Physics of Diagnostic x-ray PDF

Physics of Diagnostic x- ray CHAPTER 16 Physics of Diagnostic x- ray By; Dr.Khalid Ghanim Al ghabsha In the 1895 ,W.C Roentgen was studying cathode ray in his lab. The field of radiology has three major branches; 1.Diagnostic radiology. 2.Radi...

Physics of Diagnostic x- ray CHAPTER 16 Physics of Diagnostic x- ray By; Dr.Khalid Ghanim Al ghabsha In the 1895 ,W.C Roentgen was studying cathode ray in his lab. The field of radiology has three major branches; 1.Diagnostic radiology. 2.Radiation therapy. 3. Nuclear medicine. Production of x- ray Beams; A high speed electron convert some or all of its energy into an x- ray photon when it strikes an atom. The main components of a modern x-ray unite are; 1. A source of electrons a filament ,or cathode. 2. An evacuated space in which to speed up the electrons. 3. A high positive potential to accelerate the negative electrons. 4. A target or anode ,which the electrons strike to produce x- rays. 1 Physics of Diagnostic x- ray In modern x-ray tube the number of electrons accelerated toward the anode depends on the temperature of the filament ,and the maximum energy of the x-ray photons produced is determined by the accelerating voltage – kilovolt peak (K VP ). An X-ray tube operating at 80 K Vp will produce X- rays with a spectrum of energies up to a maximum of 80 K eV. The kilo volt peak used for an x-ray study depends on; 1. The thickness of the patient. 2. The type of study being done. X-ray studies of the breast (mammography) are usually done at 25 to 50 (K VP ) ,while some hospitals use up to 350 (K VP ) for chest x-rays. The intensity of the x-ray beam produced when the electrons strike the anode is highly dependent on the anode material. In general ,the higher the atomic number (Z) of the target ,the more efficiently x-rays are produced. The target material used should also have a high melting point. Nearly all x-ray tubes use tungsten targets. 2 Physics of Diagnostic x- ray The Z of tungsten is 74 and its melting point is about 3400 oC. Since; P=IV Where; P is the power at the target of an x-ray tube in (watt). I is in Amperes. V is in Volt. 3 Physics of Diagnostic x- ray 4 Physics of Diagnostic x- ray Line - Focus Principle; A technique is used for increasing the area on the target struck by electrons to avoid overheating without increasing the blurring of the x-ray image. The normal rotation rate of the anode is 3600 rpm and the heat is spread over a large area as the anode rotates. When a short exposure is used ,the anode does not always make a full rotation at 3600 rpm and thus its full heat capacity is not utilized. For this reason , special high speed anodes that operate at rates of up to 104 rpm where developed ,since any rotating object can cause vibration if it not carefully balanced. A resonance will occur at exactly the operating frequency of 104 rpm. Bremsstrahlung ; When one of the electrons get close enough to the nucleus of the target atom to be diverted from its path and emits an x-ray photon. It is also means braking radiation. 5 Physics of Diagnostic x- ray How x-rays are absorbed; The attenuation of an x-ray beam is its reduction due to the absorption and scattering of some of the photons out of the beam; I =IO e-µx IO is un attenuated beam intensity. I is Intensity after attenuation. e is equal to 2.718 µ is linear attenuation coefficient. x is thickness of the attenuator. 6 Physics of Diagnostic x- ray X-ray are not absorbed equally well by all materials. Heavy elements such as calcium are much better absorbers of x-ray than light elements such as carbon ,oxygen ,and hydrogen ,and as a result ,structures containing heavy elements ,like the bones ,stand out clearly. The soft tissues like fat, muscles ,and tumors all absorb about equally well and are thus difficult to distinguish from each other on an x-ray image. Half Value Layer (HVL) ; It is the thickness of a given material that will reduce the beam intensity by one half. 𝟎.𝟔𝟗𝟑 HVL = µ 7 Physics of Diagnostic x- ray The mass attenuation coefficient ( µm) is used to remove the effect of density when comparing attenuation in several materials. µ is the linear attenuation coefficient which is dependent on the energy of the x-ray photons; as the beam becomes harder, it decreases. Since the lead is used for shielding materials. µ µm = 𝝆 8 Physics of Diagnostic x- ray µ I = IO 𝒆 − (𝝆) 𝝆x I= IO e-µm ρ x Since; 𝐠𝐦 ρ= 𝐜𝐦𝟑 𝐠𝐦 ρx = 𝐜𝐦𝟐 𝐜𝐦𝟐 Mass attenuation coefficient µm = 𝐠𝐦 Linear attenuation coefficient µ = cm-1 X- RAY LOSS ENERGY ; X –ray lose energy in three ways; 1. Photoelectric Effect; All of the photon energy is given to the photoelectrons. 2.Compton Effect ; 9 Physics of Diagnostic x- ray Some energy is given to an electron and some goes into a scattered photon. 3. Pair production ; A high energy photon is converted into an electron and a positron (β+). The positron(β+) annihilates to form two photons of 511 KeV each that go in opposite direction. Since, a minimum of 1.02 MeV is necessary for pair production. 10 Physics of Diagnostic x- ray CONTRAST MEDIA ; Materials of high (Z) are often injected into different parts of the body by the radiologists. 11 Physics of Diagnostic x- ray 1. Compound containing iodine are often injected into the blood stream to show the arteries. 2. An oily mist containing iodine is sometimes sprayed into the lungs to make the airways visible. 3. Barium compounds are given by radiologists orally to see parts of the upper gastrointestinal tract (upper GI) and barium enemas to view the other end of the digestive system (low GI ). 4.It is possible to use air a contrast medium. 12 Physics of Diagnostic x- ray 13 Physics of Diagnostic x- ray The amount of scattered radiation at the film ; The amount of scattered radiation at the film depend on; 1. The energy of the x –rays. 2.The thickness of the tissue, That the x- ray beam passes through is the most important factor, so the thicker the tissue, making the greater of the scatter radiation. 3.The larger of the beam; When it is greater so, the scatter also greater , and thus one simple way of reducing radiation is by keeping the x-ray beam as small as possible. The most significant way of reducing the amount of scattered radiation striking the film is by using a grid consisting of a series of lead and plastic strips. The strips are aligned so the un scattered x-rays from the source will go through the plastic strips and strike the film, while most of the scattered radiation will strike the lead strips and be absorbed. Since the lead strips do not produce visible shadows on the image. This grid is called a focused grid. 14 Physics of Diagnostic x- ray It is necessary to increase the exposure ; 1. To obtain an optimum darkness (optical density) of the film. 2. A higher exposure must be given because the lead strips absorb some of the un scattered radiation. Intensifying screens ; Most x-ray image are made on a special film sandwiched tightly between two intensifying screens – cardboards covered with a thin coating of crystal ( Ca WO4 ) that absorb x-rays well, and give off visible or (UV) light (fluoresce) when struck by x- rays. The film is coated on both sides with a light- sensitive emulsion, and each side takes a" picture" of the light from the intensifying screen with which it is in contact. Intensifying screens are much more efficient for making x- ray image than film alone. The screens are mounted in a cassette with a compressible felt backing that holds the film and the screens in close contact. To get a better contact, vacuum cassettes have been developed. 15 Physics of Diagnostic x- ray 16 Physics of Diagnostic x- ray 17 Physics of Diagnostic x- ray 18