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Intravascular MicroBolometer
CatheterAshkan Emadi,MDSaid Siadaty,MD
Ward Casscells,MDJames T. Willerson,MDMorteza Naghavi,MD
The University of Texas-HoustonHealth Science Center
Texas Heart Institute
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Introduction to Infrared Technology
• Infrared detectors and detector arrays are used in many fields of applications today.
• Many of these are based on passive detection of thermally emitted electromagnetic radiations as described by the Planck’s law.
• In this way it is possible to image objects in darkness, or carry out contactless temperature measurement.
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Blackbody & Blackbody Radiation• Central to radiation thermometry is the concept of
the blackbody. The blackbody concept is important because it shows that radiant power depends on temperature.
• Kirchhoff defined a blackbody as a surface that neither reflects nor transmits, but absorbs all incident radiation, independent of direction and wavelength.
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• In addition to absorbing all incident radiation, a blackbody is a perfect radiating body. To describe the emitting capabilities of a surface in comparison to a blackbody, Kirchoff defined emissivity of a real surface as the ratio of the thermal radiation emitted by a surface at a given temperature to that of a blackbody at the same temperature and for the same spectral and directional conditions.
• Boltzmann showed that the radiation emitted by a blackbody is proportional to the fourth power of the absolute temperature of the surface.
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• Almost all of the real objects and surfaces have emissivities less than 1. Objects with an emissivity less than one are named graybodies. Most organic objects are graybodies, with an emissivity between 0.90 and 0.95.
• Boylan A. et al reported that emissivities of burn wound tissues were in the range 0.976-0.992, greater than those of intact skin by 0.01-0.03.
• We speculate plaque emissivity is similar to wound lesions.
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Result of emissivity measurementBurn wounds and tissueSuperficial partial thickness burn(scalp) 0.976 ± 0.006Partial thickness burn(arm) 0.992 ± 0.001Deep Partial thickness burn(buttock) 0.982 ± 0.004 Full thickness burn(hands) 0.977 ± 0.010Skin in vitro with epidermis removed 0.970 ± 0.010Skin in vivo with dermal layer removed 0.985 ± 0.007
Normal SkinIntact skin mean of 12 subjects 0.961 ± 0.007Intact skin in vitro 0.968 ± 0.003
Set of measurements on one subjectSkin dry 0.971 ± 0.001Skin with layer of moisture 0.978 ± 0.004 Skin covered by layer of cling film 0.968 ± 0.002Skin with talc applied 0.875 ± 0.011
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Emissivity Map of Volcano on Venus
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Planck’s Radiation Law Planck's distribution shows that as
wavelength varies, emitted radiation varies continuously. As temperature increases, the total amount of energy emitted increases and the peak of the curve shifts to the left, or toward the shorter wavelengths.
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Planck’s radiation law states that every object at a temperature above absolute zero emits electromagnetic radiation.
The higher the temperature the higher is the emitted intensity.
The wavelength of maximum intensity decreases when the temperature increases.
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Trends in Application & Marketing of Infrared Detectors
It is notable, that from a global perspective, for
many years all the major breakthroughs in infrared technology, and the major purchases of infrared equipment, have been funded by a military sponsor. Consequently, the technology has been developed with the military use in mind, and the emphasis been on high performance IR systems, predominantly cooled photon detectors.
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However, the main future trend will most certainly be to reexamine one of the strengths of infrared technology i. e. its suitability to applications outside the military sector, and meeting the needs of the civil customer. The civil sector can accept a lower performance, but the price per unit must be kept low, and the equipment user friendly. The medium performance, uncooled thermal detector technology is certainly suitable for this.
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A potentially important civilian market for
uncooled infrared detector arrays is :
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Research & Development / Non Destructive Testing
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Thermal Images in Security and Surveillance
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Hot 2x4s Inside Bedroom Walls (Can be used to locate secret compartments!)
Visible Image Shows Surface Only
Uncooled Image Shows Wall Structure
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Condition Monitoring / Preventitive Maintenance
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Fire Fighting
Warm body in a dark, smoke-filled room can be seen
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General Motors says it will be the first automaker to offer night vision technology when its 2000 model Cadillac DeVille goes on sale next year.
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•Cadillac's Night Vision uses an infrared heat sensor mounted in the front grille. •The sensor detects heat as low as 0 degrees Fahrenheit from objects as far as 500 yards in front of the car, five times the distance low-beam headlights reach. People, cars and other objects appear as white images in a black background, similar to a photo negative. •The images are projected onto a 4x10-inch area above the steering wheel but below the driver's line of sight.
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Medical Diagnosis
Thermal Coronary Angiography
Stenosis
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It is difficult to see the veins in this forearm with visible light.
The high sensitivity of an array can detect the increased temperature of venous blood flow in the same arm.
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Image showing different facial temperatures. Note cold nose and ears.
Cold Sweat Glands on Fingertip
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Detector Technology• Detector Types Photon Detectors Thermal Detectors
• Infrared Imaging
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Photon Detectors The absorption of long-wavelength radiation by
photon detectors results directly in some specific quantum event, such as the photoelectric emission of electrons from a surface, or electronic interband transitions in semiconductor materials. Therefore, the output of photon detectors is governed by the rate of absorption of photons and not directly on the photon energy.
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Photon detectors normally require cooling down to cryogenic temperatures in order to get rid of excessive dark current, but in return their general performance is higher, with larger detectivities and smaller response times. In most cases photon detectors need to be cooled to cryogenic temperatures, i. e. down to 77 K (liquid nitrogen) or 4 K (liquid helium). Quantum Well Infrared Photodetector (QWIP) Arrays is a type of photon detector.
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Background Limited Infrared Photodetector (BLIP)
• The current from an infrared detector may be subdivided into two parts: photocurrent and dark current. The photocurrent is the useful response of the detector, whereas the dark current is an undesired part.
• Photocurrent results from absorption of infrared photons in the detector. These photons create charge carriers which can be collected as a photocurrent.
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Dark current is by definition present even if the detector is not illuminated. The origin of dark current is usually thermal excitation of charge carriers, a process that competes with photoexcitation. Due to the thermal origin, dark current depends on the detector temperature. The most efficient way of getting rid of dark current is to cool down the detector to a temperature where the photocurrent becomes the dominant one. However, since cooling is expensive, during the detector design phase, every action should be taken to minimize dark current and maximize photocurrent.
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• When photocurrent dominates over dark current the detector is said to be background limited or BLIP (Background Limited Infrared Photodetector). Background here means the high temperature (not cooled) surroundings or scene (including imaged objects) within the detector field of view. The background scene emits infrared photons sensed by the detector giving rise to a photocurrent.
• Usually BLIP temperature is defined as the temperature where the photocurrent is ten times as large as the dark current.
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Photon Detectors Classification
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Photoconductive Detectors
The function of photoconductive detectors are based on the photogeneration of charge carriers (electrons, holes or electron-hole pairs). These charge carriers increase the conductivity of the device material. Detector materials possible to utilize for photoconductive detectors are:
*indium antimonide (InSb) *quantum well infrared photodetector (QWIP) *mercury cadmium telluride (mercad, MCT) *lead sulfide (PbS) *lead selenide (PbSe)
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Photovoltaic Detectors
Photovoltaic devices require an internal potential barrier with a built-in electric field in order to separate photo-generated electron-hole pair. Whereas the current-voltage characteristics of photoconductive devices are symmetric with respect to the polarity of the applied voltage, photovoltaic devices exhibit rectifying behavior. Examples of photovoltaic infrared detector types are:
*indium antimonide (InSb) *mercury cadmium telluride (MCT) *platinum silicide (PtSi) - silicon Schottky barrier
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Thermal Detector Arrays In contrast to photon detectors, the operation of
thermal detectors depends on a two-step process.1. The absorption of infrared radiation in these
detectors raises the temperature of the device,2. which in turn changes some temperature-dependent
parameter such as electrical conductivity.
Thermal detectors may be thermopile (Seebeck effect), Golay cell detectors, pyroelectric detectors, or bolometer.
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Bolometer• A resistive bolometer contains a
resistive material, whose resistivity changes with temperature.
• To achieve high sensitivity the Temperature Coefficient of the Resistivity (TCR) should be as large as possible and the Noise resulting from contacts and the material itself should be low.
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Resistive Materials in Bolometer Resistive materials could be metals such as
platinum, or semiconductors (thermistors). Metals usually have low noise but have low temperature coefficients (about 0.2 %/K), semiconductors have high temperature coefficients (1-4 %/K) but are prone to be more noisy. Semiconductors used for infrared detectors are,e. g., polycrystalline silicon, or vanadium oxide.
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128x128 and 64x64 Pixel Bolometric Detector Arrays Equipped with On-Chip
ROIC’s
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ROIC The electronic chip used to
multiplex or read out the signals from the detector elements are usually called readout integrated circuit (ROIC) or (analogue) multiplexer.
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Microbolometer in Cross-Section
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Bolometer Structure and Operating Principles
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Surface Micromachined Bolometric Detectors
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Surface Micromachining
is used for the fabrication of the detector elements, consisting of thin bridge structures or membranes
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NETD NETD is an abbreviation for Noise
Equivalent Temperature Difference and is a measure of the smallest object temperature difference possible to detect by an IR camera.
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Thermal Detector specification
The requirement on detector arrays comprising the detectors integrated with readout electronics is a temperature resolution (NETD)
< 100 mK, for a camera optics f-number = 1 and 50 Hz frame rate.
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Vacuum Encapsulation
The major requirement for achieving high sensitivity is an efficient thermal insulation between the detector element and the substrate. This necessitates vacuum encapsulation of the detector. Next in importance is a sensitive means of temperature detection. Semiconductor based layers (thermistors) with large temperature coefficient, and pyroelectric materials, are good choices.
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Advantage & Disadvantage of Thermal Detectors
• The major advantage of thermal detectors is that they can operate at room temperature (Uncooled Detector).
• The sensitivity is lower and the response time longer than for photon detectors. This makes thermal detectors suitable for focal plane array (FPA) operation, where the latter two properties are less critical.
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Uncooled IR Camera Based on128x128 Pixel FPA
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Infrared Imaging• There are two basic types of infrared imaging
systems: mechanical scanning systems and systems based on detector arrays without scanner.
• A mechanical scanner utilizes one or more moving mirrors to sample the object plane sequentially in a row-wise manner and project these onto the detector . The advantage is that only one single detector is needed. The drawbacks are that high precision and thus expensive opto-mechanical parts are needed, and the detector response time has to be short.
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Infrared Imaging Detector arrays operated as focal plane arrays
(FPA) (or staring arrays) are located in the focal plane of a camera system, and are thus replacing the film of a conventional camera for visible light. The advantage is that no moving mechanical parts are needed and that the detector sensitivity can be low and the detector slow.The drawback is that the detector array is more complicated to fabricate. However, with the ascent of rational methods for semiconductor fabrication, economy will be advantageous, provided that production volumes are large. The general trend is that infrared camera systems will be based on FPAs, except for special applications.
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Infrared Imaging
• The spatial resolution of the image is determined by the number of pixels of the detector array.
• Common formats for commercial infrared detectors are 320x240 pixels (320 columns, 240 rows), and 640x480. The latter format (or something close to it), which is nearly the resolution obtained by standard TV, will probably become commercially available in the next few years.
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Detector arrays are more complicated to fabricate, since besides the detector
elements with the function of responding to radiation, electronic circuitry is needed to multiplex all the detector signals to one or a few output leads in a serial manner. The output from the array is either in analogue
or digital form.
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Our Proposal
IntravascularMicrobolometer
Catheter
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Another Plan
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Intravascular Microbolometer Catheter
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Guiding Catheter
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Cost ReductionThe price of Infrared Camera:
$40000-$60000IR Fibro Optic Bundles:
The price of fiber per meter:$100-$200The average number of fibers for each bundle:100The average length of each bundle:1.5 meter
$15000-$30000
Total cost saving$55000-$90000
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Comparison between Fibro Optic Camera & Microbolometer Catheter
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Cost-Effectiveness of different Options1. Plaque Imaging Model(10 mm option)Cost: Area of each Pixel = 25 x 25 =625 µ²
Detecting Area= [10 x (2 x 3.14 x (½)] = 31.4 x 106 µ²
Interpixels Area = 10% of each pixel areaNet Detecting Area=28.26 x 106 µ²
The Number of Pixels = 45670 ~ 45500 spots(It is far more than enough because each fiber optic bundle
can visualize 100 spots of the plaque)Advantages: Instantaneous Viewing of the PlaqueDrawback: High Cost, Inflexibility (Minimal Bending Radius)
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Cost-Effectiveness of Different options2. Linear Imaging Model(1 mm)Cost:Area of each Pixel = 25 x 25 =625 µ²
Detecting Area = [1 x (2 x 3.14 x (½)] = 3.14 x 106 µ²
Interpixels Area = 10% of each pixel areaNet Detecting Area = 2.826 x 106 µ²
The Number of Pixels =4560 ~ 4500
Advantages:Low Cost, FlexibleDrawback:Needs software for the image
reconstruction
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Help YourselfSee your pocket,play with resolution
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Planck, Max: Portrait 1947
