# US 11,308,645 B2 · Apparatus and method for wide-range optical tracking during medical imaging

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- Number: US 11,308,645 B2
- Document ID: US11308645B2
- Status: Granted
- Kind: B2
- Application number: 15/977,900
- Filed: 2018-05-11
- Published: 2018-11-15
- Granted: 2022-04-19
- Earliest claimed: 2017-05-12
- Assignees: The Board of Trustees of the Leland Stanford Junior University
- Applicants: The Board of Trustees of the Leland Stanford Junior University
- Inventors: Jakob Ehrl; Julian Maclaren; Murat Aksoy; Roland Bammer
- Web page: https://patentgrep.com/patent/US11308645B2

## CPC

- G06T 7/85: {Stereo camera calibration}
- A61B 5/055: involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
- A61B 5/1127: {using markers}
- A61B 5/702: {Posture restraints}
- A61B 6/527: {using data from a motion artifact sensor}
- G01R 33/283: {Intercom or optical viewing arrangements, structurally associated with NMR apparatus}
- G01R 33/56509: {due to motion, displacement or flow, e.g. gradient moment nulling (G01R33/567 takes precedence)}
- G06T 7/246: using feature-based methods, e.g. the tracking of corners or segments
- G06T 7/73: using feature-based methods
- A61B 5/721: {using a separate sensor to detect motion or using motion information derived from signals other than the physiological signal to be measured}
- A61B 6/032: {Transmission computed tomography [CT]}
- G01R 33/385: using gradient magnetic field coils
- G01R 33/5608: {Data processing and visualization specially adapted for MR, e.g. for feature analysis and pattern recognition on the basis of measured MR data, segmentation of measured MR data, edge contour detection on the basis of measured MR data, for enhancing measured MR data in terms of signal-to-noise ratio by means of noise filtering or apodization, for enhancing measured MR data in terms of resolution by means for deblurring, windowing, zero filling, or generation of gray-scaled images, colour-coded images or images displaying vectors instead of pixels (image data processing or generation, in general G06T)}
- G06T 12/20: Inverse problem, i.e. transformations from projection space into object space
- G06T 2207/10021: Stereoscopic video; Stereoscopic image sequence
- G06T 2207/10088: Magnetic resonance imaging [MRI]
- G06T 2207/30204: Marker
- G06T: IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
- A61B: DIAGNOSIS; SURGERY; IDENTIFICATION (usefulness limited to only animals A61D)
- G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12)

## Abstract

Methods to quantify motion of a human or animal subject during a magnetic resonance imaging (MRI) exam are provided. In particular, these algorithms make it possible to track head motion over an extended range by processing data obtained from multiple cameras. These methods make current motion tracking methods more applicable to a wider patient population.

## Claims

### Claim 1

A method of determining a position and orientation of an object in a medical imaging device, the method comprising:
rigidly attaching one one or more markers to the object, wherein each marker of the one or more markers comprises three or more feature points, wherein the three or more feature points of each marker of the one or more markers have known positions in a coordinate system of the corresponding marker;
configuring two or more cameras to have partial or full views of at least one of the one or more markers;
determining a camera calibration that provides transformation matrices Tij relating a coordinate system Ci of camera i to a coordinate system Cj of camera j, wherein i and j are index integers for the two or more cameras;
forming two or more images of the one or more markers with the two or more cameras, wherein the known positions of the three or more feature points of each marker in the coordinate systems of the corresponding markers lead to image consistency conditions for images of the three or more feature points in the camera coordinate systems;
wherein the image consistency conditions are relations that are true in images of the one or more markers because of known relative positions of the three or more feature points on each of the one or more markers; and
solving the image consistency conditions to determine rigid-body transformation matrices Mk relating coordinate systems MCk of each marker k to the coordinate systems of the two or more cameras, wherein k is an index integer for the one or more markers, whereby the position and orientation of the object is provided;
wherein the solving the image consistency conditions to determine each rigid-body transformation matrix Mk is performed with a least squares solution to an overdetermined system of linear equations;
wherein the overdetermined system of linear equations for rigid-body transformation matrix Mk is a set of two equations for each feature point of marker k that is seen by each of the two or more cameras; and
wherein the overdetermined system of linear equations for rigid-body transformation matrix Mk has coefficients of the rigid-body transformation matrix Mk as unknowns to be solved for.

### Claim 2 (depends on 1)

The method of claim 1, wherein the two or more cameras are compatible with magnetic fields of a magnetic resonance imaging system.

### Claim 3 (depends on 1)

The method of claim 1, wherein the one or more markers include a position self-encoded marker.

### Claim 4 (depends on 1)

The method of claim 1, wherein the object is a head of a human subject.

### Claim 5 (depends on 1)

The method of claim 1, wherein the camera calibration is determined prior to installing the two or more cameras in the medical imaging device.

### Claim 6 (depends on 1)

The method of claim 1, wherein the camera calibration includes referencing each camera to system coordinates of the medical imaging device and enforcing consistency conditions for the camera calibration.

### Claim 7 (depends on 1)

The method of claim 1, wherein all visible feature points of the one or more markers in the images are used in the solving of the image consistency conditions.

### Claim 8 (depends on 1)

The method of claim 1, wherein fewer than all visible feature points of the one or more markers in the images are used in the solving of the image consistency conditions.

### Claim 9 (depends on 1)

The method of claim 1, wherein a frame capture timing of the two or more cameras is offset, whereby an effective rate of tracking can be increased.

### Claim 10 (depends on 1)

The method of claim 1, wherein the two or more cameras are arranged to allow a marker tracking range in a head-feet direction of a patient being imaged.

### Claim 11 (depends on 1)

The method of claim 1, further comprising applying motion correction to medical imaging data based on the position and orientation of the object.

### Claim 12 (depends on 11)

The method of claim 11, wherein the motion correction is applied adaptively.

### Claim 13 (depends on 12)

The method of claim 12, wherein two or more of the one or more markers are attached to the object, and further comprising performing analysis of a relative position of the two or more markers as a marker consistency check.