Gaspar Delso1, Mohammad Mehdi Khalighi2, Sabrina Epp3, Felipe de Galiza Barbosa3, Tetsuro Sekine3, Edwin ter Voert3, and Patrick Veit-Haibach3
1GE Healthcare, Zurich, Switzerland, 2GE Healthcare, Stanford, CA, United States, 3University Hospital, Zurich, Switzerland
Synopsis
One
challenge of PET/MR imaging is the correction of photon attenuation caused by
hardware in the field-of-view. For the particular case of MR local coil
attenuation, a solution based on stored templates is provided by commercially
available clinical PET/MR systems. This solution, however, is limited to rigid
coils docked in a pre-defined position. A
more general method encompassing other coil types would be desirable to improve
the accuracy of PET images. The goal of the present study is to investigate the
accuracy of fast ZTE acquisitions for the correction of local coil attenuation.Purpose
One of the main challenges
of PET/MR imaging is the correction of photon attenuation, caused by the
patient as well as any hardware in the field-of-view. For the particular case
of MR local coil attenuation, a solution based on stored templates is provided
by both commercially available clinical PET/MR systems (Siemens mMR and GE
SIGNA). This solution, however, is limited to rigid coils docked in a
pre-defined position (typically the head/neck coil). A more general method
encompassing other coil types would be desirable to improve the accuracy of PET
images1-4.
A recent development in
MR-based attenuation correction is the use of 3D zero echo time (ZTE) imaging
for bone tissue identification5. This sequence provides
high-resolution images of fast-decaying species without requiring preparation
pulses or multiple echoes, making it a very time-efficient acquisition. An
interesting property of ZTE is that it is also capable of capturing the signal
from certain components within the local coils, which could be used as
landmarks to register an attenuation template. The goal of the present study is
to investigate the accuracy of fast ZTE acquisitions for the correction of
local coil attenuation.
Methods
A fast ZTE acquisition,
compatible with clinical workflow, was set up at the SIGNA PET/MR system of the
University Hospital of Zurich. The sequence was tested for the localization of
the 8-Channel Breast Coil, for which an attenuation template is included in the
system. The acquisition parameters were: FOV 28cm, ST 2.5mm, 100 slices, FA 1°,
frequency 112, NEX 2, BW 62.5kHz, acquisition time 20s.
A post-processing algorithm
was implemented in Matlab (The MathWorks, Inc., Natick, MA) to eliminate all
patient tissue from the acquired images and segment the visible coil
components. A set of features were automatically extracted from each segmented
component, including volume, centroid, principal axes and dimensions. The point
clouds defined by the centroids of all identified components were rigidly
registered to evaluate the consistency of each component’s localization and
determine the optimal set of landmarks for template alignment.
The repeatability of coil
localization was tested first on 10 phantom acquisitions without repositioning,
and then on 10 acquisitions where the whole phantom setup, landmarking and
acquisition procedure was performed. The impact of realistic coil loading
conditions was evaluated on seven acquisitions of human subjects (3 healthy
volunteers and 4 patients).
Results
When only the repeatability
of the pulse sequence and post-processing were considered, the segmented
landmark centroids were in all cases obtained with sub-millimetric precision:
range [(-0.2, -0.3, -0.7)mm, (0.4, 0.1, 0.4)mm], standard deviation < (0.2,
0.2, 0.5)mm. When the entire setup and landmarking procedure was included, the
precision dropped significantly: range [(-15.3, -2.4, -7.1)mm, (7.4, 2.8,
8.9)mm], standard deviation ϵ [(0.3, 0.4, 0.2)mm, (5.7, 3.8, 3.5)mm]. Similar
results were obtained on human subjects: range [(-10.6, -25.8, -6.5)mm, (14.9,
9.3, 11.1)mm], standard deviation ϵ [(0.3, 0.5, 0.1)mm, (8.5, 13.2, 6.6)mm].
On phantom acquisitions, the
position of the coil could be registered to a common reference with a standard
deviation of (0.5, 1.2, 1.0)mm and (0.9, 0.2, 0.9)°. On human subjects, the
deviation was (1.5, 1.3, 1.3)mm
and (0.6, 0.6, 0.5)°.
Discussion
In general, the majority of
landmarks were located with a precision oscillating between 1 and 3mm, with
occasional outliers caused by mis-segmentation. After selection of the optimal
set of landmarks, the precision of coil positioning is well below the typical
voxel size of PET attenuation maps (~4mm), indicating that ZTE imaging would be
suitable for a template-based attenuation correction approach.
Alternative approaches based
on MR-visible landmarks (e.g. oil pills or micro-coils) placed on the local
coil have been published in the past, but they require hardware changes and
could be an issue in case of phase wrap-around. A
limitation of the present study is the reduced number of clinical cases
evaluated. New patients are currently being recruited through an ongoing clinical
study. Future work will be aimed at extending the method to other local coils
(e.g. GEM anterior array and Flex suite)
Conclusion
The results suggest that
proton density-weighted ZTE imaging can be effectively used for local coil
positioning in template-based attenuation correction.
Acknowledgements
No acknowledgement found.References
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MacDonald, et al.,Med Phys 2011; 38: 2948-2956.
2. Paulus, et al. Med Phys 2012;
39: 4306-4315. 3. Wollenweber et al., Magn Reson Mater Phy.
2013; 27:149-159. 4. Kartmann, et al., Med Phys 2013; 40. 5. Wiesinger et al., ISMRM 2014; 5819.