16 how to set up a gait lab (nov 2014)

How to set up a gait lab

Richard Baker

Professor of Clinical Gait Analysis

Blog: wwRichard.net

1

How to set up a gait lab

• General facilities

• Mocap systems

• Maintenance

2

General facilities

3

4

Room

5

Room

12m

8m

6

Video

7

Mocap system

8

Force plates

Plate 1 Plate 2 Plate 3

Direction of walking

9

Force plates

Plate 1 Plate 2 Plate 3

Plate 4

Direction of walking

10

Force plates

11

Physical exam

12

ReceptionAssessment Assessment

Colours

• Contrast with skin colour

• Similar tone to skin

– (auto exposure on video cameras can be

tricked if too light or too dark)

• Only matters in areas seen by video

cameras

13

Floor

• Same colour rules

• Have something with a little “give”

• Ensure force plates are mechanically

isolated

• Very useful to be able to see them

– (place operators desk with good visibility)

14

Privacy and confidentiality

• Lab should not be a general purpose space

• Do not use as office space

• Try and maintain clinical and professional

environment

• Incorportate specific spaces for privacy

– dressing and undressing

– physical examination

15

Mocap systems

16

General principles

• Maximise marker resolution

• Minimise marker masking

17

Greyscale detection

More pixels + finer greyscale > more resolution

19

Increasing strobe intensity

Increasing camera aperture

Increasing sensitivity

Dirty marker

Threshold

Marker pair

Proximity to camera

0

0.1

0.2

0.3

2 3 4 5 6 7 8 9 10

Marker diameter (pixels)

Ce

ntr

oid

re

so

luti

on

(p

ixe

ls)

edge detection

1 bit greyscale

2 bit greyscale

3 bit greyscale

4 bit greyscale

5 bit greyscale

0

0.1

0.2

0.3

2 3 4 5 6 7 8 9 10

Marker diameter (pixels)

Ce

ntr

oid

re

so

luti

on

(p

ixe

ls)

edge detection

1 bit greyscale

2 bit greyscale

3 bit greyscale

4 bit greyscale

5 bit greyscale

Resolution

Pixel size

One pixel = field of view .

linear resolution

1 Megapixel camera (1,000 x 1,000)

One pixel = field of view .

1000

Resolution

Capture volume 3m x 1m x 2mCamera 4.5 m from centre

Pixel res. = 1mm

Pixel res. = 2mm

Light intensity

1m

3m

D

Ne

arest

Relative intensity =nearest

furthest

2

Side cameras At least 2m preferably 4m

D

Light intensity

1m

3m

Nearest

Front cameras At least 4m preferably 6m

Accuracy

• 2-d

– The more grayscale resolution you get for each

marker the better

• 3-d

– The more cameras that see each marker the

better.

25

Setting up a lab

26

Steps

• Define requirements

• Obtain system

• Map out capture volume

• Place cameras

• Tune settings

27

1 Define requirements

• One type of movement

– (eg gait, sit-to-stand, posture)

• Different types of movement

• Mainly one movement but occasionally

others.

One type of movement

• Fixed camera positions

• Fixed focal lengths

• Go through this procedure once

Many types of movement

• Flexible camera positions

• Variable focal lengths

• Go through this procedure every time you

set up for a different movement

Mainly one movement

• Hybrid system

• Most dangerous

1. Define requirements

• Dimensions of capture volume

• Preferred direction?

• Location of markers

• Obstacles (to cameras)

• Force plates (number and position)

Full body walking

• Dimensions of capture volume

3m long x 1m wide x 2m high

• Preferred direction?

Both Directions

• Location of markers

Conventional gait model

• Obstacles (to cameras)

None

• Force plates (number and position)

3 (in-line)

2. Obtain system

• Resolution

– Greyscale

– High as possible

• Marker visibility

– As many cameras as possible

• As much money as possible!

2. Obtain system

• Greyscale imaging

• Sufficient cameras for each marker to be seen by

at least 3 cameras throughout movement.

• Purchase this number of cameras at as high a

resolution as you can afford.

• If you’ve still got more money purchase more

cameras.

More money

• In my university it is as easy to apply for

$400,000 of equipment as it is to apply for

$600,000 with about the same chance of

actually getting it.

Marker sizeBased on 4mm pixel width

0

0.2

0.4

0.6

0.8

1

8 16 24 32 40

Marker diameter (mm)

Ce

ntr

oid

re

so

luti

on

(m

m)

edge detection

1 bit greyscale

2 bit greyscale

3 bit greyscale

4 bit greyscale

5 bit greyscale

Marker size

• Use 14mm markers for good resolution for

walking

– (with greyscale detection)

• Safer to use 25mm for edge detection

• No obvious advantage to using smaller

markers.

3. Map out capture volume

• Yes, physically!

• Marker pairs

– Same diameter as markerset

• Cover floor of capture volume at least

– Try and cover full volume.

– Centre as well as periphery

• Map out significant obstacles

• Make centre of volume the system origin


14mm marker

Illumination too high

25mm marker

Illumination too low

4. Place cameras

• Mix of theory and practice

• Use tripods to start with

• Find what software is provided to support

camera placement

Direction

• More cameras

• Determined by

– Orientation of capture volume

– Markerset

– Movement

Location of markers Conventional gait

model – all round visibility

Angular separation of cameras

Effective Resolution

Turn a 1 megapixel camera to a 0.1 megapixel camera

Effective Resolution

Distance and focal length

• Move cameras as far back as possible

• Choose lens of focal length to maximise effective resolution

• Avoid wide angle lenses wherever possible

Height

Cameras just higher than measuring volume work well and are generally out of the way

Height

• Slightly higher than capture volume

• If reflections are a problem

– Matt flooring

– Different heights

– Different strobe frequencies

• May be required to view round obstacles

5. Tune cameras

• Back focus

• Aperture

• Gain

• Strobe intensity

• Threshold

Don’t touch unless you know what you are doing

Aperture

• Optical quality of better with smaller

aperture

Gain

• Grey scale resolution is effectively limited

by noise

• Increasing gain will increase noise

Strobe intensity

• No problem with high intensity

• (Although some current visible strobes

irritate the eyes)

Procedure

• Set strobe intensity to maximum

• Reduce aperture until good greyscale information

from markers throughout calibration volume

• Do this without increasing gain if possible

• Set threshold as low as possible to remove floor

reflections

Procedure

• Note that there might be a difference in how

camera information is processed to give

visualisation in real-time and how it is

processed offline after data capture.

Maintenance

60

Maintenance

• Very little

• Calibrate frequently

• Look at results

61

Calibrate - Cameras

• Made easy with current system

• Need an object with known dimensions

Calibrate - Cameras

• The object have to move through the entire

volume

Camera views

Calibrate - Cameras

• Inadequate mapping of the volume

Position

Orientation

1deg = 17mm at 1m

Spot checks

68

Camera settings

Remove dirty markers

Effect of time / use

Camera calibration SAMSA

Piazza SJ et al. A proposed standard for assessing the marker-location accuracy of video-based motion analysis systems. Proceedings GCMAS (2007), Springfield, MA

Poker test

Holden JP, Selbie WS, Stanhope SJ. A proposed test to support the clinical movement analysis laboratory accreditation process. Gait Posture. 2003 17(3):205-13.

Ground reaction should pass through the tip of the poker and along the shaft

Thanks for listening

Richard Baker

Professor of Clinical Gait Analysis

Blog: wwRichard.net

73

16 how to set up a gait lab (nov 2014)

Education

Transcript of 16 how to set up a gait lab (nov 2014)