time C. Decrease spatial resolution D. All of the above

All of the above Reducing the phase encoding matrix yields higher SNR, shorter scan times, but decreased spatial resolution due to the increased pixel size.

Decreasing the TR will: A. Lower SNR B. Reduce scan time C. Increase SNR D. Both A and B

Both A and B

High amplitude signals are stored in the: A. Outer edges of k-space B. Bottom line of k-space C. Top line of k-space D. Center portion of k-space

Center portion of k-space

The central lines of k-space are associated with _____________ in a fast spin echo sequence. A. The echo train length B. Echo time C. Spatial resolution D. Contrast information

Contrast information The central lines of k-space are associated with contrast information in a fast spin echo sequence.

A reduction in FOV will result in: A. Decrease in spatial resolution B. Increase in SNR C. Decrease in SNR D. None of the above E. A and C only

Decrease in SNR A reduction in FOV will result in a decrease in SNR, but will increase the spatial resolution in the image by lowering the pixel size.

An increase in TR will produce all of the following EXCEPT: A. Increase the signal-to-noise ratio B. Decrease T1 weighting C. Decrease the number of available slices D. Increase scan time

Decrease the number of available slices An increase in repetition time will produce more available slices, per TR period

Reducing the FOV (field of view) only will result in: A. Decreased spatial resolution B. Increased signal-to-noise C. Decreased signal-to-noise D. Increased T2 weighting

Decreased signal-to-noise A reduction in FOV will yield a decrease in SNR, but an increase in spatial resolution.

As the number of phase encodings is increased from 256 to 512, SNR (signal to noise ratio): A. Increases B. Decreases C. Doubles D. Is unchanged

Decreases As the number of phase encodings is increased from 256 to 512, SNR (signal to noise ratio) decreases (less protons per voxel)

SNR increases with all the following adjustments, EXCEPT: A. Increasing FOV B. Decreasing pixel size C. Increasing NEX/NSA D. Decreasing phase matrix

Decreasing pixel size

If the repetition time is doubled, the scan time will: A. Be 40% greater B. Double C. Be half of what it was D. Triple

DoubleTR is directly proportional to scan time (TR x Phase Matrix x NEX)

What formula is used to calculate the size of the pixel? A. Matrix ÷ FOV B. FOV ÷ slice thickness C. FOV ÷ matrix D. Slice thickness ÷ matrix

FOV ÷ matrix

The timing of the RF pulses in an MRI pulse sequence controls: A. How long the procedure takes B. Pixel size C. Image contrast D. Spatial resolution

Image contrast The timing of the RF pulses in an MRI pulse sequence controls image contrast.

Increasing NSA will: A. Improve SNR B. Degrade SNR C. Shorten scan time D. Have no effect on SNR

Improve SNR Increasing NSA will improve SNR.

All of the following yield a higher signal-to-noise ratio EXCEPT: A. Increase TR B. Increase TE C. Larger FOV D. Higher NEX E. Decrease Matrix

Increase TE An increase in TE will yield a decrease in SNR

K-space is typically filled: A. With a funnel B. One line at a time C. From back to front D. From right to left

One line at a time K-space is typically filled one line at a time.

_______ is when only half the views of k-space are filled in the frequency axis. A. Diffusion tensor B. Anti aliasing C. Half Fourier D. Partial or fractional echo

Partial or fractional echo Partial or fractional echo is when only half the views of k-space are filled in the frequency axis, an acceleration technique used to shorten TR and TE.

Each line of k-space is defined by the: A. Phase encoding gradient B. Frequency encoding gradient C. Slice select gradient D. Dictionary

Phase encoding gradient Each line of K-space is defined by the phase encoding gradient.

With a recFOV that is twice as tall as it is wide, and the phase direction oriented along the smaller "width" dimension of the FOV, which of the following relationships between the frequency and phase steps yields square pixels? A. Frequency steps must be half the phase encoding steps B. Phase steps must equal the number of frequency steps C. Phase steps must be half the number of frequency steps D. The pixels will always be rectangular

Phase steps must be half the number of frequency steps With a 50% Rectangular FOV, the phase matrix (rectFOV direction) must be exactly half the frequency matrix in order to yield square pixels.

Another name for a picture element is a: A. Voxel B. Pixel C. Cubicle D. Filament

Pixel

A method of reducing MRI scan time by not filling as many lines of k-space in the phase direction, while maintaining spatial resolution, is known as: A. Halfscan B. Rectangular FOV C. Partial fourier D. Fast spin echo

Rectangular FOV

All of the following would aid in reducing scan time EXCEPT: A. Utilizing the coarsest matrix possible B. Reducing NEX C. Reducing TE D. Increasing parallel imaging factor E. Setting TR to shortest

Reducing TE TE (echo time) has no affect on scan time.

Which of the following would NOT compensate for the change in signal-to-noise as a result of decreasing the FOV? A. Reducing the image matrix B. Reducing the number of echo train lengths C. Reducing the number of excitations D. Increasing slice thickness

Reducing the number of excitations

All of the following affect acquisition time EXCEPT: A. TR B. TE C. NEX D. Matrix

TE

If a sequence with a matrix of 256 x 512 is adjusted to 512 x 512, A. The SNR will stay the same B. The scan time will decrease C. The SNR will increase and the scan time will decrease D. The SNR will decrease and the scan time will increase

The SNR will decrease and the scan time will increase

Which of the following is NOT affected by changing the slice thickness? A. The signal-to-noise ratio of the image B. The T1 contrast C. The image resolution D. The anatomical coverage

The T1 contrast

With regards to k-space, the data containing signal and contrast information are located along: A. X gradient B. The phase axis C. The central lines D. The outer lines

The central lines Referring to k-space, the data containing signal and contrast information are located along the central lines.

With regards to k-space, the data containing high resolution are located along: A. The Z gradient B. The phase axis C. The central lines D. The outer lines

The outer lines Referring to k-space, the data containing high resolution are located along the outer lines.

As the number of excitations (NEX) is increased from 1 to 3, which of the following does NOT occur? A. The scan time triples B. The pixel size is increased C. The image resolution remains the same D. The signal-to-noise ratio is increased

The pixel size is increased Changes in NEX have no effect on the resolution of an image.

Reducing the number of lines filled in k-space will produce an image: A. With a decrease in spatial resolution B. With an increase in spatial resolution C. With an decrease in overall SNR D. A and C

With a decrease in spatial resolution Reducing the number of lines filled in k-space will produce an image with a decrease in spatial resolution.

Water has a __________ T1 relaxation time and a ________ T2 relaxation time. A. short; short B. short; long C. long; short D. long; long

long; long

Fat has a _______ T1 relaxation time and a ________ T2 relaxation time. A. short; short B. short; long C. long; short D. long; long

short; short