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FYS-KJM 4740
LAB 02.04.12 12-14.00 –At Intervensjonssenteret Rikshospitalet.
1.
Get to know the MR scanner. Safety aspects.
2.
Scanning of phantom.
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A. Aim: estimate T1relaxation times using two different methods
I.
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III.
IV.
Acquire a 2D (single slice) image using a T1-GRE sequence (spoiled GRE) (TR/TE = 20
ms/2 ms). Change flip angle from 5 deg to 90 deg (keeping all other parameters constant).
Measure signal intensity (SI) i each ’compartment’ in the phantom (see arrows at top of page)
as function of flip-angle. Based on the SI vs flip angle curves– estimate the T1-times of each
compartment (hint: estimate max SI from the curves).
Find an expression for T1 as function of SI, flip angle and TR measured at TWO different
flip angles (hint, use ratio of SI and assume that TR/T1 << 1). Use his expression to estimate
the T1-times in the different compartments (hint: use two flip angles <= ernst-angle).
Repeat (I) but this time use a TR of 50 ms. Comment on the different in signal intensities
obtained with TR=20 ms vs TR=50 ms. What is the advantage (in terms of SNR) of using the
longer TR? What is the disadvantage?
The standard deviation of each ROI measurement is given in the xls-sheet. Plot the SI vs flip
angle curves for TR=20 ms and add error bars for each curve reflecting the uncertainty in
each measurement. Estimate SNR as function of flip angle and T1. What other source of
error in the T1-estimation can be expected? (hint slice excitation profile).
B. Aim: Understand and avoid fold-over artifacts.
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III.
IV.
In exercise A the field of view was equal in both directions (220 x 220 mm). Repeat the T1GRE acquisition (flip = 30 deg). Reduce FoV by 50% i phase encoding (PE) direction. Turn
off’ fold-over suppression’. What happens to the image? Explain..
Reset FoV to 220 mm in PE direction and now reduce FoV by 50% in frequency encode
direction. Explain why this does not result in any artifacts.
Reduce FoV by 50% i phase encoding direction butt his time turn on the option ’fold-oversuppression’ (keep NSA = 2). Explain how you can avoid fold-over artifacts even when
FoV i PE direction is less than the object size.
In the compendium in reads: ’fold-over suppression leads to increased scan time’. The
option ’fold-over-suppression’ did, however, not affect the acquisition time in our scan
when NSA=2. Why is this? (hint NSA > 1; time for more NSA vs more PE profiles).
C. Aim: to understand ’gibbs-ringing’ artefacts and chemical shift parameters.
I. Run same sequence as in B (using full FoV) but increase the pixel size to 2 mm x 2 mm (from
1mm x 1 mm). Which artefacts are now visible? Explain.
II. On the Philips scanner there is an option called ’water-fat shift’ which is specified in pixels. This
parameter describes the amount of geometric shift (relative to true position) which will occur in a
pixel containing just fat. If we change the water-fat shift’ parameter we observe that the pixel
bandwidth (in Hz) also changes. Why? What is the relationship between water-fat shift, pixel size
and bandwidth as function of δB (difference in Bo between water and fat).
III. Repeat the scan in (I) but change the water-fat shift so that the bandwidth is increased from 217
Hz/pixel to 433 and 838 Hz/pixel. What is the effect on image quality (change in SNR). Draw
curve of SNR versus pixel bandwidth. Explain.
IV. If we subtract the images obtained with BW=217 and 838 Hz we notice that the images do not
completely overlap, especially at interface between the compartments with different relaxation
properties (see enclosed image in xls-sheet). Explain a possible explanation for this effect (hint:
different susceptibility between compartments with different relaxation-values)
D. Gradients and resolution
I. In the phantom image, the in-plane resolution is 1 mm and the FoV is 256 mm^2. If the sampling interval in
the frequency-encoding direction (ts) is 0.1 ms what is the gradient strength in the frequency encoding direction?
Specify two different methods which could be used to double the spatial resolution in the frequency-direction.
How would this affect pixel bandwidth?
E. steady state sequences
I.
Image acquisition using a ‘turbo field echo’ (T1-GRE) sequence using different number of
‘dummy-pulses’ (0, 10,100) prior to image acquisition and different k-space filling regimes
(linear versus low-high).
a. What is the definition of ‘steady state’ in relation to an MR sequence?
b. Describe the images acquired in (I) and give an explanation in relation to steady state
conditions. Hint: remember that centre of k-space is most sensitive to signal variations.
c. Eplain the relationship between time to reaching steady state and flip angle.