november 9th 2005thesis defense: tom ault1 in vivo measurement and visualization of pelvic position...
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November 9th 2005 Thesis Defense: Tom Ault 1
In Vivo Measurement and Visualization of Pelvic Position and Orientation and Changes in Soft Tissue Shape and Thickness with Respect to Changes in Seating Surface Shape
Thomas G AultRobotics InstituteSchool of Computer Science, CMU
November 9th 2005 Thesis Defense: Tom Ault 2
Abstract Pressure Ulcers
Tissue necrosis due to normal sitting or reclining Affect elderly, hospitalized and those with neurological
disorders Costly to treat & potentially fatal
Preventative measures not effective enough
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Abstract Why?
Don’t understand how ulcers form Tissue distortion is key …but no way to measure it directly
Until Now!
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Abstract Novel System To Measure Tissue Distortion
MRI Interface pressure Ultrasound Adjustable seating contour
Pilot Study Three subjects Interesting Results
May need to revise current models
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Outline Background Problem Solution Study Conclusions Future Work
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Pressure Ulcers Tissue necrosis from sitting or reclining Four Stages
Non-blanchable Erythema
Edema
Degeneration of Fat and Sweat Glands
Stage I Ulcer
Superficial ulcer
Loss of epidermis, dermis
Hemorrhagic crust
Stage II Ulcer
Extends through dermis, fat
Cellular detail obliterated
Black eschar
Stage III Ulcer
Extends to muscle, bone
Osteomyelitis
Sinus tracts undermine neighboring tissues
Stage IV Ulcer
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Pressure Ulcers Who is at risk?
Elderly Hospitalized for major surgery Spinal-cord injury (SCI)
5-8% per year Lifetime risk of 50-75% 7.5% will die
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Pressure Ulcers
Extrinsic Pressure Shear/Friction Moisture
Intrinsic Impaired Mobility Incontinence Neuropathy Malnutrition Advanced Age Illness Altered Consciousness
Risk Factors
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Pressure Ulcers Pressure/time relationship
More pressure -> less time
Contributing factors reduce tolerance
Ulcers can form with as little as 2h of exposure
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Pressure Ulcers Treatment
Stage I and II treated conservatively Stage III and IV treated surgically Costs over $1 billion annually Focus on prevention
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Pressure Ulcers Prevention
Risk assessment Keep skin clean and dry Nutritional support Pressure relief
Seat cushions and mattresses Pressure relieving exercises Turning the patient
Not effective enough!
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Pressure
Distortion
IschemiaHigh Strain Rate Injury
Reperfusion Injury
Interstitial Fluid Loss
Shear/ Friction
Damage!
Ulcers!
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Pressure Ulcers Understanding distortion is key Early work done by Chow & Odell (1978)
Axisymmetric finite element model
Submerged in water
Uniform interface pressure
Little distortion
Seated on flat surface
Uniaxial pressure distribution
Large distortion
Distribution matters more than magnitude
Equalized distribution minimizes distortion
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Pressure Ulcers But does this match reality? Hard to measure distortion in a seated subject
Crude attempt by Clark et al in 1989 B-mode ultrasound exam of sacrum No correlation between thickness and ulcers Did not (and could not) measure pressure simultaneously
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Pressure Ulcers What about indirect measurements? Brienza et al (1996)
Used stiffness as a proxy for thickness Hypothesis: Developed the CASS and ESS systems
ESS: Measure unloaded shape CASS: Create seating contour & measure pressure Augmented CASS used in thesis
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Pressure Ulcers Brienza, et al (1996)
30 elderly subjects Three contours
Flat ESS Stiffness-optimized
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Pressure Ulcers Conclusions
Stiffness-optimized contour distributed load to thicker areas of tissue
Cushions with stiffness-optimized contour shape had a more even pressure distribution
However… Did not actually measure thickness Did not measure pelvic position and orientation
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Problem Need to measure:
Pelvic position and orientation Bulk tissue thickness and volume Skin, fat and muscle thickness Interface pressure In vivo for seated subject For different seating contours
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Solution Augment the CASS!
Can already create contour, measure pressure Use registration to measure pelvis position & orientation
Based on Simon (1996) Bulk thickness and volume from pelvis + seat
Use ultrasound to measure skin, fat, muscle thickness
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Solution
11x12 Array of Actuators
Position Wand
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Solution Motors
Superior Electric M601 Stepper Motors Range of 150 mm Open loop Relative error < 2%
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Solution Pressure Sensors
Foxboro/ICT 1860-03G-LDN-B 100 sensors Range: 0-15 PSI Accuracy: 2.9 mmHg Sampling: < 10 Hz
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Solution Ultrasound
36 transducers 7.5 MHz center frequency ~6 MHz bandwidth (-3 dB) 1 kHz pulse repetition rate Time gain compensation
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Solution
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Solution Orientation Sensors
Measure normal to actuator …but not rotation about normal Rotation about normal moves sensors by about 2 mm
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Solution Process:
Data analysis Data acquisition Visualization
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MRI ofPelvis
Calibrate Wand
Flatten CASS
Measure TransducerLocations
Load Subject
Create Contour
Wait
Get Points NearSurface of Pelvis
Make US & PressureMeasurements
DataAnalysis
Once Per Subject
Once PerExperiment
Once Per Contour
UltrasonicTraces
Slices
SeatingContour
RegistrationPoints
PressureData
SensorOrientations
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Segment
Marching Cubes
Initial Segmentation
Initial Refinement
Register
Final Refinement
Compute BulkThickness
Model ofPelvis
Pose ofPelvis
SeatingContour
TissueMeshes
SensorOrientations
UltrasonicTraces
Tissue LayerThicknesses
MRI Slices
RegistrationPoints
Bulk TissueThickness
Once Per Subject Once Per Contour
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Solution But how accurate is it?
Pelvis position: 2 mm Pelvis orientation: 3 degrees Bulk Thickness: 3-5% Layer Thickness:
Skin: 3-10% Fat: 4-10% Muscle: 5-10%
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Solution But how accurate is it?
Boundary Point Location: Skin/Fat: 2-3.5 mm Fat/Muscle: 2.3-4 mm Muscle/Muscle: 3-4 mm
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Study Three subjects:
Subject #1: Male, healthy, age 40, 74 kg Subject #2: Male, healthy, age 35, 77 kg Subject #3: Make, SCI, age 60, 90 kg
Two contours Flat Pressure equalized within 25 mmHg
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Study Data for subject #3 has a bug
Vertical offset Corrected using earlier data set Relative changes still correct
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Study - Contours
Subject #1 Subject #2 Subject #3
Left Right Left Right Left Right
Mean Depth (mm) 17.3 23.2 5.0 9.3 7.3 10.1
Max Depth (mm) 58.5 64.5 34.0 51.0 31.0 46.0
Equalized Contour
Even Rt Deeper Rt Deeper
Yue Li et al (1999)
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Study – Pelvis Position
X Y Z
Subject #1 268.9 276.4 126.4
Subject #2 266.2 290.3 113.8
Subject #3 274.2 276.5 121.7
Subject #1 249.4 288.7 69.1
Subject #2 255.6 289.8 86.5
Subject #3 277.2 287.7 95.0
Fla
tE
qual
ized
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Study – Pelvis Orientation
Pitch Roll Yaw
Flat Eq Flat Eq Flat Eq
Subject #1 -6.39 -1.27 12.53 6.02 -1.84 2.56
Subject #2 -13.28 -0.26 -4.23 -4.10 -2.24 -4.12
Subject #3 -5.13 -4.34 1.26 4.53 -7.10 0.09
Equalizing pressure aligns pelvis with seat
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Study - Pressure
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Study – Thickness & Volume
Bulk Thickness and Volume – Entire PelvisSubject #1 Subject #2 Subject #3
Minimum Flat 8.8 3.0 14.1
Equalized 15.5 9.9 18.9
Mean Flat 124.7 113.7 115.3
Equalized 101.8 102.1 106.7
Volume Flat 2643.3 2294.0 2731.5
Equalized 2175.0 2188.7 2526.8
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Study – Thickness & Volume
Individual Layers –Right ButtockSubject #1 Subject #2 Subject #3
Skin Flat 2.67 2.50 3.19
Equalized 2.56 2.51 3.11
Fat Flat 2.32 3.16 10.32
Equalized 4.01 3.36 11.68
First Muscle Flat 10.90 7.56 12.15
Equalized 15.67 10.60 14.34
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Study – Ischial Tuberosities
Right IT – Bulk ThicknessSubject #1 Subject #2 Subject #3
Minimum Flat 8.7 3.1 14.1
Equalized 22.9 22.7 25.5
Mean Flat 37.9 26.7 36.1
Equalized 45.6 42.1 44.3
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Study – Ischial Tuberosities
Left IT – Bulk ThicknessSubject #1 Subject #2 Subject #3
Minimum Flat 24.5 3.0 15.0
Equalized 15.5 9.9 18.9
Mean Flat 48.4 30.3 39.0
Equalized 44.4 33.4 41.1
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Study – Ischial Tuberosities
Lef
tR
ight
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Study – Ischial Tuberosities
Individual Layers –Right ITSubject #1 Subject #2 Subject #3
Skin Flat 2.9 2.3 2.9
Equalized 2.5 2.6 2.9
Fat Flat 0.3 0.7 7.2
Equalized 5.2 2.4 12.4
First Muscle Flat 5.4 2.5 N/A
Equalized 15.0 7.1 N/A
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Study - SummaryWhat have we learned so far? Expected things:
Different-shaped seating contours Increase in minimum, decrease in mean Redistribution of soft tissue towards legs Less fat, muscle beneath IT for flat contour Equalized contour places ITs over actuators
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Study - SummaryWhat have we learned so far? Unexpected, not surprising:
Varying pelvic orientations for flat contour Equalized contour aligns pelvis with seat Redistribution of tissue occurs in deeper muscle layers
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Study - SummaryWhat have we learned so far? Unexpected & surprising:
Asymmetry between left & right ITs Thickening of soft tissue beneath right IT Slight change beneath left IT
Two-pole vs. one-pole pressure distribution No difference between SCI and healthy
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Study - ExplanationsHow can we explain these? Population not representative Left side lacks orientation sensors Asymmetric mechanical properties Different-shaped equalized contours Models too simple
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Study - Explanations Left side lacks orientation sensors
Must interpolate actuator orientations May mask changes in soft tissue thickness Can’t explain
Pressure distribution Response of SCI
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Study - Explanations Asymmetric Mechanical Properties
Right side more compressible than left Explains difference in response between left & right Expect to see left thicker than right
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Study – Explanations
Lef
tR
ight
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Study - Explanations
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Study - Explanations Different-shaped equalized contours
Right lobe deeper than left for subjects #2 and #3 Thinner tissue = stiffer But left IT not as stiff as right Assume that tilt obscures change in thickness for #1
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Study - Explanations Models too simple
Chow & Odell model = rigid core in goo Pelvis attached to spine and femurs Muscles cannot arbitrarily deform Center of gravity of torso Can explain all three
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Conclusions - ContributionsTwo main contributions Novel system that can measure:
Pelvic position and orientation Interface pressure Bulk tissue thickness and volume Individual layer thickness Seated subject in vivo Arbitrary contour
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Conclusions - ContributionsTwo main contributions Pilot Study:
Two healthy, one SCI Demonstrated usefulness of system Surprising results Models may need revision
November 9th 2005 Thesis Defense: Tom Ault 56
Conclusions - ContributionsSmaller contributions Analysis of US measurement error Effect of biased errors on registration accuracy
Errors biased in direction not different from unbiased Second position wand provides sufficient accuracy Different rotation distance metric
November 9th 2005 Thesis Defense: Tom Ault 57
Conclusions – Future WorkWhere do we go from here? Larger study Automation Clinical applications
November 9th 2005 Thesis Defense: Tom Ault 58
Conclusions – Future Work Larger study
More subjects Vary injury status and body composition Don’t assume symmetric response Measure femurs, torso center-of-gravity Acquire points from both PSIS and ASIS
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Conclusions – Future Work Automation
Easier and faster analysis Reduce bias Automate Registration Automate Segmentation of US
November 9th 2005 Thesis Defense: Tom Ault 60
Conclusions – Future Work Clinical Applications
Measure pelvic position and orientation Design cushions to align pelvis with seat Interaction between torso, femurs & pelvis