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Collaborative volumetric magnetic resonance image rendering on consumer-grade devices
Andrew Dupuis1,2, Dominique Franson1, Yun Jiang3, Jeff Mlakar2, Henry Eastman2, Vikas Gulani3, Nicole Seiberlich1, and Mark A. Griswold1,2,3

1Biomedical Engineering, Case Western Reserve University, Cleveland, OH, United States, 2Interactive Commons, Case Western Reserve University, Cleveland, OH, United States, 3Department of Radiology, School of Medicine, Case Western Reserve University, Cleveland, OH, United States

Synopsis

We present a system for intra- or post-acquisition 3D rendering of volumetric MRI datasets for independent or collaborative use on AR/VR and mobile platforms. Consumer-grade head mounted displays, phones, and computers are used to provide 3D visualizations. Datasets can be windowed and leveled in the same manner as classic visualizations, and arbitrary slices can be selected and viewed in real time in the context of the whole volume. Real world dimensionality and spatialization is retained. Using this system, multiple users can interact with a dataset collaboratively using current AR/VR platforms or any modern cellphone, tablet, or laptop.

Purpose

Medical imaging traditionally relies on planar representations of patient data without three-dimensional context. This is directly contrary to the reality of the human body, where spatial information and feature dimensionality are of critical importance to proper diagnosis and treatment. With the increasing availability of high quality stereoscopic head mounted displays (HMDs) as well as high performance non-stereoscopic mobile platforms, there exists a clear opportunity to “re-spatialize” medical images to enable direct and intuitive interaction both at the scanner and in the reading room.

Clinicians are accustomed to collaboratively diagnosing patients through group inspection and discussion. This process is impeded by traditional reading methods – collaboratively evaluating scans by scrolling through a DICOM stack can’t compare to physically searching for and pointing out anomalies. Moreover, a low latency real-time multi-user 3D environment presented intra-acquisition using AR, VR, or touchscreen offers a means of clinician engagement that better approximates traditional diagnostic practice. We have therefore developed a visualization pipeline to display both realtime and static MRI data on a group of HMDs or mobile devices for collaborative viewing.

Methods

Figure 1 details the rendering pipeline and approach. Rendering clients could import data from offline-reconstructed DICOMs or receive realtime-reconstructed data over TCP. Images were loaded into a volumetric texture system without compression, allowing the user to manipulate image window and level at runtime. Voxel sizes were calculated from scanner data to allow use of any isotropic or anisotropic dataset. Rendering was accomplished by a progressive ray tracing (“raymarching”) approach (1,2) described in Figure 1. Each voxel’s intensity value contributed to the final screenspace pixel color via the raymarching graphics shader. Noise was reduced using an adjustable noise floor filter (Figure 2).

In order to test the rendering pipeline’s accuracy, two offline magnetic resonance datasets of the brain were acquired with 3D MP-RAGE sequence with isotropic spatial resolution of 1mmx1mmx1mm, and anisotropic resolution of 1.2mmx1.2mmx2mm. The datasets were reconstructed offline to DICOMs and loaded onto client systems. Initialization and framerate performance was evaluated on a Desktop PC (3.2Ghz CPU, Quadro K4200 GPU), Oculus Gear VR (Samsung Galaxy S8+), and Microsoft Hololens for multiple matrix sizes and raymarching step counts. These devices represent, from highest to lowest, the three major performance tiers for target devices. Lag in realtime transmission was tested over Gigabit Ethernet and 802.11ac Wifi using single, 3-slice, and full volume updating of a volumetric heart acquisition. (FLASH sequence w/ undersampled radial trajectory, reconstructed using through-time radial GRAPPA on Gadgetron (3,4)).

Users were provided with runtime controls for window, level, position, and orientation as well as slice plane position and orientation through handheld controllers, gestures, or touchscreen interactions. The clipping position could also be controlled by the user’s head to allow natural data exploration. In multi-user use, render parameters were synchronized so users shared the same visualization. Additional client-side rendering parameters included raymarching step count and framerate-dependent downsampling to maintain performance.

Collaborative sessions were accomplished across all tested platforms. A host device began a “reading session” which other client devices could join. Hosts retained control of rendering parameters, while all users could explore the rendering. Participants on 2D platforms could interact via touchscreen, with a projection of their simulated “position” visualized in the AR/VR “reading room” to enhance embodiment.

Results

Figure 2 is an animated GIF showing results from the rending pipeline. It also demonstrates the noise rejection filter system in use, enhancing isolation of useful data to improve the visualization. Figure 3 maps system initialization and render times versus matrix size and volume rendering stepcount, as well as transmission lag in realtime use. Though dwarfed by the Desktop PC, both mobile devices rendered standard 128^3 datasets near full fidelity with minimal loading times. Single, three-slice, and full-volume realtime updating all supported >10fps transfer. Figure 4 is an animated GIF demonstrating accurate rendering of isotropic and anisotropic MP-RAGE datasets, dynamic clipping planes, and realtime intra-acquisition updating of a rendered volume. Figure 5 is an animated gif of a collaborative session utilizing the platform.

Discussion

Mixed Reality and mobile platforms offer highly flexible and collaborative methods for displaying MR datasets in full context. All three consumer-grade systems were capable of rendering standard matrix size isotropic and anisotropic acquisitions at acceptable framerates for both static and realtime acquisitions. Collaborative reading sessions of volumetric data sets allowed multiple users to simultaneously interact with a dataset from independent devices. This success indicates a need for further exploration of the impact a platform-agnostic volumetric rendering system could have on creating the three-dimensional collaborative reading rooms and interventional suites of the future.

Acknowledgements

Siemens Healthcare, R01EB018108, NSF 1563805, R01DK098503, and R01HL094557.

References

1. Levoy, M. Ef´Čücient ray tracing of volume data. ACM Trans. Graph. 1990; 9, 3, p.245–261

2. Zhou et al. Real-time Smoke Rendering Using Compensated Raymarching. Microsoft Research. 2007.

3. Hansen MS, Sørensen TS. Gadgetron: An open source framework for medical image reconstruction. Magn. Reson. Med. 2013;69:1768–1776.

4. Franson D, Ahad J, Hamilton J, Lo W, Jiang Y, Chen Y, Seiberlich N. Real-time 3D cardiac MRI using through-time radial GRAPPA and GPU-enabled reconstruction pipelines in the Gadgetron framework. In: Proc. Intl. Soc. Mag. Reson. Med. 25.; 2017. p. 448.

Figures

Input to the rendering pipeline is a volumetric acquisition that is reconstructed in realtime and transmitted over TCP, or reconstructed offline and saved to DICOM. On the client, data is loaded into 2D floating point full precision arrays of intensity values to retain bit depth. User-controlled window and level settings are applied at runtime to rescale intensity values. Textures are converted to a 3D Volumetric texture in memory and assigned to a representative mesh that controls rendering shape and allows dynamic slicing through the dataset. Progressive raymarching is accomplished in using a custom Cg shader with multiple pixel combination algorithms.

This animated GIF demonstrated the user-adjustable noise rejection floor in use on an MP-RAGE brain scan in real time. Noise is isolated in the raw data prior to window and level calculations and tagged with zero opacity. During the rendering loop, the raymarching shader then discards the tagged pixel and allows the non-noise patient data to dominate the volumetric rendering calculations. This process does not affect framerate performance.

Loading times and raymarching performance are dependent on a platform’s CPU and GPU characteristics respectively. Realtime data transmission is dependent on communication medium. (A) Loading time is an O(N) function of voxel count. A typical 128x128x128 matrix dataset is shown by the grey line (B) Framerates on desktops were O(n) with raymarching steps, while mobile platforms were less predictable due to forced GPU management and VSync delays. (C) Transmission delays were tested over 1000 frames with a 128x128 matrix over (Ethernet, Wifi) while updating 1 slice (16.3±3.3ms, 21.2±4.8ms), 3 slices (22.3±3.4ms, 29.11±4.7ms), or the full volume (72.0±5.0ms, 93.8±10.93ms) each frame.

The top two animated GIFs display’s the system’s ability to cope with both isotropic and anisotropic datasets without introducing errors. The datasets were acquired with an MP-RAGE sequence on a Siemens 3T scanner with 1.0mm isotropic and 1.2mmx1.2mmx2.0mm voxel sizes respectively. The bottom left GIF demonstrates arbitrary clipping plane selection. The bottom right GIF shows a FLASH sequence of the human heart (reconstructed online using Gadgetron) dynamically updating the volume in real time.

The above animated GIF demonstrates a multi-user collaborative reading session using the volumetric system. The user in the front right is dynamically controlling the clipping plane with their head position, while the other users are able to engage independently while remaining synced with the leader’s visualization parameters and clipping plane. Here two Hololenses, an Oculus Rift, a Surface Tablet and a desktop pc are sharing the same visualization. This recording was captured using a BevCam compositor system for Microsoft Hololens.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)
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