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CAS, Srping 2002 © L. Joskowicz 3

CAS clinical applications

• Neurosurgery

• Orthopaedics

• Maxillofacial, craneofacial, and dental surgery

• Laparoscopic and endoscopic surgeries

• Radiotherapy

• Specific procedures in ophtalmology, othorhinolaringology, etc.


Elements of CAS systems


Technical elements of CAS systems1. Medical images

2. Medical image visualization

3. Segmentation and modeling

4. Virtual and augmented reality, tele-surgery

5. Preoperative analysis and planning

6. Image and robot registration

7. Medical mechanical and robotics systems

8. Real-time tracking

9. Safety, man-machine interface, human factors


Key parameters for understanding and comparing solutions

• How many procedures are performed yearly?

• What is the rate of complications? What are their causes?

• In what aspects can a CAS system help?

• Does it address part of a clinically important problem?

• What stage is the system in: in-vitro, cadaver, clinical trials?

1. Medical Images


Most common imaging modalities

• Film X-ray, Digital X-ray, Fluoroscopy, Digital Substraction Angiography (DSA)

• Ultrasound -- 2D and 2.5D (stack of slices)

• Computed Tomography (CT)

• Magnetic Resonance Imaging (MRI)

• Nuclear Medicine (NM) – PET -- Positron Emission Tomography– SPECT -- Single Photon Emission Tomography


Medical images: characteristics (1)• Preoperative or intraoperative use

– depends on the size and location of imaging machine

• Dimensionality: 2D, 2.5D, 2D+time– projection, cross section, stack of projections, time

sequence

• Image quality– pixel intensity and spatial resolution– amount of noise; signal/noise ratio– spatial distortions and intensity bias


Medical images: characteristics (2)

• Field of view

• Radiation to patient and to surgeon

• Functional or anatomical imaging– neurological activity, blood flow, cardiac activity

• What it’s best at for– bone, soft tissue, fetus, surface/deep tumors, etc

• Clinical use– diagnosis, surgical, navigation,


X-ray images• Measure absorption of x-ray radiation from

source to set of receptors

• Film X-ray has very high resolution

Gray value proportionalto radiation energy


X-ray Fluoroscopy


Fluoroscopic images


X-ray image properties• Traditional, cheap, widely available

• Two-dimensional projections (at least two required)

• High resolution, low noise (more fluoroscope)– film size, 64K gray levels– fluoroscopic images: TV quality, 20cm field of view

• Relatively low radiation

• Bone and metal images very well

• Fluoroscopy used for intraoperative navigation


Ultrasound imaging (US)

• Measure refraction properties of an ultrasound wave as it hits tissue

• No radiation

• Poor resolution, distortion, noise

• Low penetration properties

• One 2D slice or several slices (2.5D)

• Relatively cheap and easy to use

• Preoperative and intraoperative use


Ultrasound imaging


Computed Tomography (CT)


Computed Tomography Images

cuts

d = 35mmd = 25mm

d = 15mmd = 5mm


Computed Tomography Principle

angle

intensity

X-rays


Computed Tomography Properties

• Sepcifications:– 512x512 12bit gray level images; pixel size 0.5mm– slice interval 1-10mm depending on anatomy– 50-200 slices per study– noise in the presence of metal (blooming)

• All digital, printed on X-ray film

• Acquisition 1sec/slice (spiral models)

• 15mins for image reconstruction

• Costs about $250-750K, each study $500


Magnetic Resonance Imaging• Similar principle and construction than CT

machine, but works on magnetic properties of matter– magnetic fields of 0.1 to 4 Teslas

• Similar image quality characteristics as CT

• Excellent resolution for soft tissue

• Costs $1-2M, each study $1,000

• Open MR: intraoperative device (only 15 to date)


Magnetic Resonance Images


Nuclear Medicine Imaging (NMI)

• Same slices principle

• Source of photons or positrons is injected in the body. Shortly after, radiation of metabolism is measured

• Poor spatial resolution

• Expensive machine AND installation ($4-5M)

• Expensive and time-consuming

• Provides functional info no other source does


Nuclear medicine images


Image Fusion: MRI and NMI

MRI (anatomy) NMI (functional)


Video images from within the body

• Used in laparoscopic and endoscopic surgery


Main medical imaging modalities

X-ray X-ray Fluoro US US Video CT MRI NMR OpenFilm Digital (2D) (2.5D) MR

Pre/Intraop2D/2.5DResolutionRadiationAnatomyProcedure

Establish a comparative table of modality properties


The imaging pipeline


2. Medical image visualization

• 3D visualization of complex structures

• image correlation and fusion

• quantitative measurements and comparisons

• visualization of medical and CAD data

Enhance diagnosis by improving the visualinterpretation of medical data


Medical image visualization


Visualization: Technical needs

• image enhancing and noise reduction

• image interpolation: images from new viewpoints

• 3D visualization from 2.5D data– volume rendering: display voxels and opacity values– surface rendering: explicit reconstruction of surface

• 3D modeling from 2.5D data

• 2D and 3D segmentation

• 3D+T visualization (beating heart)


Medical image visualization

• Much activity! Radiologists are the experts

• Commercial packages– 3DVIEWNIX, ANALYZE, IMIPS

• Main technical topics:– 3D volume rendering techniques– 3D image filtering and enhancement – surface construction algorithms: Marching cubes, etc.

• Sources: chapters 3,9, and 10 in textbook

• Related fields: computer graphics, image processing


3. Segmentation and modeling

• Isolation of relevant anatomical structures based on pixel properties

• Model creation for the next computational task– real-time interaction and visualization– simulation– registration, matching, – morphing

Extract clinically useful informationfor a given task or procedure


Segmentation and modeling


Segmentation and modeling: technical needs

• Segmentation:– landmark feature detection– isosurface construction (Marching cubes)– contour extraction, region identification

• Modeling:– points, anatomical landmarks, surface ridges– surfaces as polygon meshes, surface splines– model simplification methods (Alligator, Wrapper)


Segmentation and modeling

• Medical images have very special needs!

• Commercial packages– 3DVIEWNIX, ANALYZE, IMIPS

• Main technical topics:– Volumetric segmentation techniques for CT, MRI– 2D and 3D segmentation with deformable elements – surface and model simplification algorithms

• Sources: chapters 4 and 8 in textbook

• Related fields: image processing, computer vision


4. Virtual and augmented reality

• Create a virtual model for viewing during surgery

• Project the model on the patient or integrate with surgeon’s view

• Useful for intraoperative anatomy exploration and manipulation

• Telesurgery systems

Use images to create or enhance a surgical situation


Virtual and augmented reality


Virtual and augmented reality• Part manipulation, visual and sensory feedback

• Interaction devices: goggles, gloves, etc

• Only a handful of systems exist

• Main technical topics:– a couple of the working systems; simulators– telesurgery systems

• Sources: chapters 14 and 15 in textbook

• Related fields: computer graphics


5. Preoperative analysis and planning

• Task and procedure dependent

• Spatial and volume measurements

• Stress and fracture analysis

• Implant and tool selection and positioning

• Surgical approach planning: bone rearrangement, angle evaluation, radiation dose planning, etc

Use images and models to assist surgeons in planning a surgery and evaluate options


Preoperative analysis and planning


Preoperative analysis and planning• About a dozen planners exist for different

procedures

• Main technical topics:– planning systems for orthopaedics, neurosurgery– application of engineering analysis techniques: finite-

element methods, stress analysis, etc

• Sources: chapters 11, 25, 33, 41, 52--56in textbook

• Related fields: CAD, computational geometry, engineering analysis


6. Image and robot registration

• Define correspondance features– point-to-point, point-to-line, surface-to-surface

• Establish correspondances between features

• Establish a similarity measure

• Formulate and solve dissimilarity reduction problem

• Related tasks: image fusion, morphing, atlas matching

Establish a quantitative relation between different refererence frames


Multimodal registration problems• Great differences depending on

– the type of data to be matched– the anatomy that is being imaged– the specific clinical requirements of procedures

• Feature selection and extraction: stereotactic frame, implanted fiducials, anatomical landmarks and surfaces, contours and surfaces in

• Manual vs. automatic feature selection, pairing• Rigid vs. deformable registration• Nearly similar vs. dissimilar images• Noiseless vs. noisy images (outlier removal)


Registration chain example

3D surface model

X-rays

CT

Patient

Tracker

Instruments

Infrared tracker


Image and robot registration• Rich topic of very great importance!

• Types of registration methods vary widely

• Main technical topics:

• rigid registration methods: three points and more– deformable registration: local and global methods– intensity-based registration

• Sources: chapters 5-7 in textbook, many papers

Book on Medical Image Registration

• Related fields: vision, robotics


7. Medical robotics devices

• Task and procedure dependent

• Accurate, steady, and repeatable 3D positioning

• Navigation and localization aids

• Cutting and milling, biopsies

• Key issues are:– kinematic design, trajectory planner– controller, safety provisions

Semi-active and active mechanicaldevices for improving surgical outcome


Medical robotics devices


Medical robotics devices

• Mostly passive and semiactive devices

• Rich topic of very great importance!

• Main technical topics:– compare features and functionalities of systems– discuss and compare design considerations – devices for specific surgeries laparoscopy)

• Sources: chapters 16-18, 22, 29, 34, 39, 45, 47, and 48 in textbook

• Related fields: robotics, mechatronics


8. Real-Time tracking devices

• Ideally, an accurate Global Positioning System!

• Current technologies offer only partial solution

• Based on different principles– video: follow known objects– optical: follow light-emitting diodes– magnetic: measure the variation of – acoustic: works like a radar

Hardware to follow in real time the precise position and orientation of anatomy and instruments during surgery


Optical and video tracking devicescamera

instrument

Passive markers

Instrument has infrared LEDs attached to it Active markers


What kind of accuracy?


9. Safety, man-machine interfaces

• Medical systems have very stringent safety requirements

• Reported cases of radiation overdose due to faulty system design

• Important issues in man-machine interfaces

• Ideas for presentations– the radiotherapy accident– chapters 12-15 and 19 in textbook


10. Systems integration• Complete systems that address specific clinical

problems in domains

• Use available technology to develop the system

• The hard part: make it all work!

• Main technical topics:– systems in orthopaedics, neurosurgery, etc

• Sources: chapters in each section of

• Related fields: all!

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