A Review and Adaptation of Methods of Object Tracking to ... - CiteSeerX

Vol. 1, No. 1, Issue 1, Page 35 of 46 Copyright © 2007, TSI® Press Printed in the USA. All rights reserved

A Review and Adaptation of Methods of Object Tracking to Telemetry Capsules Khalil Arshak*, Francis Adepoju1, David Waldron2 1

Electronic & Computer Engineering Department, University of Limerick, Plassey Technological Park, Limerick, Ireland. 2 Mid Western Regional Hospital, Limerick, Ireland. Received 1 January 2007; revised 2 February 2007, accepted 3 March 2007 Abstract This review considers techniques employing radio frequency (RF) as well as ultrasound signals for tracking. Medical capsules have been employed since the 50s to measure various physiological parameters in the human body. Examples are temperature, pH, or pressure inside the gastrointestinal (GI) tract. The development and subsequent incorporation of new technology into reasonably priced, commercially available devices have made ultrasound and RF devices readily accessible for medical diagnosis. Some applications for telemetry capsules are drug delivery, and collection of tissue/fluid samples. Samples are taken from the GI to understand or treat diseases where diagnosis can only be made by taking a biopsy from the intestinal walls. Such biopsies have traditionally been performed using customized endoscopes. In order for a telemetry capsule to be effective in the above named tasks, accurate knowledge of the location of the capsule within the body during tests is necessary. As such, methods for calculating the position of an object based on geometry, i.e. triangulation, time of flight, time difference of arrival and angle of arrival, etc, are presented in detail. Keywords Wireless Endoscopy, Capsule Tracking, GI tract, Ultrasound pulse, RF signals

1. INTRODUCTION

Table 1. The GI tract.

The Human GI tract The various GI tract nodes and average transit times of objects moving through them (propelled by peristalsis) are as highlighted in Table 1. Motility disorder refer to abnormal intestinal contractions, such as spasms and intestinal paralysis, that can result in the inability to eat (which may require patients to receive intravenous nutrition) and severe abdominal pain, nausea, vomiting, maldigestion, weight loss, diarrhea, constipation and incontinence. Motility Disorders encompass a variety of conditions, including: gastroparesis; achalasia; gastroesophageal reflux disease; chronic intestinal pseudo obstruction; constipation and irritable bowel syndrome. Irritable bowel syndrome is the most common of the disorders, resulting in pain and diminished quality of life. Serious disorders produce excruciating pain and make it impossible to eat ordigest food. In order to diagnose and treat such disorders, physicians have over the years employed various methods, notably among them are: *

Approximate length(cm)

Transit time (min) 2

Small Intestine

25 1500ml (Average capacity) 400-600

36-65 (0.5-1hr) 194-246 (3-4 hr)

Large Intestine

200

2160-4500(36-75hr)

Description Esophagus Stomach

Barium follow-through - This test is done in radiology (x-ray department) and involves drinking a chalky substance called barium [1] that can be seen on x-ray. The radiologist (physician) has the patient drink the barium and observes by x-ray how the barium moves through the esophagus and into the stomach. If backwashing of stomach contents into the esophagus (GE reflux) occurs during the test, it can

Khalil Arshak, ECE Dept, University of Limerick, Ireland. Tel +353-61-202267, [email protected]

Double-Balloon Endoscopy (DBE) [3] - Due to inaccessibility and also due to the aforementioned problems, the small intestine is usually bypassed by flexible endoscopy. Push enteroscopy is now becoming obsolete with the introduction of capsule endoscopy (CE). In the last 3 years, wireless capsule endoscopy and DBE have been introduced. Developed by Yamamoto et al. in Japan, DBE incorporates greater insertability and manoeuvrability [4], allowing for a risk-free insertion and easy reach of the areas of diagnosis. The new DBE features two balloons, one attached to the distal end of the scope and the other attached to a transparent tube sliding over the endoscope. When inflated with air, the balloons can grip sections of the small intestine and "shorten" the small intestine by pleating it over the endoscope. Sequential shortening of the small intestine over the endoscope and advancement of the endoscope enables a comprehensive examination of the entire small intestine. This further positions the DBE as a suitable replacement for push enteroscopy. Obscure gastrointestinal bleeding can now be explained and treated in the majority of cases. Biopsy sampling, hemostasis, polypectomy are now all possible to accomplish in the small intestine. The safety and efficacy of DBE have been demonstrated, but the method is not fully known and tested.

be seen as barium backing up from the stomach into the esophagus. Esophageal narrowing (strictures) can also be seen as well as some ulcers and tumors. The problem with this test is that it can miss small abnormalities in the esophagus such as small erosions or ulcers. Another problem is that GE reflux may occur in normal individuals. The advantage of this test is that it can be ordered by a primary care physician and if erosions, strictures, ulcers or tumors are seen, these are highly specific for esophageal disease. Esophagogastroduodenoscopy (EGD) - also known as upper endoscopy, is a procedure usually performed by a gastroenterologist (GI or intestinal doctor). This test involves passing an endoscope, a long, flexible black tube with a light and video camera on one end, through the mouth to examine the esophagus, stomach and the first part of the small intestine called the duodenum. The advantages of this test over the barium follow-through (x-ray test) are that the lining of the upper digestive tract can be directly viewed by the doctor and very small abnormalities seen [1]. Endoscopic therapies (treatments) can also be performed at the time of the procedure. Biopsies (taking small pieces of tissue) of any abnormality may also be done directly through the endoscope. Push enteroscopy - Push enteroscopes usually have a working length 210-cm or more and have standard and therapeutic sized biopsy channels that will accommodate a full range of accessories. They are similar to standard upper endoscopes in that they have up/down and right/left angulation along with a field of view of 120° or greater. To aid in achieving the greatest possible depth of advancement, an overtube is usually placed over the endoscope prior to its insertion. Once the scope is advanced into the second or third portion of the duodenum, the instrument is straightened along the greater curve of the stomach by gentle withdrawal. At this time the overtube is gradually advanced over the scope until it rests within the duodenum. The overtube reduces looping of the scope within the stomach. Gradual insertion and withdrawal movements of the enteroscope help to pleat as much small intestine onto the scope and improve the depth of insertion. Using these techniques it is possible to examine up to 100 cm of small intestine. In most cases, the diagnostic yield for obscure gastrointestinal bleeding ranges from 45% - 80% for dedicated push enteroscopses. [2] Complications from push enteroscopy appear to be more frequent than for standard upper endoscopy. Perforation, possibly related to the use of an overtube, is also very common.

Capsule endoscopy [5] or video capsule endoscopy (VCE) is a new technology which has proven to be effective for screening of the entire small bowel in those with hereditary colon cancer-prone disorders that predispose to small bowel polyps (a small vascular growth on the surface of a mucous membrane) and small bowel cancer (SBC). VCE are a more superior and future alternatives to both barium follow-through and push enteroscopy for diagnosis of small bowel lesions in these disorders. The danger of intestine perforation and the limitation of not being able to view all sections of the small intestine have motivated the shift from the use of traditional endoscopic methods to the more modern capsule endoscopy method to diagnose GI tract malfunction. However for a successful diagnosis, accurate tracking of the capsules is a very important aspect of the endoscopic process. Research shows that GI physicians will be quite happy with a tracking system that is capable of cm-range resolution. But in reality due to the difficulty posed by the intestinal organs to acoustic signal transmission and reception, it is generally acceptable if the section/quadrant of the intestine where the capsule is situated at a point in time can be accurately determined. It is quite possible to achieve this with the system described in section 4.

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Sub Sonic Range

0

Low Frequency/ Airborne/ High Power

Audible Range

10

1k

100

100k

10k

High Frequency Acoustic microscopy

Conventional/ Industrial Ultrasonic Range

1M

10M

100M

1G (Hz)

Figure 1a. Acoustic Frequency Spectrum.

Radio Waves

Microwave

3G

Infrared

300G

Ultra Violet

Optical

400T

750T

Gamma Ray

X- Ray

> 30x10 18 (Hz) 30E

30P

9

12

15

18

Legend: G=10 ,T=10 , P=10 , E=10

Figure 1b. Electromagnetic Frequency Spectrum. Ultrasound & Radio Frequency waves.

Table 2. Velocity and Half-Power distances of Acoustic wave.

Ultrasound or Radio Frequency (RF) waves are commonly employed in medicine to transmit and receive signals into the human body [6]. Figure 1 shows the approximate frequency range of the various regions of the electromagnetic spectrum [7] spanning the range from audible through radio waves. Sound generated above the human hearing range (typically 20 kHz) is called ultrasound. However, the frequency range normally employed in ultrasonic non-destructive testing is 100 kHz – 50 MHz. Although ultrasound behaves in a similar manner to audible sound, it has a much shorter wavelength. This means it can be reflected quite easily. Attenuation is of major concern in ultrasonic systems. For practical purposes, velocities of ultrasound waves in various mediums are as shown in Table 2. Table 2 also shows that ultrasound energy can travel in water 380 cm before its power decreases to half of its original value.

Tissue Mean Velocity (m/s)………….. Air 330 Fat 1450 Humantissue(mean) 1540 Brain 1541 Blood 1570 Skull bone 4080 Water 1480

Materials and their Half Power Distance (cm). Water 380 Blood 15 Fat 6 Soft tissue 5 to 1 Muscles 1 to 0.6 Bone 0.7 to 0.2 Air 0.08

water, it is noted that the speed of radio wave is approximately equal to (1/9)th of the speed of light in free space i.e. (1/9)x 3x108ms-1 [8]. Radio waves belong to the lower frequency part of the electromagnetic spectrum, which includes x-rays, gamma rays, ultraviolet, infrared, microwave, and audio waves. In the RF region, the electromagnetic energy is too low to break molecular bonds. RF waves are essentially electric and magnetic energy moving together through space at the speed of light.

Attenuation is greater in soft tissue, and even greater in muscle [6], so a thick muscled chest wall will offer a significant obstacle to the transmission of ultrasound. Non-muscle tissue such as fat does not attenuate acoustic energy as much. The half power distance for bone is still less than muscle, which explains why bone is such a barrier to ultrasound. Air and lung tissue have extremely short half-power distances and represent severe obstacles to the transmission of acoustic energy. In pure

Some detailed research that has been conducted concerning the interaction between human bodies and the electromagnetic spectrum was discussed in Alex Hariz et al [9]. It was shown that if the wavelength of a signal is significantly larger than the cross-section of the human body being penetrated, the body has little effect on the signal. These

37

wavelengths occur at frequencies below 4 MHz. Above 4 MHz, the absorption of RF energy increases and the human body may be considered to be essentially opaque until roughly 1 GHz, when the dielectric properties of the human body begin to introduce a scattering effect on the RF signal. However, at frequencies near the body's natural resonant frequency, RF energy is absorbed more efficiently, and maximum heating of tissue cells occurs. In adults, this frequency is about 35 MHz if the person is grounded and about 70 MHz if the person's body is insulated from the ground. Also, different body parts may be resonant; the adult head, for example is resonant at around 400 MHz, while a baby's smaller head resonates near 700 MHz [9]. Body size thus determines the frequency at which most RF energy is absorbed. As the frequency is increased above resonance, less RF heating generally occurs. Common sources of noise with RF are interference from external sources while noise due to the effect of multi-path echo caused by some of the signals bouncing off another object in the local area is of major concern with ultrasound signals [7].

B=

2. THEORETICAL BASIS OF OBJECT TRACKING METHODS Telemetry systems are normally built from commercial-off-the-shelf products. But while they all have many common elements, they are each uniquely configured to meet specific application requirements. Data acquisition begins when sensors (transducers) measure the amount of a physical attribute and transform the measurement to an engineering unit value. Sensors attached to signal conditioners provide a basis for the sensors to operate or modify signals for compatibility with the next stage of acquisition [11]. Since maintaining a separate path for each source is cumbersome and costly, a multiplexer is employed to serially measure each of the analog voltages and outputs a single stream of pulses, each with a voltage relative to the respective measured channel. The rigorous merging of data into a single stream is called Time Division Multiplexing or TDM.

At the heart of many positioning methods is the concept of triangulation: Triangulation is essentially the use of the properties of triangles to calculate distances. Because of its significance in modern positioning methods, a basic description is given here. Originally, triangulation was used for surveying [10] and civil engineering purposes, and later for finding the range of targets for artillery strikes. Given any two reference points it is possible to calculate the distance from one reference point to an object with knowledge of the angles between both references and the object and also the distance between the reference points. Figure 2 illustrates this. The distance B for example, can be found by using the sine rule for triangles. Equation (1) shows this formula rearranged to find this distance. Note that a, b, and C are all known.

Two basic quantities are considered in object tracking or range measurements [12, 13], these are: (1). Time of flight (TOF) or the round trip time for sound to travel through the sample, for methods employing ultrasound waves. This is based on velocity and round trip time of flight through the material. The distance between the transmitter and reflector can be calculated as follows: R=

A

B

Reference 1

vt s 2

(2)

R = distance, υ = velocity of sound in material, ts= Time of Flight. (2). Amplitude of received signal as a ratio of amplitude of transmitted signal, for methods employing either ultrasound or RF signals. Measurements of the relative change in signal amplitude can be used in sizing flaws or measuring the attenuation of a material. The relative change in signal amplitude is commonly measured in decibels. This is a logarithmic value of the ratio of two signal amplitudes and can be calculated using the following equation [8].

Object

b

(1)

where b is the angle opposite the side with length = B; a = the angle opposite the side with length = A and; c = (180-a-b) is the angle opposite side with length = C.

Triangulation

c

C. sin(b) sin(180 − (a + b))

Reference 2

a C

Figure 2. Position Ref for triangulation.

38

dB = 20. log10 (

A1 ) A2

placed on the object can be reduced considerably if not removed completely. This type of system is therefore feasible in situations where it is either inefficient or impossible to reach the object, i.e. ingestible capsule.

(3)

dB = Decibels, A1 = Amplitude of signal in media 1, A2 = Amplitude of signal in media 2.

Another advantage of this method is realized when using a laser as the signal carrier; because a laser can be made to be highly directional, interference from the environment is reduced. Ultrasonic waves are used in much the same way as lasers. Wolf [15] proposes a system for using ultrasonic waves to measure the position of an object in 3-D. In a TOF system that uses multiple base stations, the position of the object (which has a transmission node attached to it) is found by plotting the locus of points where the signal could have originated for each base station (BS). The true position of the object is the point where all of the loci intersect. Figure 3 illustrates this method.

Due to the need for tracking in a wide range of applications, many methods of implementation have been proposed to suit the requirements of a particular task. This section summarizes the literature regarding these methods, specifically the theory of operation and the advantages and disadvantages of each method.

Positioning Methods (Time of Flight) As explained above, this method uses the time taken for a signal to travel between the object and base station. The distance of the object from the base station can be found by using the time, distance and velocity relationship. If only one base station is employed, the position cannot be directly measured, only a distance from the reference point. However, the position of the source can be found in one of two ways: (1) By measuring the angle deviation from a reference by using a system that is sensitive to direction, such as a directional antenna or a system in which the receiver must be pointing directly at the source for the signal to be received; or, (2) By using multiple base stations at different known locations (at least three are required to find the position) [14]. A Time of flight (TOF) system that is used for calculating distance only can work in two ways. The signal could be sent from a node on the object to the base station and the time it takes for that signal to reach the station is used to work out the distance.

Figure 3. TOF positioning method. This type of system is best used with an omni-directional signal carrier, such as radio waves. This allows the object to be anywhere within range of all the base stations. Also, the object can be moving if there is ability to make multiple measurements with the system. One of the disadvantages of this system is that for the arrangement to have a wide range, the nodes and base stations must be omni-directional. Also, the system is vulnerable to interference from outside sources or, more importantly, multi-path fading.

One major disadvantage of this system is that the node and base station must be synchronized so that a timer starts exactly when the signal is sent. This is necessary to ensure accuracy of time measurement. Note that if the signal is an electromagnetic wave then an error in time measurement of 1ns will cause an error of about 0.3m in the calculated length [7]. Because of this, most TOF systems send the signal from the base station (receiver) and time how long it takes for the signal to travel to the object, be reflected, and return (called the round trip time). Assuming that the reflection time is negligible the TOF can be calculated by halving the round trip time.

Time Difference of Arrival (TDOA) In this system a base station measures the difference in time of the arrival of two or more signals. Using this data and the time, distance and velocity relationship, the base station can calculate its position relative to the signal sources. For this method to work the locations of the signal sources must be known. As an example, Global Positioning System (GPS) uses this method. Timing accuracy is attained in GPS systems through the use of synchronized atomic clocks on board each satellite. TDOA

This system has the advantage that all of the timing is done at the base station and as such no remote synchronization is necessary. Also, with all of the transmitting and receiving equipment at the base station, the amount of equipment that must be

39

system would work in the same way as the GPS system. The range of such system is the area in which the signal from the node can be received by at least three base stations. This range could be expected to be of the order of meters to hundreds of meters depending on the terrain and equipment being used. Since the time measurement at the base station begins when the first signal arrives, this method does not require the node and the base station to be synchronized with each other. However, this method does require that the time measurement system be accurate. Inaccurate measurements of the time difference will result in erroneous calculations of distance and inaccurate measurements of position. A solution to this problem in GPS systems is to measure the TDOA of the signal from a fourth satellite. This extra information allows the GPS receiver to fine tune its local time. The principle of using redundant information to increase accuracy can be applied to any TDOA system. An alternative to using redundant information for timing is to have an atomic clock in the base station. Atomic clocks are currently in development that will fit in an integrated circuit and have similar accuracy to GPS clocks, but for now, it is very expensive [16].

signal with respect to a reference. An example of such a receiver might be a directional antenna that can calculate what direction an RF signal originated from. One example of a directional antenna is the phased array antenna [20]. Here, the incident angle of the incoming signal can be calculated by measuring the phase difference of the incoming signal at several receiving elements aligned in a known pattern. Another example is the Doppler antenna in which the Doppler shift [21] of an incoming signal is measured with respect to an antenna array that simulates a single antenna moving in a circle [22]. The AOA method can be used to calculate position in one of two ways. In the first, the angle of arrival information is combined with data from the time of flight distance method. This method has the distinct advantage that it requires only one base station. In the second method, two or more AOA stations are set up at known distances from each other. The AOA of an incoming signal is calculated at each station as shown earlier on and the theories and equations of triangulation are used to calculate the position of the source. The main advantage of using a purely angle based system is that there is less need for accurate timing in the system. Therefore the oscillator jitter, gate propagation delay and other factors that affect timing in TDOA, have less of an effect on the position uncertainty [10]. Another advantage is that by increasing the number of sensing stations, the redundant data can be used to reduce the position uncertainty. It should also be possible to have the positions of the extra sensing stations calculated by using the AOA method from the stations of known position. The major disadvantage of this method is that the sensing stations must have a fixed reference for the calculated angles to have any meaning. The implication of this disadvantage is that the AOA antennas must be static unless some way of measuring the orientation of the station with respect to a reference is included in the base station system [10]. The major factor that affects the accuracy of an AOA positioning system is the angular resolution of the antennas used. As the minimum angle that can be resolved increases so also will the error associated with each measurement. Furthermore, for a given angular resolution the boundaries of the error of positioning will increase when the object is further away from the antenna. By minimizing the angular resolution and also the range of an object to the nearest base stations the positioning error can be minimized.

For systems that do not have the advantage of highly accurate atomic clocks, other methods of increasing the accuracy of timing are required. One of the major sources of inaccurate timing comes from jitter in the oscillator of a system. Jitter is small variations in the frequency of the output signal of an oscillator. This variation can come from a range of sources including: (1) Substrate and power supply noise, which causes switching components to change state at the wrong time [17]. (2) Interference from environmental sources, (including changes in temperature, which has an effect on the propagation delay of circuits), (3) Analogue signal jitter [18], (4) Vibration of the crystal from external sources, and (5) Variations in the manufacturing process of the crystal oscillators [19]. TDOF is suitable for use in wireless sensor networks that are either static or mobile. The major operational requirement is the provision of at least three base stations of known position that are within range of all of the other nodes (transmitter).

Angle of Arrival (AOA) This method involves using a receiver that can calculate the angle of incidence of an incoming

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ber of nodes in the system is increased. Anti-collision algorithms are common in multiple access RFID systems [24].

Laser or Ultrasound - Based Tracking (Unique Identification Solution) In some cases, it becomes unavoidable to track multiple objects. An example is the GI tract where two or more capsules needs to be tracked in real-time. This might be in order to locate the site of bleeding within the intestine and to consequently deliver medication to such site. Tracking multiple objects (capsules) using a laser or ultrasonic based system, which can individually identify each object is feasible by using unique identification tags. The solution of Unique ID involves having some form of identification on each of the objects, such as radio frequency identification (RFID) tag or a Dallas 1-wire ID tag [23]. These transmit a unique bit stream when prompted by an electronic interrogation device (called a reader). This method is appealing because it is very easy to include an ID into most types of wireless sensor nodes and is cheap to implement. This ID signature could be used in conjunction with any of the methods presented here to calculate the positions of each object in turn. One limitation is that the system could only handle a finite number of objects. But for medical application where only a few objects need be tracked, this is not much of a concern.

Self-Positioning In this method each of the nodes calculate their positions based on signals received from known references in combination with one of the methods described in previous sections. The GPS uses this type of configuration. Because the signals in this system are all broadcast to all of the nodes, there is the advantage that this scheme will allow an unlimited number of nodes to track their own positions so long as they are within range of enough power of the reference signals (in most cases three are required for two dimensional positioning). The disadvantage is that each object to be tracked must have a node and that the node must be interrogated for the position data to be used by an outside agent.

3. METHODS OF TRACKING CAPSULES IN REAL-TIME Some of the problems of traditional endoscopes were discussed in Section 1. These include the danger of intestine perforation and the limitation of not being able to view all sections of the GI tract for diagnosis or observation. What is needed however is a solution that is relatively comfortable for patients, easy to use, inexpensive, and that which provides a reasonable level of visual imaging for the detection of abdominal abnormalities. Different methods have been used to diagnose the motility disorders and detect the abnormal activities of the GI tract like radiology, manometry [25, 26], and radio telemetry [4]. Radiology offers qualitative information on the behavior of the alimentary canal, but at the same time it carries the risk of radiation exposure and therefore only few and short lasting events can be observed. Radiology is able to detect wall movement caused by contraction of the circular muscle layer, but it is inadequate in the investigation of the longitudinal layer events.

Another major disadvantage is that there may be an uncertain delay in the query signal being received and the reply transmission being sent (the reflection time). As has been stated, this delay will affect time-based calculations of distance [23, 24]. It is easy to take account of this delay if it is a constant (by subtracting the reflection time before halving the round trip time). However, this delay will be affected by all sources of jitter and propagation delay. Accurate models of the relationship of this jitter and propagation delay would help this type of system to correct for this uncertain reflection time. It should also be noted that this disadvantage would not affect a pure angle of arrival system since this method is not as dependent on time. Another method of using the ID information is to have each of the nodes transmit on a pseudo-random basis. This would remove the need for the nodes to be interrogated by one of the base stations, thereby reducing the complexity of the nodes. The drawback of this solution is that without control over when a particular node transmits its ID there is a possibility of collisions. That is two or more transmissions occurring at the same time, making all of the signals unreadable because of interference. Pseudo-random transmission is very similar to an early network protocol for computers called the Aloha protocol. Like Aloha, the performance of the system will degrade exponentially as the num-

Manometry is a second method, which is a technique for recording mechanical activity of the bowel by detecting changes in pressure caused by contractions of the gut wall. The system is an established procedure for the evaluation of GI motility, including diagnostic studies in the GI tract using a perfuse tube. As shown in Figure 4, a patient can be examined by inserting a manometry tube [25,27] from the mouth down through the GI tract. The tube is attached from the other side to a pressure transducer, which is connected to a recorder device. The equipment for manometry consists of thin tubing with

41

Figure 4. The manometry tube [29]. openings at various locations. When this tube is positioned in the esophagus, these openings sense the pressure in various parts of the esophagus. As the esophagus squeezes on the tube, these pressures are transmitted to a computer analyzer that records the pressures on moving graph paper. A number of limitations are associated with this technique. First the manometry in the upper digestive tract and colon is disturbed by solid food components. Secondly the pressure changes are measured within a sealed cavity, which makes it difficult to get good signals [26, 28]. The process of inserting the probe into the patient is inherently uncomfortable.

Figure 5. Capsule tracking using RF signal. ceived signal strength indicator (RSSI) [33] outputs of the receivers in order to compute the real-time capsule position. Receivers (pods) are spaced over the abdomen uniformly at known distances from each other. The pods are capable of receiving electromagnetic signals. The pods communicates with a processor located in the controller which converts the RSSI outputs to digital values, timestamp them and store them for later upload to the PC for analysis by the triangulation algorithm. Effects of noise, tissue attenuation, etc. are considered in the solver routines in order to determine the capsule position.

N - Sensor Network A direct application of geometry with the power of ultrasound transducers was employed by Jiang Pingping [30] to achieve a rough estimate of capsule position in the GI tract. A number of ultrasonic detectors (pods) are fixed on the abdominal surface. These points are linked to a data logger carried by the patient on her waist. The detectors will respond to the passage of the capsule whenever the capsule is within the vicinity of the sensors. When the capsule reaches a particular sensor, echoes are received from the capsule and processed by the receiver. A trigger pulse to a microcomputer initiates recording of TOF [31] data. At the end of test, the data is read into a computer and processed by software.

4. INTEGRATED ULTRASOUND MODEL FOR CAPSULE TRACKING In addition to drug delivery, telemetry capsules with tissue/fluid sampling capability are extremely useful for GI related diseases where diagnosis can only be made by taking a biopsy from the intestinal walls. Such biopsies have traditionally been preformed using an endoscope, which has been customized so that it is capable to taking tissue or fluid samples [7]. Although, physiological structures can be used to determine the location of a moving capsule based on the acquired images to identify a site of interest, accurate knowledge of the location of telemetry capsules is necessary if the tasks mentioned above are to be accomplished within a reasonable range of accuracy. Typically, transducers operating at ultra-sound frequencies e.g. 1 MHz and higher are used to perform a single distance measurement [6]. One crystal will transmit a burst of ultrasound, and a second crystal will receive this ultrasound signal. As discussed in section 2, the elapsed time from transmission to reception is a

RF Signal-Power Detection Method As discussed in Section 2, a variation of self-positioning method is employed in [32]. The relative amplitudes of RF signals received by the receiving antennas are used to determine relative location of the capsule based on the correlation between the capsule to antenna distance and RF amplitude (signal strength) at the receivers. As shown in Figure 5, four or more antennas are employed to track the location of the capsule in 2–D. Triangulation algorithm is then applied to the re-

42

direct and linear representation of the physical separation of the crystals. The resulting transit time is easily converted to a distance if the speed of sound in the material being measured is known. This is typically 1540 ms-1 in human tissue.

processing. All control and timing logic are coded into the microprocessor. When echoes are detected, the timing circuit will precisely measure an interval corresponding to a round trip of the sound pulse in the test piece. Usually, this process is repeated for all instances of echo received by the transducer to obtain the required position information as a function of time. The microprocessor then passes this time interval measurement, along with sound velocity and any offset information to a PC running a triangulation algorithm in order to compute and present the position information graphically.

The resolution of any ultrasound system depends on its ability to accurately detect the received ultrasound signal and on its ability to measure transit time. Advanced high-gain, low noise circuitry allows for the detection of the received ultrasound signal and precise tracking of the received signal regardless of its frequency. This allows for complete independence of crystal frequency and measurement resolution. The choice of 1MHz frequency of transmission is also informed by the fact that ultrasound signals suffer less attenuation at lower frequencies. A rule-of-thumb for attenuation of ultrasound signals in soft tissue is 1 dB/cm/MHz [34]. For example, a 5-MHz signal will have an attenuation factor of 5dB/cm. If the target is at a depth of 10 cm, the reflected signal will be attenuated by 100 dB. This shows that it is usually better to use frequencies lower than 5 MHz when sending ultrasound waves deeper into the body.

Most ultrasonic measurements are done with one of four types of transducer and appropriate electronics: These are contact, delay line, immersion, and dual element transducers. Of course, each type of transducer has advantages and limitations [7]. Echo sensing is initiated by first creating an acoustic ping at 1MHz frequency. In this method, the pulser circuit produces the ping. This pulse train is passed on to the excitation circuits to produce the necessary voltage to excite the transducer. The circuit diagram shown in Figure 6 detect a reflected wave from the object after sending out ultrasonic pulses at a controlled interval. By measuring the round trip time after emitting the ultrasonic pulse wave, a distance to the object is measured. This operation is executed repeatedly for the duration of the objects motion. Usually, the chirp moves away radially from the transducer through the couplant into the body at approximately 1540 m/s, the speed of sound in tissue [7]. This speed is only slightly affected by humidity and virtually not affected at all by pressure and therefore is almost independent of altitude

Absorption (conversion of sound to heat), reflection and scattering of the sound at heterogeneous tissue interfaces are usually the sources of attenuation to ultrasound signals [9]. Figure 6 shows a generalized block diagram [7] of a microprocessor-based ultrasonic position tracking system. An ultrasonic position tracker will generally include a pulser/receiver circuit, control and timing logic, computational circuits, optional display circuit, and a power supply. The pulser, under the control of the microprocessor, provides an excitation pulse to the transducer. The ultrasonic pulse generated by the transducer is coupled to the test piece. Echoes returning from the inside surface of the test piece are received by the transducer, converted to electrical signals, and fed to the receiver amplifier for Amplifier and Excitation circuits

When the chirp reaches the capsule, it is reflected in varying degrees dependent on the shape, orientation, and surface properties of the capsules surface. The transducers are used in a slightly different way in this architecture so that strong signals are returned to the receivers situated outside the abdomen. Automation is achieved by a microcontroller. Noise and unwanted echoes are filtered off as shown in Figure 6 at the receivers input amplifier stage. Once an echo signal hits the transducer, a voltage is created which is fed to a stepped-gain amplifier. Since the acoustic signals decreases in strength with distance [11], a high gain amplifier is used. This helps give the best sensitivity across the detection range. Once the module "sees" the permitted band [7, 14] of the reflected signal, it changes its echo output to reflect the received reflected signal or echo. All that is left to do is to

Regulated Power ( 5v )

1-10MHz Crystal Chip

Pulser circuit

uController

Tx ----Rx

Pulse Detector Circuit

Amplifier Bandpass Filter

Memory RS232 UART

Signal Rectifier

Figure 6. Architecture of Novel Prototype for Capsule Tracking.

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technical standards for the specific radio frequency that the Given System uses to redefine the period of time during which their device can operate continuously in a defined frequency, known as the “duty cycle.” This change permits the Given System to transmit for more than 10% of the time that it is in operation. This was necessary for the Given System to function correctly without any known health hazards. Ultrasound is used in many medical diagnostics and consumer products with little or no information about their potential health effects. They are generally thought to have a negligible effect on human tissue [38] upon exposure.

measure the time from the initiation of the ping to the received echo. This time (divided by 2) corresponds directly to the distance traveled by the ping. Using the capture feature of the microcontroller, this can be accomplished.

5. CONCLUSION For effective tracking application, the main requirement for an ultrasound tracking system is to complete the round-trip computation in minimum amount of time. We have discussed in earlier sections that signal attenuation due to tissue and organ characteristics are a source of concern to ultrasound tracking systems. As stated in section 1, most GI physicians will be quite happy for a tracking system that is capable of cm-range resolution. But because of the difficulty posed by the intestinal organs to acoustic signal transmission and reception, it is generally acceptable if the section/quadrant of the intestine where the capsule is situated at a point in time can be accurately determined. Based on a selection of various tracking solutions discussed in this paper, a wide variety of tracking methods have been identified and proposed for medical application. However, a great deal of restrictions is imposed by the unusual nature of the body terrain, which makes impracticable, the adaptation of most of these popular tracking methods for medical use. The N-Sensor network is a concept that is already in use for RF method of tracking, based on Received Signal Strength Indicator (RSSI) detection. However, a novel solution based on Ultrasonic transducers, employing the TOF measurement to compute the length of traversed segment, based on successive accumulation [35] was discussed. If successfully implemented, this solution will be of interest to gastroenterologists and patients alike because the implementing circuitry can be built with simple and commercially off-the-shelf components. This will no doubt have a positive effect on the price of the final product [36]. It is to be noted that current international guidelines [37] are available for the protection of individuals from RF hazards. The basic restriction is that the peak Specific Absorption Rate (SAR) level (averaged over 10 g of contiguous tissue) within the trunk and head of the user must be below 2 W kg. This extremely low power level and low duty-cycles used in short-range telemetry (typically 1mW – 2 mW peak radiated power, with a 1% duty-cycle) suggest that most devices will comply with the guidelines. Really since the tracking system operates for a few microseconds before going back to sleep mode, effects of ultrasound exposure at 1MHz can be safely ignored. In February 2002, the Short Range Device Maintenance Group [35] modified the

6. ACKNOWLEDGEMENTS This review was supported by the Enterprise Ireland commercialization Fund 2003, under technology development phase, as part of the MIAPS project, reference no. CFTD/03/425. Funding was also received from the Irish Research Council for Science, Engineering and Technology: funded by the National Development Plan.

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AUTHOR INFORMATION Prof. Khalil I. Arshak received a BSc from Basrah University, Iraq, in 1969; an MSc from Salford University, UK, in 1979; and the PhD and DSc from Brunel University, UK, in 1986 and 1998 respectively. He joined the University of Limerick in 1986 where he leads the Microelectronic & Semiconductor Research Group. He has authored more than 250 research papers in the area of microelectronics and thin- and thick-film technology. His current research interests include lithography process modeling, Top Surface Imaging processes characterization, mixed oxide thin - and thick - film sensor development, and application specific integrated circuit design.

Francis Adepoju is currently a PhD researcher at the University of Limerick, Ireland.

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David Waldron is a consultant surgeon with the Mid-West Health Board, Ireland

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