Arathi Sreekumari1, K S Shriram1, Uday Patil1, Ersin Bayram2, Dattesh Shanbhag1, and Rakesh Mullick1
1GE Global Research, Bangalore, India, 2GE Healthcare, Houston, TX, United States
Synopsis
In this work we are focusing
on automating the scan coverage and FOV for liver MRI acquisitions. We
demonstrate that using fast scout images, we can achieve very good localization
of liver FOV, irrespective of anatomy differences and hand-up / hands-down
positioning.Purpose
MRI is becoming the primary
imaging modality for liver oncological assessment and diffuse liver disease
evaluation such as fibrosis and fatty liver disease [1]. Wide
range in body habitus and also variations in liver shape and size between
patients make it impossible to utilize a one size fits all protocol for
patients. Technologists therefore end up customizing protocol for each patient
and in the process lose valuable scanner time and patient comfort is
compromised with longer exam times. This also leads to operator dependent
results and could result in inconsistent image quality due to operator errors.
This work focuses on addressing these challenges by automating the scan
coverage and field of view (FOV) using the localizer/scout images.
Methods
MRI Data: MRI data for this
study were obtained on 1.5T GE Optima MR 450w and 3T GE DISCOVERY MR750 systems
(GEHC) from 21 healthy volunteers. MR scout images was acquired using
tri-planar 2D SSFSE sequence to generate 6-11 coronal, 5-10 axial and 2-6
sagittal slices. Parameters were: TE/TR = 80-83ms/435-1500 ms, FA = 90°,
FOV = 410 to 480 mm2, TH = 8-10 mm, 512x512 matrix.
Localization: Localizing the
organ of interest involves feature generation followed by detection.
a.
Feature Generation: Histogram of Oriented Gradients (HOG) has in the
past been used for object detection in 2D images [2]. As the name suggests,
information contained within a patch is represented as a histogram of gradient
orientations. In this instance, the HOG has been extended to 3D, which means
that the gradient orientation at any voxel location has both an azimuth and an
elevation component making it a 2D histogram. Additionally, we divided the
patches into left-right and top-bottom cells and computed the HOG vector
separately for each of the portions (see Figure 1). Since region descriptors
based on histograms lose the spatial context, the division of patches into
regions for feature vector computation and their subsequent concatenation
provides a semblance of spatial arrangement for detection.
b. Detection: We used a
learning based algorithm to detect the bounding box of interest. Random forest
classifier [3] was used to create a model of the liver bounding box from a set
of training images. The model was used to detect the liver bounding box on a
set of previously unseen images. The organ model obtained from the training
provides a matching probability score for each of the evaluated patch locations
on the test images. To test the performance of the algorithm, we computed the
distance of the bounding box from the most superior and most inferior points of
the liver surface.
Experiment
Design
We
used 16 randomly selected MR scout volumes for training and remaining 5 scout volumes
were used for testing. The test images were chosen randomly and typically FOV
ranged from top of the lungs to the pelvis. A trained radiologist marked the bounding
box around the liver from which we computed the HOG feature vector in the
training phase. Also, we randomly extracted a set of regions in the training
images (excluding liver) to obtain negative examples. The feature vectors
obtained were fed to the random forest classifier along with their labels and
generated a model for testing. For
detection of the liver bounding box in test images, we used the average
bounding box size from training to sweep through the image and computed HOG
feature for each patch location at different scales. With an appropriate
threshold for the probability score we obtained the centroid of the liver
bounding box. The execution time taken for detection was approximately 100 ms.
Results
In
all the cases, we could always localize on liver FOV (Figure 2-4). On 4 test
images with no anomalous anatomy, the localization error near the liver dome
ranged between 0.14 c m and 3.4 cm, while the error near the liver apex ranged
from -0.4 cm to 1 cm (see Figure 2). In Figure 3, subject had excess fat
in and around the pericardium (white arrow), which created a curved
surface with a lung above. This is exactly similar to what the algorithm has
been trained for (i.e. mimic’s diaphragm) and as a result, error for bounding
box on superior was large (4cm).
Conclusion
We demonstrate that using
fast scout images, we can achieve very good localization of liver FOV,
irrespective of anatomy differences and hand-up / hands-down positioning.
Acknowledgements
No acknowledgement found.References
1.
Santhi Maniam , World J Radiol. 2010 Aug 28;
2(8): 309–322
2.
N. Dalal et.al. CVPR 2005, 886–893
3.
A Criminisi et.al, Med. Comp. Vis. Recog. Tech.
Appl. In Med. Imag., pp. 106 –117, 2011