Shivani Kumar1,2,3, Robba Rai2, Daniel Moses1,4, Armia George2,3, Lois Holloway1,2,3,5,6, Shalini VInod1,2, and Gary Liney1,2,3,7
1The University of New South Wales, Sydney, Australia, 2Liverpool Cancer Therapy Centre, Liverpool, Australia, 3Ingham Institute of Applied Medical Research, Liverpool, Australia, 4Prince of Wales Hospital, Randwick, Australia, 5Institute of Medical Physics, University of Sydney, Sydney, Australia, 6Centre for Medical Radiation Physics, University of Wollongong, Wollongong, Australia, 7University of Wollongong, Wollongong, Australia
Synopsis
Dynamic contrast enhanced (DCE) MRI is becoming an increasingly important tool for assessing tumour response, however it's application in lung is limited by respiratory motion. We propose the use of radial acquisition technique to minimise motion by oversampling the centre of k-space albeit with reduced temporal resolution. The initial results show that the radial k-space trajectory is a suitable method for motion compensation which provided a DCE scan of sufficient image quality and temporal resolution which can be used as part of a complete free breathing lung MRI protocol. Purpose
Dynamic contrast enhanced (DCE) MRI is
becoming an increasingly important tool for assessing tumour response.
Important characteristics are spatial and temporal resolution and in lung this
is further complicated by the effects of respiratory motion. A common approach
is to acquire fast gradient-echo image utilising k-space sharing to provide accelerated
temporal resolution and collect data during short ‘windows’ of breath-holds
over the time course. However patient compliance during breath-hold manoeuvres
can lead to tumour displacement and introduce error in analysis. Radial
acquisitions can alleviate motion by oversampling the centre of k-space albeit
with reduced temporal resolution. The purpose of this study was to evaluate
whether such a ‘stack-of-stars’ acquisition can achieve high enough resolution
for the DCE sequence to provide a complete free breathing protocol for lung
cancer patients.
Methods
Institutional review board approval was
obtained. All images were acquired on a wide bore 3 Tesla system (Skyra,
Siemens, Erlangen Germany) utilising an 18 channel surface coil and 32 channel spine
coil. Breath hold (BH) DCE sequence was performed using a fast gradient-echo
sequence employing k-space sharing (TWIST) acquired as 5 breath-hold periods of
20 seconds each with a spatial and temporal resolution of 1.5 x 1.5mm and 3
seconds. The free breathing (FB) protocol was performed using a radial stack-of-stars
acquisition (StarVIBE) with a spatial and temporal resolution of 1.8 x 1.8mm
and 14 seconds respectively. An in-house
developed MR compatible motion phantom was first used to quantify motion
sensitivity and signal variation in the two sequences. Image quality was subsequently
assessed in vivo by imaging a healthy volunteer and subtracting sequential dynamic
frames for both techniques. The final sequence was tested using contrast
enhancement on a patient with lung cancer and compared to a patient using the
previous breath hold technique. The DCE datasets were acquired for a period of 6minutes
for both sequences. For in-vivo scans, two rapid pre-contrast measurement of T1
was acquired using two flip angle variations (2 o and 15o)
of each specific sequence was acquired. Calculation of T1 map and a
two-compartment model fit to the data (Tissue4D, Siemens) was done to provide
pixel-by-pixel maps of the perfusion rate constant. Image quality was reviewed
by a thoracic radiologist based on a 4 point scale (1, excellent to 4, poor)
for volunteer and patient scans.
Results
Phantom data demonstrated a reduction in signal
variation from 40% using TWIST compared to 4% using StarVIBE for the same
amplitude and frequency of motion. The volunteer images showed the impact of
respiratory motion to be larger during the TWIST acquisitions versus StarVIBE
(Fig1). The optimum StarVIBE protocol used 300 radial views. Figure 2 shows
images and analysis taken from patient DCE scans. Viewing DCE data in a cine
loop revealed large movement between frames for TWIST compared to StarVIBE. A
comparison of signal-time plots shows a typical result where failure to
maintain and reproduce breath hold has produced large variation and discontinuities
in the dataset. As a result the goodness-of-fit (chi2) was better
for StarVIBE (0.2) than the corresponding value using TWIST (0.16). Although
temporal resolution is much poorer with the StarVIBE sequence, it was
sufficient to sample the early upslope phase of the contrast agent. Overall the StarVibe sequence performed better
for both patient and volunteer scans (Fig 3).
Conclusion
These
initial results show that use of a radial k-space trajectory as a method of
motion compensation provides a DCE scan of sufficient image quality and
temporal resolution which can be used as part of a complete free breathing lung
MRI protocol.
Acknowledgements
No acknowledgement found.References
No reference found.