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Effects of imaging gradients in sequences with varying longitudinal storage time-Case of diffusion exchange imaging
Samo Lasič1,2, Henrik Lundell1, Daniel Topgaard3
1Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, Hvidovre, Copenhagen, Denmark.
Purpose:
To illustrate the potential bias caused by imaging gradients in correlation MRI sequences using longitudinal magnetization storage (LS) and examine the case of filter exchange imaging (FEXI) yielding maps of the apparent exchange rate (AXR).
Methods:
The effects of imaging gradients in FEXI were observed on yeast cells. To analyze the AXR bias, signal evolution was calculated by applying matrix exponential operators.
Results:
A sharp threshold for the slice thickness was identified, below which the AXR is increasingly underestimated. The bias can be understood in terms of an extended low-pass diffusion filtering during the LS interval, which is more pronounced at lower exchange rates. For a total exchange rate constant larger than 1 s-1 , the AXR bias is expected to be negligible when slices thicker than 2.5 mm are used.
Conclusion:
In correlation experiments like FEXI, relying on LS with variable duration, imaging gradients may cause disrupting effects that cannot be easily mitigated and should be carefully considered for unbiased results. In typical clinical applications of FEXI, the imaging gradients are expected to cause a negligible AXR bias. However, the AXR bias may be significant in preclinical settings or whenever thin imaging slices are used. Magn Reson Med 79:2228-2235, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
Insights
Imaging gradients in filter exchange imaging (FEXI) can cause bias in apparent exchange rate (AXR) measurements. Using thicker slices (>2.5 mm) minimizes this bias in most clinical applications.
Area of Science:
- Magnetic Resonance Imaging (MRI)
- Biophysical Techniques
Background:
- Correlation MRI sequences utilize longitudinal magnetization storage (LS) for advanced tissue characterization.
- Filter exchange imaging (FEXI) is a specific correlation MRI technique used to map the apparent exchange rate (AXR).
Purpose of the Study:
- To investigate potential bias introduced by imaging gradients in FEXI.
- To quantify the impact of these gradients on AXR measurements.
Main Methods:
- Simulated signal evolution in FEXI using matrix exponential operators.
- Analysis of AXR bias as a function of slice thickness and exchange rate.
Main Results:
- A critical slice thickness threshold was identified below which AXR is underestimated.
- Bias is linked to low-pass diffusion filtering during the LS interval, more prominent at lower exchange rates.
- Negligible AXR bias is expected for slices >2.5 mm when the exchange rate is >1 s⁻¹.
Conclusions:
- Imaging gradients in FEXI can cause significant bias, particularly in preclinical research or when using thin slices.
- Careful consideration of gradient effects is crucial for accurate AXR quantification.
- Clinical FEXI applications with standard slice thicknesses are likely to have negligible bias.
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