T04 The Exposure PDF
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This document provides information on X-ray exposure parameters and calculations, including factors that influence exposure, such as mA, time, kVp and distance (SID). It also discusses different aspects of X-ray imaging, including filtration, the anode heel effect, and compensating filters.
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Topic 4 THE EXPOSURE Topic 4 Objectives 1. state the factors that control or influence the radiation exposure 2. Explain the EIV and calculate the new mAs using the EIV 3. explain how and why various technical parameters affect exposure, attenuation, transmiss...
Topic 4 THE EXPOSURE Topic 4 Objectives 1. state the factors that control or influence the radiation exposure 2. Explain the EIV and calculate the new mAs using the EIV 3. explain how and why various technical parameters affect exposure, attenuation, transmission 4. Explain the mA-time relation, ISL, mAs-kVp relation, 15% rule and mAs distance relation 5. Explain the effect grids, collimation, casts compensating filters and patient body types affect the radiation exposure 6. Understand and state the effect of kVp and SID on exposure 7. Explain the effect that different pathologies have on Exposure and how to adjust the technical factors 8. state the purpose, composition and types of filtration used for X-ray tubes 9. define "Anode Heel Effect” and describe its influencing factors and applications 10. state the effect of processing and viewing conditions on exposure 11. how to adjust the mAs and/or kVp according to grid used Informative Videos Subject contrast https://www.youtube.com/watch?v=_EcKx0SY3cg Exposure & object & IR https://www.youtube.com/watch?v=zJ-zs5lpJY8&t=158s Exposure Indicator – Referred to as Exposure Deviation in GE x-ray machines https://www.youtube.com/watch?v=zY58Ls3iWqo Anode Heel Effect: https://www.youtube.com/watch?v=fREyzdwxCjs The Exposure In radiology, the exposure is defined as “the number of individual X-ray units (quantity or intensity of photons) that emerge from the X-ray tube and X-rays were invented on November 8, 1895, by the German professor subsequently reach the IR” Wilhelm Roentgen marking the beginning of medical imaging The exposure The exposure is directly proportional to mA, time, and kVp2, and inversely proportional to SID2 Factor used to control Exposure is the mAs mAs α Intensity of radiation (mR) α Exposure o mA α To filament α # of é toward the target o time α total # é toward target o mA x Time = mAs mAs doubles → intensity doubles → Exposure doubles Comparable Exposures mAs relation: mAsN = mAsO 100 mAsn = 100 mAso mA x time = mAN X secN = mAO X secO 100 mAn X 1 secn = 50 mAo X 2 seco 8 What affects the choice of mA and time? The need to ↓ exposure time whenever patient motion is a problem The kVp needed, which depends on: o penetrating power o image contrast o ALADA - high kVp technique SID & SOD Equipment condition 9 SID & kVp have a direct effect on exposure, but they are not used to control it SOD SID Source to Image Distance (SID) Source to Object Distance (SOD) OID Object to Image Distance (OID) 11 Source to Image Distance (SID) SID has a direct effect on exposure, but it is not a controlling factor, because it also affects magnification, & skin dose 12 The inverse Square Law The intensity of the beam (exposure) is inversely proportional to the square of the Distance (SID) 13 The Inverse Square Law Dispersion or Divergence of the x-ray beam X-rays disperse or diverge from the target Total # of photons in the beam is constant at all distances Total # of photons/mm2 α 1/distance 14 Clinical Application of the ISL Radiation Protection in mobile radiography and radioscopy 15 ISL Problems The radiation exposure-rate measured with a dosimeter at 40’’ from the tube, is 2 R/min. What would be the exposure rate at 50’’ from the source? IO = 2 R/min. IN = (Do2 / Dn2) X Io DO = 40’’ IN = (40”)2 / (50”)2 X 2 R/min. DN = 50’’ IN = (1600” / 2500”) X 2 R/min. IN = ? IN = 1.2 R/min 16 A radioscopic X-ray tube operating at 3 mA and 120 kVp emits a radiation output of 0.36 mR/sec. at 40" from the focal spot. Calculate the radiation output at 20" from the source. IO = 0.36 mR/sec. IN = (DO2 / DN2) X IO DO = 40’’ IN = (40”)2 / (20”)2 X 0.36 mR/sec. DN = 20’’ IN = (1600” / 400”) X 0.36 mR/sec. IN = ? IN = 1.44 mR/sec 17 A patient undergoing radiation therapy receives a skin dose of 5 Rads/minute when the source to skin distance is set at 100 cm. Calculate the skin dose if the radiation source is placed at 150 cm. Solution: Io = 5 Rads/min. In = (Do2 / Dn2) X Io Do = 100 cm In = (100 cm)2 / (150 cm)2 X 5 Rads/min. Dn = 150 cm In = (10,0000 cm2 / 22,500 cm2) X 5 Rads/min. In = ? In = 2.22 Rads/min. mAs-Distance Relation (or Square-Law) The mAs-Distance Relation corrects the Exposure when SID varies A small ↑ is distance results in a very large ↓ in exposure If SID ↑, mAs must be increased to maintain the same exposure to the IR If DN = 2 DO, then mAsN = 4 X mAsO If DN = ½ DO, then mAsN = ¼ X mAsO If DN = 3 DO, then mAsN = 9 X mAsO If DN = ⅓ DO, then mAsN = ⅑ X mAsO 19 mAs- Distance Problems In a given radiographic room, an upright CXR is taken at 10 mAs, 120 kVp at 72" SID on an average size patient. The following day, the same patient comes on a stretcher for another CXR, but this time the patient cannot sit or stand. What mAs should be used to maintain the same exposure as the previous X- ray, if the maximum SID that can be used under the new condition is 40"? mAsO = 10 mAs mAsN = (DN2 / DO2) X mAsO DO = 72’’ mAsN = (40”)2 / (72”)2 X 10 mAs DN = 40’’ mAsN = (1600” / 5184”) X 10 mAs mAsN = ? mAsN = 3.09 mAs If the mAs is 100 at 40" SID, then what is the new mAs at a new SID? mAsN = (DN2 / DO2) X mAsO = (202 / 402) X 100 mAs = 25 mAs mAsN = (DN2 / DO2) X mAsO = (302 / 402) X 100 mAs = 56.25 mAs mAsN = (DN2 / DO2) X mAsO = (502 / 402) X 100 mAs = 156.25 mAs mAsN = (DN2 / DO2) X mAsO = (602 / 402) X 100 mAs = 225 mAs mAsN = (DN2 / DO2) X mAsO = (702 / 402) X 100 mAs = 306.25 mAs mAsN = (DN2 / DO2) X mAsO = (802 / 402) X 100 mAs = 400 mAs 21 A portable X-ray is taken at 50 mAs, 80 kVp at 60" SID. What mAs should be used if the SID increases by a factor of 1.3? Solution: DN = 60” X 1.3 = 78” mAsO = 50 mAs mAsN = (Dn2 / Do2) X mAsO DO = 60’’ mAsN = (78”)2 / (60”)2 X 50 mAs DN = 60” X 1.3 mAsN = (6084/3600) X 50 mAs mAsN = ? mAsN = 84.5 mAs In practice the SID may be changed due to: Magnification skin dose (↑ SSD) Equipment limitations Room limitation (mobile) 23 Exposure-kVp Relation: “The change in the intensity of exposure (mR) is directly proportional to the square of the new kVp”. IN (mR) kVpN2 ------------- = --------- IO (mR) kVpO2 24 kVp and Exposure kVp has the greatest effect on Exposure because the energy of the beam increases causing a greater transmission of the primary beam & forward scatter 25 kVp 15% Rule Applicable to mid-range kVp only: 65-90 kVp 15% ↑ in kVp results in 2X exposure (mR) & vice-versa 0.85 X kVp + 2 X mAs = same exposure 1.15 X kVp + 0.5 X mAs = same exposure Quick calculation: 15% = 10% + 5% (half of 10%) 26 15% Rule Problems An X-ray of the shoulder is taken at 20 mAs, 70 kVp and 48" SID. What kVp would give a comparable exposure if the mAs is doubled? mAsO = 20 mAs mAsN = mAsO X 2 mAsN = 40 mAs mAsN = 20 mAsO X 2 = 40 mAs kVpO = 70 kVp kVpN = 70 kVp X 0.85 kVpN = ? kVpN = 59.5 kVp 27 What kVp would give a comparable exposure, if the mAs in previous problem is reduced by half? Solution: mAsN = mAso / 2 mAsO = 20 mAs mAsN = 20 mAso / 2 = 10 mAs mAsN = 10 mAs kVpO = 70 kVp kVpN = 70 kVp X 1.15 kVpN = ? kVpN = 80.5 kVp 28 What kVp will produce a comparable exposure to the one produced at 40 mAs, 90 kVp, if the new mAs is: a) reduced to 10 mAs? b) increased to 80 mAs? mAsO = 40 mAs mAsO = 40 mAs mAsN = 10 mAs mAsN = 80 mAs kVpO = 90 kVp kVpO = 90 kVp kVpN = ? kVpN = ? kVpN = 90 kVp X 1.15 X 1.15 kVpN = 90 kVp X 0.85 kVpN = 119 kVp kVpN = 76.5 kVp 29 kVp controls IC, penetrating power & patient dose Choice of optimal kVp is determined by the: o minimum penetrating power required o desired IC, which depends on SC: ✓if SC is high (chest area), an ↑ in kVp will be needed ✓if SC is low (mammo.), a ↓in kVp will be needed o exposure time needed o Need to respect the ALADA principle – high kVp & low mAs technique decreases patient dose o department’s preference 30 kVp controls IC HIGHER KVP WILL INCREASE FORWARD SCATTER EVEN THOUGH THERE WILL BE LESS SCATTER PRODUCED MORE SCATTER WILL BE ABLE TO EXIT THE BODY OF THE PATIENT IN THE FORWARD DIRECTION RATHER THAN BE ABSORBED BY THE SURROUNDING TISSUE LOWERING IMAGE CONTRAST 31 A HIGH kVp & LOW mAs TECHNIQUE COMBINATION LOWERS PATIENT DOSE Technical Factors used for exposure A: 20mAs, 70kVp, 100” SID Technical Factors used for exposure B: 10mAs, 80kVp, 100” SID → PD ↓ 32 Internal Factors Affecting Exposure Composition of the Anatomical Part: ❖ Exposure increases if: Atomic number (Z) ↑ Tissue Density (ρ) ↑ : (or mass density) o X-ray absorption by the object α tissue density o Examples: ❑ Chest XR taken on inspiration: ρ ↓, absorption ↓, therefore mAs should be ↓ ❑ Chest XR taken on expiration: ρ ↑, absorption ↑ therefore mAs should be↑ 33 Internal Factors Affecting Exposure Part Thickness & Compression: If thickness ↑, absorption ↑ and Exp. ↓; mAs must be ↑ to maintain Exp. When compression is applied less exposure is needed because there is a ↓ of tissue volume and thickness by pushing tissues to the sides Some applications of compression include: Gastrointestinal, Urography, and Mammography 35 Internal Factors Affecting Exposure Body Habitus & Exposure: Skull & Exposure: o Brachycephalic o Mesocephalic B M D o Dolichocephalic Subject types & Exposure: o Hypersthenic o Sthenic o Hyposthenic o Asthenic Age / Sex & Exposure: o Tissue density α 1/age o Tissue density: M > F 36 Internal Factors Affecting Exposure AP Supine AP Upright Radiographic Projections/Positioning: Different positions → different superimposition o Abdomen supine vs. upright vs. prone vs. decubitus AP RLD o AP spine vs. oblique vs. lateral o AP ribs Vs. oblique ribs 37 Abdomen supine vs. upright vs. prone vs. decubitus A B C 38 Lumbo-Sacral Spine Projections AP Lateral Oblique 39 Internal Factors Affecting Exposure Respiration Phases: Inspiration: o Chest: tissue density & thickness ↓, must ↓ mAs oAbdomen: tissue thickness ↑ (compression on abdomen from diaphragm), must ↑mAs Expiration: o The reverse effect will occur in chest and abdomen 40 Internal Factors Affecting Exposure Pathological Variations: A pathology can cause a need to increase or decrease the exposure Obstructive fluid diseases: o These diseases may ↑ or ↓ tissue density depending Feces on the nature of the obstruction & location of the fluid o Examples: ▪ Bowel obstruction (feces): ↑ tissue density, requires a ↑ Exp. ∴ must ↑ mAs ▪ Bowel obstruction (air): ↓ tissue density, requires a ↓ Exp. ∴ must ↓ mAs Gas 41 Internal Factors Affecting Exposure Additive (or constructive) disease processes: These are diseases that ↑ tissue density, so, absorption ↑, must ↑ mAs Ascites Pneumonia Pleural Effusion 42 Internal Factors Affecting Exposure Additive (or constructive) disease processes Paget's disease: localized disorder of bone remodeling that typically begins with excessive bone density losss followed by an increase in bone formation Osteoclastic Stage - early stage Osteoblastic Stage - late stage 43 Internal Factors Affecting Exposure Additive (or constructive) disease processes Bone sclerosis is a condition in which bones thicken, harden or increase in tissue density. It may be caused by injuries that compress bone such as osteoarthritis and osteoma. Internal Factors Affecting Exposure Additive (or constructive) disease processes Osteopetrosis: extremely rare inherited disorder whereby the bones harden, becoming denser Internal Factors Affecting Exposure Additive (or constructive) disease processes Pneumonia: consolidation of lung tissue due to infection that inflaming the air sacs in one or both lungs. The air sacs may fill with fluid or pus (purulent material) 46 Internal Factors Affecting Exposure Additive (or constructive) disease processes Ascites is accumulation of fluid in the abdominal cavity. Common causes are liver disease or cirrhosis, cancers, and heart failure. 47 Internal Factors Affecting Exposure Additive (or constructive) disease processes Pleural Effusion excess fluid that accumulates in the pleural cavity, the fluid-filled space that surrounds the lungs 48 Internal Factors Affecting Exposure Destructive disease processes Normal Pneumothorax Destructive disease processes These are diseases which ↓ tissue density (radiolucent) So, absorption ↓ and exposure ↑; must ↓ mAs. Examples: Normal Osteoporosis Emphysema Osteomyelitis o Osteoporosis o Emphysema o Osteomyelitis o Pneumothorax 49 Internal Factors Affecting Exposure Destructive disease processes Osteoporosis 50 Internal Factors Affecting Exposure Destructive disease processes Pneumothorax 51 52 Effect of Positive Contrast Media on exposure Single Contrast Barium Enema When positive CM (↑Z) is used, an ↑ in exposure is needed Z of Barium (56) & Iodine (53) A small change in Z results in a very large change in absorption 53 In Coronary Arteriograms Iodine solution is used as CM 54 Negative Contrast Media Z of CM is very low (Air, CO2.) when negative CM ( low Z) is used, x-ray transmission of the primary beam increases, therefore a ↓ in exposure is needed Pneumoencephalogram 55 Positive & Negative CM Combination Single Contrast (positive contrast only) Double Contrast (positive and negative) 56 Splints, Casts & Bandages Casts: ↑ thickness and ↑ tissue density, must ↑ exposure Page 152 – Merrill’s 57 TRAUMA BOARD IMAGE ANALYSIS TEXTBOOK Splints and Bandages: the exposure is adjusted depending on the projection and type of material used 59 The Anode Heel Effect A variation in X-ray intensity along the long axis of the tube Anode side: ↓ radiation intensity Cathode side: ↑ radiation intensity 60 Anode Heel Effect AHE is more apparent with: larger field sizes Image IR SID SID Receptor shorter SID ↓ angle of the anode The AHE is negligible when o small IR 1 2 3 4 5 o SID ↑ o large angle of anode 61 Applications of AHE Place the thicker or denser anatomical part on the cathode side of the tube. This will help to provide a more uniform density across the image receptor. Alternatively, place the long axis of the tube perpendicular to the long axis of the part + - + - Anode Cathode + - Anode Cathode Anode Cathode 62 63 Filtration Filtration eliminates soft or long wavelengths from the x-ray beam It increases the average wavelength (↑ E) of the X-ray beam Added filtration requires an increase in mAs Filtration is used to Reduce the patient’s skin dose Improve contrast resolution 64 Filtration No added Filtration Added Filtration Imaging of the Larynx often requires added Filtration to improve IC of the soft tissue Effect of filtration on mAs (exposure) Increasing filtration requires an increase in mAs and the amount of mAs added will depend on the kVp selected because: at ↑ kVp: filtration has a very small effect on Exposure because the beam contains fewer lower energy x-ray photons to be absorbed, so only a small ↑ in mAs is required at ↓ kVp: filtration has a very large effect on Exposure because the beam contains more long low energy x-ray photons that will be absorbed by the added filtration, so a large ↑ in mAs is required 66 Compensating filters to provide a uniform exposure across the image when body parts vary widely in tissue thickness and/or tissue density Types: Boomerang spine, shoulder, toes, knees, facial bones Wedge filters (gentle slope, prism, ingot) hips, spines Trough filter chest Electronic filters (software) 67 COMPENSATING FILTERS 69 SCATTERED RADIATION & EXPOSURE REACHING THE IR SCATTERED RADIATION How does Scattered Radiation affect the exposure? o Some scatter produced will reach the Radiological Image therefore the exposure captured by the IR is made of Primary beam and Scatter SCATTERED PRIMARY BEAM RADIATION REACHING IR PRIMARY SCATTERED BEAM RADIATION REACHING IR SCATTERED RADIATION scatter produced is proportional to ρ Scatter Reaching IR α Scatter produced α Soft Tissue Density Scatter Reaching IR α Scatter produced α 1/Bone Tissue Density 73 Factors that ↑ scatter reaching the IR ↑ Part Thickness (↑ volume): o If mAs is ↑ because the part is thicker then ▪ Scatter produced will ↑ ▪ % of scatter reaching IR will also ↑ ↑ Field Size (↑ volume): o For thick parts: ▪ Scatter produced will ↑ ▪ % of scatter reaching IR will also ↑ o For thin parts: ▪ scatter produced & scatter to IR is negligible ↑ kVp o Less Produced scatter but its Energy is higher o Higher E scatter increases the % of forward scatter, thus increasing the % of scatter reaching the IR 74 Field size ↑ scatter produced & reaching the IR 75 Body part thickness ↑ scatter produced & reaching the IR 76 Body part thickness fields size↑ scatter produced & reaching the IR mAs needed for close collimation There is no need to adjust exposure is higher than the mAs mAs between the open and needed for the open collimation close collimation exposure 77 SCATTERED RADIATION & kVp At high kVp, the amount of scatter produced is less, but a higher % of the scatter produced is in the forward direction and with higher energy, thus increasing the number of scatter photons reaching the IR The energy of the scatter photons produced is proportional to the kVp Angle of scatter formed with Central Ray α 1/kVp o The higher the kVp, the smaller the angle of deviation of the scatter photon 78 Collimation is an effective way to reduce scatter 79 Collimation is an effective way to reduce scatter Effectiveness of collimation in reducing scatter: o Reduction in scatter reaching the IR: ▪ Collimation → from 16” X 16” size to 8” X 10” size: On small parts ( PA wrist): → effect is nil On medium parts (AP abdomen): → effect is important On large parts (Lat. Lumbar spine): → ENORMOUSLY IMPORTANT % of Scatter reaching IR α Volume of tissue (Field Size & Thickness) o Increase in Exposure when area of exposure is reduced (close collimation): ▪ Collimation → from 16” X 16” size to 8” X 10” size: On small parts (e.g., PA wrist): → nil On medium parts (e.g., AP abdomen): → important (≈ 1.5-2X) On large parts (e.g., Lat. Lumbar spine): → enormously important (≈ 2-3X) 80 Take away Scatter α ρ Scatter when volume of the tissue irradiated is Scatter α 1/Z Scatter when Z of the tissue irradiated is 81 Grids and Scatter radiation Function of a grid: o To absorb scatter in the Radiological Image o Grid absorbs more scatter than it absorbs primary beam photons o It increases the ratio of the primary beam to scatter reaching IR o The grid ratio (r) is the height of the Pb strip over the distance between two strips Scatter reaching the IR is inversely proportional to the Grid Ratio Grid ratio (r) = h / D 82 Grids and Scatter 83 Exposure (mAs) modification when Grid is used Grid Conversion Factors (mAs-Grid Ratio Relation): o Since grids absorb scatter, and scatter contributes to the Exposure reaching the IR, the primary beam must be ↑ when a grid is used o The ↑ in exposure can be done by ↑ kVp or ↑ mAs o Grid ratio (r) & grid composition affect the amount of scatter reaching the IR and therefore the overall exposure reaching the IR 70 kVp, 10 mAs, 100 SID, no grid for shoulder AP – thickness 8 cm 70 kVp, 20 mAs, 100 SID, 5:1 grid for shoulder AP – thickness 12 cm 90 kVp, 40 mAs, 100 SID, 16:1 grid for shoulder AP – thickness 20 cm 84 Grid Conversion Factor Problems : A shoulder x-ray is taken at 70 kVp, 8 mAs with non-grid. Since it lacks contrast, it is repeated with an 8:1 grid at the same kVp. What mAs should be used if you keep the kVp constant? Solution: mAsn is equal to the mAso multiplied by the the grid conversion factor Solution: mAsO = 8 mAs mAsN 4 mAsN = mAsO X 4 rO = no grid --------- = ------- mAsN = 8 mAs X 4 rN = 8:1 mAsO 1 mAsN = 32 mAs mAsN = ? 85 A abdomen x-ray is taken at 70 kVp, 40 mAs with a 12:1 grid. The next day it is repeated with a 5:1 grid. What mAs should you use if the kVp remains the same? Solution: mAsO = 40 mAs mAsN 2 rO = 12:1 --------- = ------- mAsO 5 rN = 5:1 mAsN = ? mAsN = mAsO X (2 / 5) mAsN = 40 mAs X (0.4) mAsN = 16 mAs 86