Medical Physics - Heat and Cold in Medicine PDF

Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 Heat and Cold in Medicine o Introduction The matter is composed of molecules that are in motion, in gas or liquid the molecules move about hitting one another. In fact, that the molecules move me...

Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 Heat and Cold in Medicine o Introduction The matter is composed of molecules that are in motion, in gas or liquid the molecules move about hitting one another. In fact, that the molecules move means that they have kinetic energy is related to temperature. To increase the temperature of a gas it is necessary to increase the average kinetic energy of its molecules. This can be done by putting the gas in contact with a flame. The energy transferred from the flame to the gas causing the temperature rise is called heat. The reverse, heat can be removed from substance to lower the temperature are referred to as the cryogenic region. Heat: Is the energy transferred to the molecules causing the temperature rise. Solid liquid heat gas heat ions heat The ultimate in cold is "absolute zero"(-273.5oC) a temperature that is experimentally unattainable. o Temperature scales 1. Fahrenheit ( 𝒐𝑭) scale: According to this scale water freezes at 32 "𝐹 and boil at 212 "𝐹 and normal body temperature is about 98.6 "𝐹 . 𝑇# = 9 𝑇 + 32 5 $ 1 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 2. Celsius ( 𝒐𝑪) scale: Formally called centigrade scale and it is the most common scale. According to this scale water freezes at 0 "𝐶 and boil at 100 "𝐶 and the normal body temperature is about 37 "𝐶 . 3. Kelvin ( 𝒐𝑲) or absolute scale: It is important scale used for scientific work. It has the same degree intervals as the Celsius scale. The 0 "𝐾 is called absolute zero equal to -273.15 "𝐶 . On the absolute scale water freezes at 273.15 "𝐾 and boil at 373.15 "𝐾 and normal body temperature is about 310 "𝐾 . This scale is not used in medicine. 𝑇% = 𝑇$ + 273.15 Example 1/ In a hot room, a person skin temperature is about (35o C),find his skin temperature on the kelvin and Fahrenheit scales? Sol/ T(K)=T(oC)+273.15 T(K) =35+273.15=308.15oK T(oF)=1.8T(oC)+32 T(oF)=1.8´35+32=95oF Example 2/ A pan of water is heated from(30o C)to the boiling point. What is the change in its temperature on the Kelvin and Fahrenheit scales? SOL/ T(K) =T(oC) +273.15=30+273.15=303.15 2 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 T(oF) =1.8T(oC)+32=1.8´30+32=86 T(K) =T(oC) +273.15=100 boiling +273.15=373.15 T(o F ) = 1.8T(o C ) + 32 = 1.8 ´100 boiling + 32 = 212 \ DoC=100-30=70o Do K = 373.15 - 303.15 = 70o DoF =212-86=126o F o Thermometry: Temperature is difficult to measure directly, so we usually measure it indirectly by measuring one of the physical prosperities that change with temperature. Then we relate the physical property to temperature by a suitable calibration. There are three main types of temperature measurement devices: 1. Glass fever thermometer containing mercury or alcohol. The principle of this thermometer is that the increase in temperature of different materials causes them to expand differently; when the temperature increase, alcohol or mercury expands more than the glass thus the level of liquid increase in the glass. Figure 1. the temperature scales and thermometry 3 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 • The change in temperature ( DT a DV ) change in volume mercury because : a. it uses The volume expansion(DV)with (DT) is very small (1cm3Hg) increase volume by (1.8 %) when the temperature changes from (0to100 o C) . b. It`sclearcolor . c. It has low adhesion force with the wall of glass . • The expansion of liquid in thermometer is not large. Therefore, the smaller the diameter of the capillary the greater sensitivity of the thermometer. Fever thermometer has diameter less than 0.1mm. • Two things increase the visibility of the capillary: 1) The glass case acts as a magnifying glass. 2) An opaque white backing is used. • The capillary of fever thermometer has a restriction just above the bulb so that after the liquid is forced in to the stem by expansion it does not return when the temperature falls. 2. Thermistor: The change in temperature (DTa DR) change in the electrical resistance. It is a special resistor that changes its resistance rapidly with temperature (about 5% /1 oC). Initially, the four resistors are equal. Where the bridge is balanced by symmetry, and the voltages at each end of the meter are equal, and there is no current flows through the meter. • A temperature change causes the thermistor resistance to change. The voltage at each end becomes unequal, causing current to flow and the meter deflection can be calibrated for temperature. 4 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 • Thermistors are used in medicine because of their sensitivity. It can measure temperature changes of 0.01 oC. • Because of its small mass, a thermistor has little effect on the temperature of the surrounding tissues and responds rapidly to temperature change. • Thermistors are placed in the nose to monitor the breathing rate of patient by showing the temperature change between inspired cool air and expired warm air. An instrument of this type called pneumograph. Figure 2. The resistance of a thermistor T can be measured with a simple bridge circuit to determine the temperature. 5 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 Figure 3. Thermistors Types 3. Thermocouple: It consists of two junctions of two different metals. If the two junctions are at different temperature, a voltage is produced that depends on the temperature difference. One of the junctions is kept at reference temperature such as in an ice-water bath and the other for temperature measurement. It measure temperatures from -190 to 300oC. It can be made small enough to measure the temperature of individual cells. voltage (e.m.f) a DT . Figure 4. Schematic diagram of a thermocouple 6 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 4. Infrared thermometer: Each body with a temperature above the absolute zero (– 273.15 °C = 0 Kelvin) emits an electromagnetic radiation from its surface, which is proportional to its intrinsic temperature. Infrared thermometers measure temperature by sensing the infrared energy. The most basic design of infrared thermometers consists of a lens to focus the infrared thermal radiation onto a detector, which converts the radiant energy into an electric signal. This configuration facilitates temperature measurement from a distance, without the need for contact with the object to be measured. Figure 5. Schematic diagram of infrared thermometer 7 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 o Applications of Heat in Medicine A. Thermography Measurements of body surface temperature indicate that the surface temperature varies from point to point depending upon external physical factors and internal metabolic and circulatory processes near the skin blood flow near the skin is the dominant factor, since the variations in these conditions, many researchers have attempted to accurately measure the surface temperature of the body and relate it to pathologic conditions. Figure 6. A thermogram of a hand Thermography is a simple routine method of obtaining a surface temperature map. Thermography is most commonly used to detect breast cancer by comparing the heat pattern of the two breasts. It was found that most breast cancer could be characterized by an elevated skin temperature in the region of cancer, the surface temperature above the tumor was typically about 1co higher than that above nearby normal tissue. Thermogram can detect small tumor (less than 1 cm 2 in diameter) in breast cancer. 8 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 Figure 7. Thermograms of a) normal breast, b) with breast cancer One very appealing method of obtaining a thermogram is to measure the radiation emitted from the body. In the infrared (IR) region, the basic equation describing the radiation emitted by a body was given by Max Plank for our purposes the Stefan-Boltzman law for total radiative power per surface area W is more useful, it is: 𝑾 = 𝒆 𝝈𝑻𝟒 Where: W: is total radiative power per surface area. T: is the absolute temperature. e: is the emissivity =1 for radiation from the body σ: is the Stefan Boltzman constant= 5.7x10-12 W/cm2.K4 9 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 Example 3: A. What is the power radiated per cm 2 from skin at temperature of 33o C? W = esT4 T = 33+273= 306K W =1´5.7´10-12 ´ (306)4 =0.05W cm2 B. What is the power radiated from a nude body (1.75´104 cm2 ) in area? 𝑊= ! " , P=W´A P= 0.057 w cm2 ´1.75´104 cm2 =875W C. If the radioactive power received from the surrounding walls (background) =735 watt, what is the net power? 875-735=140 watt Note: the commercial instrument used in clinical thermography can measure the temperature differences of DT = 0.2 oC and record a thermogram in 2 second. To get a good thermogram : Before thermograph : a. Clothes must be removed because clothing affects skin temperature . b. It`s necessary to keep the temperature of the thermograph room at 20 o C and cool uniformity to enhance the temperature difference and contrast thermograph image . 10 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 B. Heat Therapy Two primary therapeutic effects take place in a heated area: 1) There is an increase in metabolism resulting in vasodilation. 2) There is an increase in blood flow as blood moves to cool the heated area. The physical methods of producing heat in the body are: 1. Conductive heating: Heat transfers from the warmer object to the cooler by conduction. The total heat transferred will depend upon : a. The area of contact . b. The temperature difference . c. The time of contact (duration) . d. The conductivity of materials . Heat conduction by: hot bath, hot pack, electric heating pad, hot paraffin, etc. Conductive heating is used in treating conditions such as : 1. arthritis 2. neuritis 3. Sprains 4. Strains 5. Contusions 6. sinusitis 7. back pain. 2. Infrared (IR) radiant heating: It is also used for surface heating of the body. The used IR wavelength is 800-4000 nm. The waves penetrate the skin about 3mm and increase the surface temperature. Excessive exposure causes reddening(erythema) and sometimes swelling (edema). This type of heating is used to treat the same condition of conductive heating, IR is considered to be more effective than conductive heating because the heat penetrates deeper. 11 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 3. Radio wave heating (Diathermy): When alternating electric current passes through the body, tissue heating and electric shock take place. The amount of heat increases as the frequency of the current increases. Heat from diathermy penetrates deeper in the body than radiant and conductive heat. Short wave diathermy utilises electromagnetic waves in the radio range (wavelength=10m). It has been used in relieving muscle spasms, pain from protruded intervertebral discs, degenerative joint disease, and bursitis. Two different methods are used for transferring the electromagnetic energy in to the body : a. The first method: The part of the body to be treated is placed between two metal plate-like electrodes energized by the high-frequency voltage .The body tissue between the plates acts like an electrolytic solution. The charged particles are attracted to one plate and then the other depending upon the sign of the alternating voltage on the plates . b. The second method: It is a magnetic induction . In induction diathermy, either a coil is placed around the body region to treated or a (pancake) coil is placed near that part of the body.The alternating current in the coil results in an alternating magnetic field in the tissues . 4. Microwave Diathermy: It uses waves in the radar range (wavelength = 12cm). It is another form of electromagnetic energy . It is easier to apply than short-wave diathermy . The frequency closer to 900 MHz is effective in therapy, causing uniform heating around body regions. It is used in the treatment of fractures, sprains and strains, bursitis, injuries to tendons. 12 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 5. Ultrasonic wave heating (ultrasonic diathermy): Ultrasonic waves are also used for deep heating of body tissue. They produce mechanical motion like audible sound waves except the frequency is much higher (~1MHz) . US waves vibrate tissues producing heating . Ultrasonic heating has been useful in relieving the tightness and scarring that often occur in joint disease, aids joints that have limited motion . It is useful for deposition heat in bones because they absorb ultrasound energy more effectively than does soft tissue. • Cold in Medicine Cryogenics is the science and technology of producing and using very low temperature. The study of low temperature effects in biology and medicine is called cryobiology. Cryopreservation: - Low temperature has been used for long-term preservation of blood, sperm, bone marrow and tissues. - For medical uses the preservation at the temperature of liquid nitrogen (-196oC) is much better than at the temperature of solid carbon dioxide (-79oC). - Cells survival after freezing is more dependent upon the cooling rate during the freezing cycle than on the warming rate during the thawing cycle. - The survival of some cells can be helped by adding a protective agent before cooling. 13 Medical Physics Lecture. 4 Heat and Cold in Medicine. Dr. Sara Ahmed Medicine & Dental College Stage 1 Blood storage: 1) Non-cryogenic method: The conventional non-cryogenic method of blood storage involves mixing whole blood with an anticoagulant and storing it at 4oC. 2) Cryogenic method: Blood can be stored for a much longer time if it is frozen after mixing with cryoprotective agents which are either intracellular or extracellular. Two techniques are used for this method slow cooling technique and the other one is the rapid cooling technique. The skin, bone, muscle and organs are harder to preserve than blood because: 1. The large physical dimensions limit the cooling rate. 2. Adding and removing protective agents is difficult. Cryosurgery: Cryogenic methods are also used to destroy cells, this application is called cryosurgery. One of the first uses of cryosurgery was in the treatment of Parkinson's disease. Cryosurgery is also used in the treatment of tumors and warts. Cryosurgery has several advantages: 1. There is little bleeding in the destroyed area. 2. Destroyed tissue volume is controlled by temperature of cryosurgical probe. 3. There is little pain sensation because low temperatures desensitise the nerves. Cryosurgery is used in several types of eye surgery: 1.In retinal detachment, a cooled tip is applied to the outside of the eyeball in the vicinity of the detachment a reaction occurs that acts in weld the retina to the wall of the eyeball 2.Extraction of the lens, remove the darkened lens, in this procedure the cold probe is touched to the front of the lens, the probe sticks to the lens making the lens easy to remove. 14