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Detection of an object inside a

phantom tissue using a spatial filter

Dawn V. Stephens, Frank K. Tittel, Lihong V. Wang,

Andreas H. Hielscher, Steven L. Jacques

Dawn V. Stephens, Frank K. Tittel, Lihong V. Wang, Andreas H. Hielscher,

Steven L. Jacques, "Detection of an object inside a phantom tissue using a

spatial filter," Proc. SPIE 2135, Advances in Laser and Light Spectroscopy to

Diagnose Cancer and Other Diseases, (19 May 1994); doi:

10.1117/12.175994

Event: OE/LASE '94, 1994, Los Angeles, CA, United States

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Detection of an Object in a Phantom Tissue

Using a Spatial Filter

Dawn V. Stephens

Clark Atlanta University, Department of Physics

Atlanta, Georgia 30314

Frank Tittel

Rice University, Department of Electrical & Computer Engineering

Houston, Texas 77251

Lihong Wang, Andreas Hielscher, Steve Jacques

UT MD Anderson Cancer Center, Laser Biology Research Lab

Houston, Texas 77251

ABSTRACT

We report the detection of an object inside a phantom tissue using

a spatial filter and a 5 mW He-Ne Laser. The phantom tissue is

composed of 8% scattering Polystyrene spheres (particle size 579

nm) and is diluted to different concentrations in water.

The

solution is placed inside of a cuvette of length 5 cm and width 5

cm. The spatial filter, composed of a 4 cm plano-convex lens and

a 10 urn pinhole, is able to extract ballistic and quasi-ballistic

photons from the transmitted light. A photomultiplier tube is used

for detection, and a lock-in amplifier is used to reduce the amount

of noise in the signal. We are able to detect the object in a

phantom tissue of 20 mean free paths [mfp) (concentration .016%)

with a contrast of 99.0%. The contrast in a tissue with 30 mfp

(concentration .024%) is 22.7%.

1. INTRODUCTION

1.1. Current cancer detection techniques

X-rays, Ultrasonography, and Nuclear Magnetic Resonance are

methods that are used today for the detection of cancerous tissue.

The X-ray technique requires the use of extremely high amounts of

energy (on the order of MeV). As a result, the X-rays can ionize

(remove electrons) from tissues in the body and therefore be

harmful to the patient. Despite the energy used, X-rays can fail

to produce a sufficient amount of contrast thereby blurring the

image.

X-rays may also cause discomfort, as well as require a

period of rest for the patient.

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Ultrasonography is a technique that enables the visualization

of objects in the body by recordi,ng the reflections of mechanical

waves directed at the tissue.

Since ultrasonography uses

mechanical waves, it requires a density difference between tissues

to produce a good image.

This creates a problem because some

cancerous tissues may have a density approximately equal to the

surrounding tissue.

The third technique used today is Nuclear Magnetic Resonance

Imaging (NMRI). This technique subjects an odd number of protons

to a magnetic field. and measures the deflections as a means of

producing the image.L NMRI is an asset in providing images of the

heart as well as soft tissues. However, it is costly and requires

the patient to remain immobile for a lengthy period.

1.2. Purpose and significance of prcdect

The purpose of this research project is to develop an

alternate efficient optical method for the detection of cancerous

tissues in the body. Although there are other techniques (X-rays,

Ultrasound, NMRI) that are currently used, this technique has the

potential to be more effective because it is potentially safer,

does not requires a difference in density, and is more cost

efficient. It is a non-ionizing approach which subjects the body

to less possibility of injury to the surrounding tissue without

sacrificing the contrast of the image. This method uses light as

a means of detection which enables the differentiation between

tissues solely by their optical properties. Finally, this method

of spatial filtering will be more cost efficient because its

components are relatively inexpensive.

The significance of this research is that it could lead to

significant improvements of cancer detection. One such improvement

could be the aid in the search for an efficient method of

determining whether a cancerous tissue is malignant or benign by

using optical properties. With the application of this method, the

safety of the cancer patient would increase during treatment while

the cost of health care decreases.

1.3. Project basis

The basis of this technique hinges on the use of a spatial

frequency filter. A spatial frequency filter restricts light that

encounters it at certain angles. A simple filter is composed of a

lens and a pinhole. The light that is parallel to the optical axis

is focused onto the pinhole where it enters.

However, the light

that enters the lens at any other angle is focused at a different

point and since the pinhole is very small (10 pm) the light does

not get through. When light is incident upon tissue, a very small

amount travels straight through without scattering, ballistic

light.

However, the majority of the light, diffuse light, is

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randomly scattered.

As a result, light leaves the tissue at

various angles. Since the spatial frequency filter rejects light

that enters the lens at wrong angles, the diffuse light which

distorts the image is rejected allowing the creation of an image

with little distortion (see Figure 1).

Figure

SPATIAL

FREQUENCY FILTER

Incoming

Ught

Pinhole

Filur,

1:

An il1ustr, of th. propertj.s of a spatial filt.r

1.4. Project objectives

There are three major objectives in this research project.

The first is to determine the maximum number of mean free paths

that are attainable with this technique. When light encounters

tissue, it undergoes a series of scatterings while some of it is

absorbed. The average length between these interactions is called

the mean free path (mfp). In the body, the length is typically on

the order of one hundredth of a centimeter. Imaging capability of

an approximately 5 cm thick tissue is desired. This corresponds to

500 mean free paths (mfp).

The second major objective is to optimize the spatial

frequency filter in order to achieve the best possible resolution

of the image. The optimization is performed by using the same

concentration of phantom tissue and measuring the image of an

object with various sizes of the pinhole.

The use of this technique in actual tissue is the final

objective. Although phantom tissue (essentially a milk solution)

is initially used for a preliminary study, an object will be buried

in a chicken breast and scanned to determine if detection in tissue

is possible.

A mean free path is the average length between which light is

either scattered or absorbed. A continuous wave (cw) laser is one

in which the light is continuously emitted at some specified

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