Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Jacqueline Nerney Welch, Jeremy A. Johnson, Michael R. Bax, Rana Badr, Ramin Shahidi

IEEE Ultrasonics Symposium 2000

October 24, 2000

A Real-time Freehand 3D Ultrasound System for Image-guided Surgery

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Overview

• Design motivations and decisions– 3D ultrasound

– Freehand scanning

– Optical tracking

– Volume rendering

– Simultaneous acquisition and visualization

• Methods– Equipment

– Spatial calibration

– Volume construction and maintenance

• Results

• Future Work

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Ultrasound

• Ultrasound versus other imaging modalities (CT, MR, X-ray)– Least expensive

– No ionizing radiation

– Compatible with existing surgical instruments

– Widely available and commonly used

– Real-time, interactive nature

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

3D Visualization of Ultrasound

• Compared to 2D, 3D provides:– More intuitive and comprehensible images

– More accurate volume estimation

– Shorter scanning times

– Improved sharing of information

2D Ultrasound Image Volume Rendered 3D US

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

3D from Conventional 2D Ultrasound

Volume Volume Construction Construction

EngineEngine

WorkstationWorkstation

Volume Volume Rendering Rendering

EngineEngineji

k

(x,y,z)

2D Images

Position Data

US Probe

Tracking Device

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Optically Tracked Freehand Acquisition

• Freehand versus other scanning techniques (mechanical)– Greatest freedom of movement

– Compact

– Least cumbersome

– Requires probe position measurements

• Optical versus other position tracking methods (magnetic, mechanical, speckle decorrelation)– Insensitive to metallic surgical equipment

– Allows volume localization

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

• Volume rendering versus other visualization methods (slice projection, surface rendering)– Truest to the data set

– Easiest to interpret

– Segmentation not required

– Computationally expensive but feasible with current technology

Interactive Volume Rendering

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Volume Construction

Engine

Visualization

Static Volume

Simultaneous Acquisition & Visualization

(x,y,z)

ji

k

DataStorage

Acquisition

(x,y,z)

ji

k

Volume Construction

Engine

Simultaneous Acquisition & Visualization

Dynamic Volume

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Equipment

• Image Guided Technology FlashPoint™ 5000 optical tracking system with 580 mm camera

• Sonosite handheld ultrasound scanner with 5MHz linear probe

• SGI 320 Visual Workstation with a single processor running Windows NT

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Image to Volume Mapping

SSPPTTWW pTTTp

IISS pSp

WWVV pSp

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Calibration Parameters

• 6 extrinsic parameters

– Rotation (Ri , Rj , Rk)

– Translation (ti , tj , tk)

• 2 intrinsic parameters

– Image scale (si , sj)

• Can be written as

jS, v

iS, u

Slice Coordinates

Probe Tracking Device Coordinates

iP jP

kP

v

uj

i

S

PSSPSP

p

ps

s

00

0

0

x

txRx

(Ri , Rj , Rk)(ti , tj , tk)(si , sj)

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Calibration Phantom

Ultrasound Phantom(1/16” Acrylic)

Image of Phantom During Calibration

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Calibration Method

• Obtain feature positions• Align ultrasound probe• Capture US image and probe position• Localize features in image• Calculate calibration parameters

• Scale factor

• Rotation and Translation

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Volume Construction and Maintenance

Insertion of New Slices Removal of

Old Slices

Overwrite Existing Slices

Interpolate with Nearby

Slices

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Results

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Results

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Future Work

• Quantify and improve system performance– Spatial and temporal accuracy

– Data rates

• Display position and trajectory of surgical instruments

• Apply system to clinical situations

Image Guidance Laboratories, Stanford University Image Guidance Laboratories, Stanford University School of MedicineSchool of Medicine

Acknowledgements

• Dr. Thomas Krummel’s lab• DOD Graduate Research Fellowship• CBYON, Inc.