Physics Lecture Terminology PDF

Terminology Assistant Professor Dr. Salim Attia Objective learnings:  Medical Physics terms  Physical Therapy  Health Physics  Modeling  Homeostates  Measurement  Measurements of repetitive processes  Measurements of nonrepetitive processes  Accuracy & Precision  False...

Terminology Assistant Professor Dr. Salim Attia Objective learnings:  Medical Physics terms  Physical Therapy  Health Physics  Modeling  Homeostates  Measurement  Measurements of repetitive processes  Measurements of nonrepetitive processes  Accuracy & Precision  False Positive and False Negative -1-  Nis Medical Physics & I I What about the forces of the body during various activities, or how much useful work can be done by the body, or the relationship of an electrocardiogram to the heart's electrical activity, or how a medical x-ray works, or how much radiation you receive from an x-ray? These questions and many others like them involve applications of physics to medicine and are answered in this subject. While the roles of chemistry and biology in medicine are well accepted, the role of physics is usually not as obvious. Even though all medical and paramedical students take the principles of physics, they often see little or no relationship between physics and medicine. This communication gap is primarily because their studies did not include sufficient coverage of physics applied to medicine. The term medical physics refers to two major areas: - 1. The applications of physics to the function of the human body in health and disease. O 1909 2. The applications of physics in the practice of medicine. · : joi The first of these could be called the physics of physiology or the physics of the various organ systems such as the eyes, ears, lungs, and the heart and circulatory system.; the second includes such things as the physics of the stethoscope, the tapping of the chest (percussion), and the medical applications of lasers, ultrasound, radiation, mechanics, heat, light, sound, electricity, and magnetism to medicine. -2-  os apes - S bis ↑ ↑ -i ·? The word physical appears in a number of medical contexts. Only a generation ago in England a professor of physic was actually a professor of medicine. The words physicist and physician have a common root in the Greek word Physike (science of nature). Today the first thing a physician does after taking a medical history of a patient is to give him a physical examination. During this examination he uses the stethoscope, taps the chest, measures the pulse rate, and in other ways applies physics. & The branch of medicine referred to as physical medicine deals with the &=5-41 2.&1 Y diagnosis and treatment of disease and injury by means of physical agents such as manipulation, massage, exercise, heat, and water. 1/274141b552 Physical therapy is the treatment of disease or bodily weakness by physical - means such as massage and gymnastics rather than by drugs. - -3-  branch of Knowledge I I Biophysics includes medical physics as a narrower sub-discipline. In fact, s 880- biophysics is a relatively broad specific field that is not limited to medicine. It -- is mainly involved in the physics of various organisms, including microorganisms such as viruses, etc., although it approaches and overlaps with medical physics in many areas such as the transport of substances across cell membranes. The field of medical physics has several subdivisions: - &wayI swi 1. Most medical physicists work in the field of radiological physics. This % involves the applications of physics to radiological problems and includes - - = & the use of radiation in the diagnosis and treatment of disease as well as the - - - - use of radionuclides in medicine (nuclear medicine). · 2. Another major subdivision of medical physics involves radiation protection of patients, workers, and the general public. In the United States - this field is often called health physics. Health physics also includes radiation - - protection outside of the hospital such as around nuclear power plants and in industry. 3. Very often an applied field of physics is called engineering. Thus, medical z - physics could be called medical engineering. : 4. In some areas, such as the applications of ultrasound in medicine and the use of computers in medicine, you are likely to find medical physicists and medical engineers in nearly equal numbers. (The word medical is sometimes replaced with the word clinical if the job is closely connected with patient problems in hospitals, i.e., clinical engineering or clinical - physics). - boy4 - - -i-2991s -4-  ex- $1 dis ~ & i ly  Modeling -4 I Even though physicists believe that the physical world obeys the laws of physics, they are also aware that the mathematical descriptions of some physical situations are too complex to permit solutions. If you tore a small corner off this page and let it fall to the floor, it would go through various gyrations. Its path would be determined by the laws of physics, but it would be almost impossible to write the equation describing this path. Physicists would agree that the force of gravity would cause it to go in the general direction of the floor if some other force did not interfere. Air currents and static electricity would affect its path. st In trying to understand the physical aspects of the body, we often resort to analogies. Physicists often teach and think by analogy. Keep in mind that - J analogies are never perfect. For example: eye is analogous to a[ In many ways theE camera; however, the analogy is St s % +j5s t poor when the film, which must be developed and replaced, is compared to the. I t - retina, light detector of the eye. So analogies are often used to help explain j5- some aspect of the physics of the body. All explanations are incomplete to i · some degree although the success of this method. The real situation is always more complex than the one we describe. Some models involve physical phenomena that appear to be completely unrelated to the subject being studied; For example: A model in which the flow of blood is represented by the flow of electricity - is often used in the study of the body's circulatory system. This electrical - - -5-  model can simulate very well many phenomena of the cardiovascular system. Of course, if you do not understand electrical phenomena the model does not help much. Also, as mentioned before, all analogies have their limitations. [ Blood is made up of red blood cells and plasma, and the percentage of the blood occupied by the red blood cells (the hematocrit) changes as the blood - flows toward the extremities. This phenomenon is difficult to simulate with the - electrical model. Other models are mathematical; equations are mathematical models that can be used to describe and predict the physical behavior of some systems. In the everyday world of physics, we have many such equations. While the laws of physics are involved in all aspects of body function, each situation is so complex that it is almost impossible to predict the exact behavior from our knowledge of physics. Nevertheless, a knowledge of the laws of physics will help our understanding of physiology in health and disease. Some are of such general use that they are referred to as laws. For example, the relationship between force F, mass m, and acceleration a, usually written as F = ma, is known as Newton's second law. There are other mathematical expressions of this law that may look quite different to a lay person but are recognized by a physicist as other ways of saying the same thing. Newton's second law can be expressed in the form F = Δmv/Δt where v is the velocity, t is the time, and Δ indicates a small change of the quantity. The quantity mv is the momentum, and the part of the equation Δ/Δt means rate of change (of momentum) with time. Many functions of the body are controlled by homeostasis, which is &analogous to feedback control in engineering. An engineer who wants to -6- control some quantity that changes with time, will take a sample of what is being produced and use this sample as a signal to control the production to some desired level. So the production will increase or decrease according to the level of this sample. The process is described as negative feedback that it produces a stable control. Negative feedback control is common in the body. For example, one important function of the body is to control the level of the calcium in the blood. If the level drops too low, the body releases some calcium from the bones to increase the level in the blood. If too much calcium is released, the body lowers the level in the blood by removing some via the kidneys. While many of the control mechanisms of the body are not yet understood, various diseases have been found to be directly related to the failure of these mechanisms. For example, as the body grows, its cells keep increasing in number until it reaches adult size, and then the body remains almost constant in size under some type of feedback control. Occasionally some cells do not respond to this control and become tumors. -7-  s error # S ↳ B S is Is. 9 s Measurement ·! One of the main characteristics of science is its ability to reproducibly measure quantities of interest. The growth of science is closely related to the growth of the ability to measure. In the practice of medicine, early efforts to measure quantities of clinical interest were often scorned as detracting from the skill of the physician. Even though body temperature and pulse rate could be measured during the seventeenth century, these measurements were not routinely made until the nineteenth century. In this century there has been a steady growth of science in medicine as the number and accuracy of quantitative measurements used in clinical practice have increased. The following figure illustrates a few of the common measurements used in the practice of medicine. Some of these measurements are more reproducible than others. -8-  For Example: - An x-ray gives only qualitative information about the inside of the body; a repeat x-ray taken with a different machine may look quite different to the ordinary observer. There are many other physical measurements involving the body and time. We can divide them into two groups: - 1. Measurements of repetitive processes, such as the pulse. 2. Measurements of nonrepetitive processes, such as how long it & Ss takes the kidneys to remove a foreign substance from the blood. ↓ Measurements of the repetitive processes usually involve the number of repetitions per second, minute, hour, and so forth. - - 5 -9- For Example: - The pulse rate is about 70/min The breathing rate is about 15/min. Nonrepetitive time processes in the body range from the action potential of a nerve cell (1msec) to the lifespan of an individual. In science accuracy and precision have different meanings: -  Accuracy - Refers to how close a given measurement is to an accepted standard. · So ja set For Example: - s) - # A person's height measured as 1.765m may be accurate to 0.003m (3mm) compared to the standard meter.  Precision Refers to the reproducibility of a measurement and is not necessarily related to the accuracy of the measurement. are For Example: - how the measured values close to each other An ill person measured her temperature ten times in a row and got the following values in degrees Celsius: 36.1, 36.0, 36.1, 36.2, 36.4, 36.0, 36.3, 36.3, 36.4, and 36.2. The precision was fairly good, with a variation of 0.2°C from the average value of 36.2°C. Fig Pressions distined i s it 158 -10- 9 It is an accepted fact in science that the process of measurement may significantly alter the quantity being measured. This is - especially true in medicine. change The process of measuring the blood pressure may introduce errors (uncertainties). Although the data are scarce, it is generally believed that when an attractive woman is performing the measure- ment, the blood pressure of a young man will increase. Similarly, a handsome man may affect the blood pressure measurement of a female patient. When the physician decides if the patient is ill or not? After he or she has reviewed a patient's: - 1. Medical history. 2. The findings of the physical examination. 3. The results of clinical laboratory measurements. It is not surprising that sometimes wrong decisions are made. These wrong decisions are of two types: - 1. False Positives. 2. False Negatives. A false positive error occurs when a patient is diagnosed to have a particular disease when he or she does not have it. -11-  A false negative error occurs when a patient is diagnosed to be free of a particular disease when he or she does have it. Note: - In some situations a diagnostic error can have a great impact on a patient's life. For Example: (ggdel 500 I A young woman was thought to have a rheumatic heart condition and spent several years in complete bed rest before it was discovered that a false positive diagnosis had been made-she really had arthritis. *-/185db In the early stages of many types of cancer it is easy to make a false negative diagnostic error because the tumor is small. Since the probability of cure depends on early detection of the cancer, a false negative diagnosis can greatly reduce the patient's chance of survival. Diagnostic errors (false positives and false negatives) can be reduced by: - 1. Research into the causes of misleading laboratory test values. 2. Development of new clinical tests and better instrumentation. -SN Errors or uncertainties from measurements can be reduced by: - 1. Using care in taking the measurement. 2. Repeating measurements. -12-  to be trusted -able iss rose 3. Using reliable instruments. 4. Properly calibrating the instruments. d)5 In summary: - - gS &i dish 1. All measurements are uncertain and inaccurate. - 14 s 2. With special effort we can reduce the error and the uncertainty. => = 3. In many cases there is no need to improve the measurement because the quantity being measured is variable. = -13-