Microwave Medical Imaging State of the Art and …cost-emf-med.eu/wp-content/uploads/2015/09/Prague...Microwave Medical Imaging – State of the Art and Challenges in to Dielectric

Microwave Medical Imaging –State of the Art and Challenges in to Dielectric Properties

Dr. Martin O’Halloran

ERC Research Fellow,

College of Medicine and Engineering

National University of Ireland Galway

Monday, November 16th 2015

Needs-driven Medical Device based on Dieleletric Properties

Diagnostics:• Electrical Impedance Imaging (KHz)

• Microwave Imaging (GHz) - CMI

Therapeutics:

• Radio-Frequency Hyperthermia

• RF & Microwave Ablation

Key Features:• Non-ionising (safe)• Non-invasive • Low cost• Aligned with the goals of H2020

Martin O’Halloran - Monday Nov. 16th 2015

Medical Device Ecosystem at NUIG


Basic Science

Engineering

Clinical Evaluation

Technology Exploitation

New Translational Research Facility

New Engineering Facilities

SFI Medical Device Research Centre

HRB Clinical Research Facility

BioInnovate Ireland

Insight InPrime Research Centre

School of Psychology

Confocal Microwave Imaging


Focussing ∑

Basics:• Illuminate the breast with UWB

pulses

• Gather reflected radar signals

• For each voxel, synthetically focus the signals and calculate energy

• High energy indicates a dielectric scatterer

Confocal Microwave Imaging


Advantages and Disadvantages• Fast and simple image

reconstruction

• Based on many assumptions:

• Breast is primarily homogeneous

• Allows for coherent addition of signals

• Tissue not overly lossy

• Sufficient dielectric contrast

• Qualitative information only

• Difficult to relate image to tissue composition

Coronal CMI Image of the breast showing the presence of a dielectric scatterer

Microwave Tomography


Basics• Reconstructs entire dielectric

profile

• Iterative reconstruction:• Record reflections from patient (S1)

• Run numerical simulation (S2)

• Compare S1 and S2

• Update simulated model

• Stop when S2~=S1

• Simulated model ~= patient’s breast

Microwave measurements

Numerical estimate

Update

CompareError > threshold

YES



Advantages and Disadvantages• Map of tissue structure

• Accurate quantitative information (size, shape, location, dielectric properties)

• Complex inverse problem

• Slow numerical solution

• Sensitive to initial estimate

Tomographic Image of the breast showing glandular and cancerous scatterers



Landscape of Tomographic Algorithms

Microwave Tomography: Global Optimization, Parallelization and Performance Evaluation –Noghanian et al.



Variants – Linear Inversions• Non-linear relationship between

scattered field and dielectric distribution

• Approximate with a linear function (Born Approximation)• Assumes dielectric contrast is low

• Small number of dielectric objects

• Still ill-posed and requires regularization

• Not suitable for microwave breast imaging

Variants of Microwave Tomographic Algorithms



Variants – Iterative Deterministic Algorithms• Minimize cost function using

Newton-type minimization

• Minimization guided by local gradient

• Application-specific regularization to address ill-conditioned problem

• Converges “quickly” • Can get trapped in local minima• Can be over-smoothed by

regularization




Variants – Iterative Deterministic Algorithms• Use Frequency-hopping

• Low Frequency - More likely to converge; but less detailed image

• Solve the tomography problem

• Move up to next frequency and use the initial solution as “seed”

• Use local gradient method to solve at each frequency

• Each image has finer detail




Variants – Iterative Deterministic Algorithms• Process all frequencies

simultaneously

• More likely to search in right direction

• Less chance of local minima (only true solution will be a minima across all frequencies)

• Use Debye models to model frequency-dependence of tissue properties

• Alternatively ignore the dependence




Variants – Stochastic Methods• Global optimization approach

• Typically:

• Genetic algorithms

• Particle swarm methods

• Wavelet-based methods

• Local minima problems less likely

• Regularization less important – less smoothing

• Computationally very costly

• Not currently used in prototype systems


Prototypes and Results


Dartmouth College• Largest academic patient study

(~400 patients)

• Circular array of monopole antennas (2D images)

• Seven frequencies between 500 and 1700 MHz

• Gauss-Newton Iterative Approach (iterative-deterministic)

• Complex permittivity reconstructed

• 1 minute reconstruction time

Breast Imaging System at Dartmouth College



Dartmouth College• Eight patients undergoing

chemotherapy

• Imaged with CE-MRI at start

• Imaged Microwave Tomography in ROI during treatment

• Seven images created (7 planes)

• Changes in properties during treatment were monitored Permittivity and Conductivity at 1300 MHz for

Planes 5-7. ROIs are also shown

Meaney, Paul M., et al. "Microwave imaging for neoadjuvant chemotherapy monitoring." Antennas and Propagation, 2006. EuCAP 2006. First European Conference on. IEEE, 2006.



Dartmouth College• Prior to treatment

Prior to treatment – Tumour clearly visible with significant contrast



Dartmouth College• 44 days into treatment

44 days into treatment – Tumour still clearly visible with significant contrast (2-3:1)



Dartmouth College• 229 days into treatment

229 days – Significant drop in dielectric properties in the tumour region



Dartmouth College• Similar results for other patients

Similar contrasts in permittivity and conductivity existed for other patients in the study

Conclusions –Dielectric Properties


Based on this small-scale study, it appears:• There is a difference in the dielectric properties

of normal and cancerous breast tissue at microwave frequencies

• The contrast is sufficient to detect relatively large tumours (study only examined women under-going chemotherapy)

• Relative Permittivity at 1300MHz – 20 (healthy), 50 (cancerous)

• Conductivity at 1300MHz - <1 (healthy) 1.5-2 (cancerous)

• How does this compare with dielectric studies?

Historical Dielectric Studies –Chaudary et al. (1984)


• 15 patients• Ex-vivo dielectric properties of fatty and cancerous breast tissues• 3MHz – 3GHz @ 25°C• Summary: Significant contrast (300-500% difference)– greatest in

permittivity below 100MHz

Historical Dielectric Studies –Surowiec et al. (1988)


• 7 patients

• Healthy tissue, tissue at margins of the tumour, and cancerous tissue

• 20 kHz – 100 MHz

• Summary: Significant dielectric contrast even at tumour margins; small tumours could reflect a lot of microwave energy.

Historical Dielectric Studies –Campbell and Land (1992)


• 63 tissue samples (22 normal and 41 diseased)

• Normal, fatty and benign and malignant tissues

• 3.2GHz

• Summary: Large range of properties for “normal tissue”. Overlap in properties between malignant and benign tissue; correlation with water-content of tissue.

Historical Dielectric Studies –Joines et al. (1994)


• 12 Normal and 12 malignant tissue samples

• 50-900 MHz

• Summary: Average difference in conductivity and permittivity of 233% and 577% respectively

Historical Dielectric Studies –Meaney et al. (2000)


• Measured healthy in-vivo using prototype tomographic imaging system

• 5 patients

• 900MHz

• Summary: Average properties of normal were higher compared to Joines; Correlation between radiographic density and dielectric properties

Recent Dielectric Studies –Choi et al. (2004)


• 34 samples from 12 patients

• Healthy and cancerous lymph nodes

• 500MHz – 30 GHz

• Summary: Again, significant dielectric contrast was evident in the lymph nodes

Recent Dielectric Studies –Lazebnik et al. (2007a)


• First of two studies - healthy breast tissue only (reduction)

• 500MHz – 20GHz• Largest number of samples (354)• Two site study (UW & UC)• Careful histological

categorisation:• 0-30% adipose tissue• 31-84% adipose tissue• 85-100% adipose tissue

• Physics-based exclusion method• Kappa analysis• Measurements taken within ~90

minutes of excision

Recent Dielectric Studies –Lazebnik et al. (2007a)


Results of healthy breast tissue study:

• No significant difference between within-patient and between patient variability

• Dielectric properties of healthy tissue spanned a much greater range

• Properties primarily a function of adipose (fat) content

Recent Dielectric Studies –Lazebnik et al. (2007b)


• Second study- healthy, benign and malignant breast tissue

• 500MHz – 20GHz

• Large number of samples (155)

• Two site study

• Physics-based exclusion method

• Careful histological categorisation:• 0-30% adipose tissue

• 31-84% adipose tissue

• 85-100% adipose tissue

• Benign tumour tissue

• Malignant tumour tissue

• Kappa analysis

• 18 – 27°Celsius



Results of Lazebnik’s second breast tissue study:

• Malignant tissue – largely in agreement with smaller historical studies

• Chaudhary et al. (circle), Surowiec et al. (triangle), Joines et al.(asterisk)



Malignant Tissue compared to healthy tissue:

• A 10:1 contrast between adipose and malignant tissue But…

• Only a 10% contrast between malignant and low-adipose breast tissue

• Most tumoursoriginate within glandular tissue

Microwave Breast Imaging –“A divided community”


Two distinct groups:

• ~40 IEEE publications since 2009 relating to the use of contrast agents in microwave breast imaging

• Need to find a specific contrast agent

• More complex system

• System would no longer be non-invasive

and

• Those with clinical prototypes continuing to produce positive results (Bristol, Dartmouth, Calgary)

• All without the use of contrast agents

Possible Explanations (1)


Open-ended Probe Sensing Volume• Lazebnik categorised tissue based on

a sensing volume of 3mm

• Depth established by Hagl (2003)

• Hagl determined minimum homogeneous volume required to determine dielectric properties

• But this conclusion was used to categorise heterogenous samples…

Adipose

Glandular

Probe



Open-ended Probe Sensing Volume• Meaney experimented with water

and teflon

• Measurement only represented teflon when within 300 microns of sample

• Sensing volume < 300 microns, rather than 3mm

• Lazebnik dielectric measurements could have been “mislabeled”

Teflon

Probe

water



Linear Versus Logarithmic Measurements

• Lazebnik measurements taken on a linear frequency scale (50 points)

• Fewer measurements below 2 GHz compared to logarithmic scale

• Tomographic contrast greatest below 2GHz

Summary and Conclusions


• Support required from the dielectric community

• Dielectric Literature details a contrast varying between 10:1 to 1.1: 1.

• 10% contrast between tumour and glandular tissue (where tumours arise).

• However tumours visible in tomographic image.

• Suggested explanations:• Dielectric Probe Sensing Volume

• Histopathological categorisation of tissue

• Linear/Log acquisition

Thank you for your attention!


Recent Dielectric Studies –Kikkawa et al. (2014)


• 102 samples from 35 patients

• 0.5 – 20GHz

• Divided into adipose, Stroma and Cancer cells.

Documents

Microwave Medical Imaging State of the Art and …cost-emf-med.eu/wp-content/uploads/2015/09/Prague...Microwave Medical Imaging – State of the Art and Challenges in to Dielectric