Alexander J Daniel1, Eleanor F Cox1, Charlotte E Buchanan1, and Susan T Francis1
1Sir Peter Mansfield Imaging Centre, University of Nottingham, Nottingham, United Kingdom
Synopsis
Renal T2 mapping is still in its
infancy with little consensus on methodology between studies, this leads to a
variation in T2 measurements between studies. Here four T2
mapping methods (Spin Echo-Echo Planar
Imaging (SE-EPI), Multi-Echo Turbo Spin Echo (ME-TSE), Gradient Spin Echo (GraSE), and
Carr-Purcell-Meiboom-Gill T2 preparation (T2 prep)) are compared on both a calibrated
phantom and in-vivo kidneys. The GraSE technique was found
to produce the most accurate maps relative to the phantom and form the clearest
maps of the kidneys in-vivo, showing clear differences between cortical and
medullary tissues.
Introduction
T2
weighting is a fundamental image
contrast, T2 mapping has been shown to provide sensitivity to oedematous tissue and ischaemia in cardiac imaging. However T2 mapping in renal
MRI is still in its infancy with only a few studies measuring this parameter1. Studies that have measured T2
show potential for evaluation of renal transplant function2 and early diagnosis of Autosomal Dominant Polycystic Kidney Disease (ADPKD)3. The variation in T2
measurements between studies is large, this is partly due to physiological
factors such as hydration level, but also due to the variation in T2
mapping methodology. To date, no study has compared T2 mapping
methods within the same subjects. Here, four renal T2 mapping methods
are verified for accuracy on a calibrated phantom and the measured T2
values of the kidneys compared in humans. Methods
All data was collected on a Philips 3T
Ingenia system and the sequence accuracy verified using a QalibreMD System
Standard Model 130. The four methods compared were Spin Echo-Echo Planar
Imaging (SE-EPI) (Repetition time - TR = 5000 ms, Echo time - TE = 20 – 70 ms in 5 ms steps, voxel size = 3 x 3
x 5 mm3), Multi-Echo Turbo Spin Echo (ME-TSE) (TR = 3000 ms, TE = 13 –
130 ms in 13 ms steps, TSE factor = 10, voxel size = 3 x 3 x 5 mm3),
Gradient Spin Echo (GraSE) (TR = 3000 ms, TE = 11.2 – 173.3 ms in 5.6 ms steps,
TSE factor = 30, voxel size = 3 x 3 x 5 mm3) and Carr-Purcell-Meiboom-Gill
T2 preparation (T2 prep) (TR = 3000 ms, TE = 5.3 ms,
effective TE = 0, 40, 80 and 160 ms, voxel size limited by EPI factor= 3 x 5.65
x 5 mm3, EPI Factor 17). All protocols had a consistent field of view
(288 x 288 x 25 mm3) and flip angle (90°); all sequences were respiratory
triggered and collected within 6 to 9 minutes depending on breathing rate.
Each sequence was used to generate a T2
map on the calibrated phantom, Figure 1, the mean T2 of each sphere
was compared to their standard. Five subjects were scanned using each of the
four sequences. A T1–weighted image with clear cortical
medullary contrast was used to define cortex and medulla regions of interest of
each subject allowing an average T2 for each tissue to be calculated.Results
Figure 2 shows the signal decay curves and
T2 fit across the T2 spheres in the QalibreMD
System which range from 5.35 ms to 645.8 ms. The SE-EPI
method overestimates T2 below 20 ms due to the long minimum TE
compared to other methods. More accurate measurement of the shorter T2
spheres was recorded using the ME-TSE protocol however the raw data is noisier,
this manifests itself as inaccuracies in the longer T2 measurements
due to their smaller dynamic range over the TE sampled. GraSE produced the most
accurate measurements of the four methods, although still struggles to accurately
measure the 5.35 ms sphere. Data collected using the T2 prep method
does not fit the standard values below 100 ms due to its small number of data
points.
In-vivo data collected using each method
from a representative subject is shown in Figure 3. The SE-EPI method generated
maps with little blurring, but showed lack of differentiation between the
cortex and medulla. Maps generated using the ME-TSE method suffer from a large
amount of blurring due to the relatively long echo train length. This blurring
leads to structures being obscured in the map and only a very small
differentiation between cortex and medulla. Using the GraSE method, there is a
clear difference between the cortex and medulla and the data fits well to a T2
decay, the short echo-spacing made possible by GraSE means more TEs can be
sampled and therefore leads to a more accurate fit. The map made using the T2
prep method suffers from noise in the raw data. Some of the areas with a longer T2 do match with the medulla seen in the GraSE map however due to
the degree of noise, it is un-usable on its own.
In Figure 4, the mean T2 of the
cortex and medulla measured over five subjects with each method is shown. The
two methods that have delivered the highest image quality, SE-EPI and GraSE,
produce substantially different values of T2 in-vivo despite their
accuracy over the range of T2 in the kidneys when used on the
phantom. This disparity is due to the additional confounding factors of
diffusion and flow present in the body; these do not affect the GraSE sequence whilst,
as expected, are considerable for the SE-EPI sequence. This shortening in T2
measured with SE-EPI, and a slight shortening when using the ME-TSE method
compared to GraSE and T2 prep is consistent with literature1,4.Conclusion
The ME-TSE and GraSE sequences produce the
most accurate T2 maps of the quantitative phantom however in-vivo
ME-TSE suffers from blurring. As such, GraSE produces the most accurate in-vivo
renal T2 maps.References
- Wolf, M. et al. Magnetic resonance imaging T1- and T2-mapping to assess renal structure and function: a systematic review and statement paper. Nephrol. Dial. Transplant. 33, ii41–ii50 (2018).
- Mathys, C. et al. T2’ Imaging of Native Kidneys and Renal Allografts – a Feasibility Study. RöFo - Fortschritte Auf Dem Geb. Röntgenstrahlen Bildgeb. Verfahr. 183, 112–119 (2011).
- Franke, M. et al. Magnetic resonance T2 mapping and diffusion-weighted imaging for early detection of cystogenesis and response to therapy in a mouse model of polycystic kidney disease. Kidney Int. 92, 1544–1554 (2017).
- Ke, Z. et al. Validation of SE-EPI-based T2 mapping for characterization of prostate cancer: a new method compared with the traditional CPMG method. Abdom. Radiol. 44, 3432–3440 (2019).