november 9th 2005thesis defense: tom ault1 in vivo measurement and visualization of pelvic position...

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


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


Abstract Why?

Don’t understand how ulcers form Tissue distortion is key …but no way to measure it directly

Until Now!


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


Outline Background Problem Solution Study Conclusions Future Work


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


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


Pressure Ulcers

Extrinsic Pressure Shear/Friction Moisture

Intrinsic Impaired Mobility Incontinence Neuropathy Malnutrition Advanced Age Illness Altered Consciousness

Risk Factors


Pressure Ulcers Pressure/time relationship

More pressure -> less time

Contributing factors reduce tolerance

Ulcers can form with as little as 2h of exposure


Pressure Ulcers Treatment

Stage I and II treated conservatively Stage III and IV treated surgically Costs over $1 billion annually Focus on prevention


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!


Pressure

Distortion

IschemiaHigh Strain Rate Injury

Reperfusion Injury

Interstitial Fluid Loss

Shear/ Friction

Damage!

Ulcers!


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


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


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


Pressure Ulcers Brienza, et al (1996)

30 elderly subjects Three contours

Flat ESS Stiffness-optimized


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


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


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


Solution

11x12 Array of Actuators

Position Wand


Solution Motors

Superior Electric M601 Stepper Motors Range of 150 mm Open loop Relative error < 2%


Solution Pressure Sensors

Foxboro/ICT 1860-03G-LDN-B 100 sensors Range: 0-15 PSI Accuracy: 2.9 mmHg Sampling: < 10 Hz


Solution Ultrasound

36 transducers 7.5 MHz center frequency ~6 MHz bandwidth (-3 dB) 1 kHz pulse repetition rate Time gain compensation


Solution


Solution Orientation Sensors

Measure normal to actuator …but not rotation about normal Rotation about normal moves sensors by about 2 mm


Solution Process:

Data analysis Data acquisition Visualization


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


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


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%


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


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


Study Data for subject #3 has a bug

Vertical offset Corrected using earlier data set Relative changes still correct


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)


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


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


Study - Pressure


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


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


Study – Ischial Tuberosities

Right IT – Bulk ThicknessSubject #1 Subject #2 Subject #3


Equalized 22.9 22.7 25.5

Mean Flat 37.9 26.7 36.1

Equalized 45.6 42.1 44.3



Left IT – Bulk ThicknessSubject #1 Subject #2 Subject #3


Equalized 15.5 9.9 18.9

Mean Flat 48.4 30.3 39.0

Equalized 44.4 33.4 41.1



Lef

tR

ight



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


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


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


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


Study - ExplanationsHow can we explain these? Population not representative Left side lacks orientation sensors Asymmetric mechanical properties Different-shaped equalized contours Models too simple


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


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


Study – Explanations

Lef

tR

ight


Study - Explanations


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


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


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


Conclusions - ContributionsTwo main contributions Pilot Study:

Two healthy, one SCI Demonstrated usefulness of system Surprising results Models may need revision


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


Conclusions – Future WorkWhere do we go from here? Larger study Automation Clinical applications


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


Conclusions – Future Work Automation

Easier and faster analysis Reduce bias Automate Registration Automate Segmentation of US


Conclusions – Future Work Clinical Applications

Measure pelvic position and orientation Design cushions to align pelvis with seat Interaction between torso, femurs & pelvis

november 9th 2005thesis defense: tom ault1 in vivo measurement and visualization of pelvic position...

Documents

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tom ault13 pressure

tom ault7 pressure ulcers

ulcer slide

prevention slide

cmu slide

ulcers form tissue distortion

tolerance ulcers