Radiographic Principles 1 - Digital Technique & Image Acquisition PDF

Radiographic Principles 1 Digital Technique & Image Acquisition Julie Kloft, MSRS, RT (R) Clinical Coordinator & Professor of Radiography  With film-screen, kVp controlled image contrast.  With digital, you will use a kVp within the textbook kVp range and manipulate mAs to pro...

Radiographic Principles 1 Digital Technique & Image Acquisition Julie Kloft, MSRS, RT (R) Clinical Coordinator & Professor of Radiography  With film-screen, kVp controlled image contrast.  With digital, you will use a kVp within the textbook kVp range and manipulate mAs to provide sufficient radiation to create an image with the exposure/deviation index within range. The two most important characteristics of an imaging modality: 1. Spatial Resolution 2. Contrast Resolution  Spatial Resolution The ability to image small objects with high subject contrast. (In DR, limited by pixel size!) Most people can see objects as small as 200 um! Describing Spatial Resolution  In medical imaging, spatial resolution is described by the quantity “spatial frequency.”  The most common way of measuring spatial resolution is spatial frequency (defined as the number of details that fit in a space).  Spatial Frequency Measure of resolution, usually in line pair per millimeter (lp/mm).  Doing the math… The smallest object that can be imaged is inversely proportional to ½ the spatial frequency (the width of a line). 1 1 Minimum Object Size = 2 sf  Practice Math If the spatial frequency is 3 lp/mm, the smallest object you can image with that equipment is: 1 1 Minimum Object Size = 2 sf 1 1 Minimum Object Size = 2 3 Minimum Object Size =.5 x.3 (0.166 mm)  One TINY Problem In digital imaging, if the object is smaller than one pixel, it cannot be visualized. (more to come on this a little later)  Spatial Frequency Film-Screen Resolution is approximately 10 lp/mm. CR is limited to approx 2.5 to 5 lp/mm (less detail). However, dynamic range in digital imaging is much higher, so it is more difficult to discern. Film-screen imaging has the best spatial resolution of all imaging modalities, yet it is now obsolete.  Spatial Frequency As spatial frequency is larger, the objects it can image become smaller. Imaging System Spatial Resolution Gamma Camera 0.1 lp/mm MRI 1.5 lp/mm CT 1.5 lp/mm Ultrasound 2 lp/mm Fluoroscopy 3 lp/mm DR 4 lp/mm CR 6 lp/mm Film-Screen 8 lp/mm Mammography 15 lp/mm Higher spatial frequency = better spatial resolution.  Clarification Objects with HIGH spatial frequency are more difficult to image than those with LOW spatial frequency.  Small things are harder to image than large things!  Modulation Transfer Function The ability of a system to record available spatial frequencies. A system that produces an image identical to the object would have a MTF of 1 (non-existent). Film-Screen Mammo used a single screen Film-screen and smaller focal spot (0.1 mm). Film-screen Film-screen used two screens (more blur) and larger focal spots Large objects Small objects (0.5-2.0 mm). Spatial resolution is determined by system MTF.  Digital Imaging MTF Digital Radiography has a high MTF at low spatial frequencies AND has an abrupt cutoff due to pixel size.  One TINY Problem (part two) In digital imaging, if the object is smaller than one pixel, it cannot be visualized. Currently, 7 lp/mm (or 70 um) is the smallest available pixel size in DR, so that is the limiting factor. Focal Spot Size WAS our limiting factor (.5-1.5 mm).  Photographs to Compare To “see” how MTF impacts digital images, compare these two images. The one on the left represents film-screen mammography. The one on the right digital radiography. DR has reduced spatial resolution but improved contrast resolution. Contrast Resolution The ability to distinguish between and to image similar tissues. CR & DR have better contrast resolution than film-screen because of wider dynamic range.  Dynamic Range Range of values that can be displayed by an imaging system; shades of gray. Film-screen: CR & DR: 30 shades of 10,000+ gray. shades of Dynamic gray. range of 1000.  Optical Density  OD originates from film-screen.  A densitometer is used on film to measure the OD on film.  In theory, those numbers “could” range from 0-4. Zero (clear like glass) and 4 (black like blind) were both impossible. The practical application range was.25 to 2.5 OD.  Dynamic Range A 14-bit Dynamic Range DR system has 16,384 shades of gray. The human eye can only discern about 30 shades. Window/Leveling allows us to fully visualize each region. The exact number of shades of gray is determined by bit depth. 12-bit dynamic range: 2bit depth = 212 = 4096 14-bit dynamic range: 2bit depth = 214 = 16,384 16-bit dynamic range: 2bit depth = 216 = 65,536 Seeing Dynamic Range When comparing an AP knee on film-screen (left) to one taken with CR (right), more tissue densities are recorded on the digital image. This gives the appearance of more detail. This is actually wider dynamic range, not additional detail.  Signal-to-Noise Ratio The signal in an image is the image-forming x- rays representing anatomy. Noise includes scatter and anything that might limit contrast resolution. The highest SNR Increasing mAs (with the least increases SNR dose) is desired. and dose. Contrast resolution is limited by SNR.  Patient Dose With digital imaging, dose can be reduced by 20- 50%, depending 50 kVp, 0.2 mAs 50 kVp, 1.2 mAs 50 kVp, 6 mAs on the procedure. Sadly, the opposite has occurred with dose creep.  Image Receptor Response With digital imaging, image contrast does not change with dose. (Therefore, kVp is less important in digital imaging.) A very low Ideally, a technique in higher kVp, digital lower mAs imaging, technique may require should be used a repeat to reduce due to low dose. SNR. Curve/line represents contrast resolution, not spatial resolution.  Dose Reduction in DR o Digital images should NOT be repeated because of brightness/contrast concerns. o DR cannot compensate for excessive noise caused by quantum mottle (insufficient mAs). o Over-exposed images DO NOT need to be repeated, but corrective action should be taken for subsequent images.  Detective Quantum Efficiency The percentage of x-rays absorbed by the IR. The probability that an x-ray will interact with the IR is determined by the thickness of the capture layer and its composition. The high DQE in DR should result in a lower patient dose. Image Acquisition Principal Exposure Technique Factors 1. kVp 2. mA (tube current) 3. Exposure time (in seconds 4. SID  Exposure Factors Exposure factors influence and determine the intensity and energy of x-radiation the patient will be exposed. Radiation quantity = radiation intensity in mGya Radiation quality = beam energy and penetrability, measured by HVL  220 volts Kilovoltage peak (kVp): represents the energy and penetrating power of the x-ray beam Review Primary factor that controls beam energy and penetrability.  kVp Selection When selecting the kVp for an image, you are determining the energy of the photons produced.  A kVp in the low to moderate range, produces photons that are likely to be absorbed by the body.  A kVp in the high range, produces photons that possess a lot of energy, which may interact with tissue atoms and be scattered.  Low kVp Using low kVp reduces excessive scatter; however, the number of photons absorbed by the body increases. This increase in x-ray photon absorption is patient dose and the higher the absorption, the higher the patient dose. High kVp Using a kVp that is too high for the body part results in an unnecessarily high amount of scatter production. Excessive scatter fogs the image, reduces image contrast and resolution, and increases patient dose.  So… How do I decide!?!? Optimal penetration Desired scale of contrast High kVp, low mAs technique = low contrast image. Low kVp, high mAs technique = high contrast image.  Changing kVp Kilovoltage affects: X-ray beam quality and quantity X-ray beam penetration and absorption in anatomic tissues: Increasing kVp increases penetration and decreases absorption. Decreasing kVp decreases penetration and increases absorption.  Review  Primary factor that controls density. It is the measure of current flowing through the filament of the x-ray tube.  The mAs setting literally determines the number of electrons boiled off of the filament of the x-ray tube mAs and available for interaction with the anode target to produce x-rays. HOW does mAs control density?  The number of electrons boiled off of the filament (quantity) determined by the mAs selected.  The number of electrons that travel from cathode to anode are all that is available to produce x-rays.  Each electron will interact with atoms of the anode and create either heat (a waste of the energy) or x-rays.  mAs and Patient Dose Because mAs is the quantity of x-ray photons exposing the patient, this affects patient dose.  The higher the mAs, the greater the number of photons and the higher the patient dose. mAs does not influence other areas (like kVp). The proper mAs selection is based solely on the desired density.  If an image needs to be lighter or darker, an adjustment in mAs is required.  mAs mA x time = mAs 300 mA x.05 sec = 15 mAs 20 msec =.02 sec 600 mA x 20 msec = 12 mAs mA = mAs/time time = mAs/mA  mA Options In general, inexpensive radiographic imaging systems have a maximum tube current of 600 mA. (Interventional Radiography typically uses 1500 mA to reduce time.) Typically, the following mA stations are available: mA ms mAs 100 100 10 200 50 10 300 33 10 400 25 10 600 17 10 800 12 10 1000 10 10 Table 15-3 on Bushong pg 224 provides the common mA stations (above). In clinic, you will see some variations from this list--this is a guideline. mA Stations and Focal Spot Size The lowest mA station(s) will come with the option of a smaller focal spot size, but the higher mA stations will only offer the larger focal spot size. More info to follow on focal spot size a little later!  Exposure Time We use exposure time to our advantage to minimize or exaggerate motion.  Use a higher mA and a shorter time to minimize motion.  Use a lower mA and a longer time to exaggerate motion.  Time Generally, exposure times are kept as short as possible to minimize patient motion. Patient dose is NOT affected by the time selected.  The dose at 10 mAs is the same for the patient who receives 500 mA for.02 seconds, as it is for the patient who receives 25 mA for.4 seconds.  Units of Time Time can be expressed via fraction or decimal. (½ second is the same as.5 seconds and 500 milliseconds.)  Usually, older equipment uses fractions, and newer equipment uses milliseconds (ms).  Portable equipment usually does NOT allow you to select the time.  Ma/Time Relationship mA and Time are inversely proportional: as one increases the other decreases. Formula: mA1 = T2 mA2 T1 300 = T2 600.5 600 x T2 = 150 T2 = 150 ÷ 600 T2 =.25  Minimum Exposure Time Three-phase or high- frequency generators can typically provide an exposure time as short as 1 ms. Single-phase generators cannot produce an exposure time less than 8- 10 ms.  Millisecond (msec) The prefix “milli-” means it is 1000 times smaller, so… 50 msec = 50 or.05 sec 1000.35 sec =.35 x 1000 or 350 msec  Reciprocity Shorter time: less motion Longer time: breathing technique Focal Spot: detail 20 mAs = 100 mA x.2 sec 20 mAs = 400 mA x.05 sec 100 mAs = 400 mA x.25 sec 100 mAs = 25 mA x 4 sec Primary Exposure Factors Kilovoltage (kVp) Directly related to radiation quality and quantity. Directly related to radiographic density. Inversely related to radiographic contrast. Milliamperage (mA) Directly proportional to radiation quantity. Directly related to radiographic density. Inversely related to exposure time to maintain density. Exposure Time (time) Directly proportional to radiation quantity. Directly related to radiographic density. Inversely related to mA to maintain density. mA × time (seconds) = mAs  Image Receptor Exposure To increase exposure to image receptor: Increase mA, exposure time, or kVp. To decrease exposure to image receptor: Decrease mA, exposure time, or kVp. To maintain exposure to image receptor: Increase mA and proportionally decrease time. Increase time and proportionally decrease mA. Increase kVp 15% and decrease mAs ½. Decrease kVp 15% and increase mAs × 2.  Distance The distance between the radiation source and the image receptor (SID) affects the amount of radiation reaching the patient. The divergent beam causes the intensity to vary at different distances (Inverse Square Law).  Increasing the SID decreases beam intensity, so mAs must be increased to maintain exposure.  SID SID and radiation intensity are inversely related. Inverse square law: I1 = D22 I2 D12 Increasing SID decreases the intensity of radiation reaching the image receptor. Decreasing SID increases the intensity of radiation reaching the image receptor. SID affects size distortion (magnification) and recorded detail and spatial resolution. Increasing SID decreases magnification and increases recorded detail and spatial resolution. Decreasing SID increases magnification and decreases recorded detail and spatial resolution.  SID and Intensity Inverse square law: double the distance = ¼ the intensity  SID Different procedures require the use of different SIDs to balance magnification, image detail, and technique.  Most procedures use a 40 inch SID although some procedures (such as the chest and lateral cervical spine) use 72 inches.  SID and Distortion  A long SID creates less magnification than a short SID.  A short SID creates more magnification than a long SID. 40 inch SID vs. 72 inch SID  Direct Square Law (Density Maintenance Formula) Changing SID requires a change in mAs to maintain exposure to the image receptor. mAs2 = (SID2)2 mAs1 (SID1)2 or new mAs = old mAs x (new distance2 ÷ old distance2)  OID Increased Object to Image receptor Distance causes decreased intensity at the IR.  As the exit radiation continues to diverge, less overall intensity reaches the IR.  OID Distance between the anatomic part and image receptor affects: Radiation intensity reaching the image receptor Amount of scatter radiation reaching the image receptor Magnification Recorded detail and spatial resolution An air gap decreases the intensity of radiation and scatter reaching the image receptor.  OID (more vs. less) Increasing the OID decreases the exposure to the IR, increases contrast and magnification, and decreases recorded detail and spatial resolution. Decreasing the OID increases the exposure to the image receptor, decreases contrast and magnification, and increases recorded detail/spatial resolution. Compare no OID to 3” OID Less pronounced at 72” SID  Focal Spot Size Most x-ray tubes are equipped with two focal-spot sizes to choose from (small or large). Some common combinations for diagnostic tubes include: Small Large 0.5 mm 1.0 mm 0.6 mm 1.2 mm 1.0 mm 2.0 mm  Interventional Radiography typically uses 0.3 mm and 1.0 mm focal spots.  Mammography uses microfocus tubes with 0.1 mm and 0.3 mm focal spot sizes (better detail!).  Focal-Spot Size Again, the small focal spot size is limited to the lower mA station. Using too much mAs on a small focal spot will lead to burn out. The small focal spot should be reserved for fine-detail radiography when the quantity of x-rays is low. Small focal spots are always used for magnification radiography.  Filtration Remember, there are 3 types of filtration: 1. Inherent 2. Added 3. Compensating Required total filtration of 2.5 mm of Aluminum equivalent.  Generator Options There are three basic types of generators available: 1. Single Phase 2. Three Phase 3. High Frequency Three-phase power results in higher x-ray intensity and energy.  AEC vs. APR Automatic Anatomically Exposure Programmed Control Radiography Automates the exposure time factor, Essentially a computerized but the Radiographer technique chart, but can must determine all include pre-programmed other factors: mA, AEC factors. kVp, distance, and sensor(s). Automatic Exposure Techniques  Technologists are often able to select Automatic Exposure Control (AEC) to utilize computer-assisted technology.  Patient positioning MUST be accurate because the specific body part MUST be placed over the AEC detector (cell) to ensure proper exposure.  Automatic Exposure Systems Ionization Chambers are used to control the exposure time. Therefore, precise positioning is very important. Most equipment uses the three-cell set-up. Automatic Exposure Control RT will select the best cell(s) for the projection. RT will select optimal density for patient (size). Federal regulations require that AECs have a 600 mAs safety override.  Automatic Exposure Systems Collimation Concerns Primary beam collimation near ionizing chamber locations will create an overexposed image. Under-collimating can create problems because the primary beam will produce increased scatter which may cause the AEC to terminate the exposure too early, causing an underexposed image.  Automatic Exposure Systems Backup Time  Most AECs have a minimum response time of approximately 0.001 second.  Some procedures and some equipment may need less than the MRT to produce a diagnostic quality image. In these instances, the mA should be adjusted to account for the MRT.  The backup timer should be set at 150% of the anticipated exposure time. Automatic Exposure Systems  Federal regulations require that AECs have a 600 mAs safety override.  Poor positioning or technical errors result in patient over-exposure.  Technique charts are necessary to consult for appropriate kVp, mA, and back up time, as well as the proper AEC sensors and density selections. Automatic Exposure Systems The control panel does NOT relieve the Radiologic Technologist’s responsibility to identify variables (patient anatomy/pathology or equipment) affecting the selection of the technical factors. Anatomically Programmed Radiography  The principle of APR is similar to that of AEC, with the radiographic technique chart programmed into the microprocessor of the control unit.  The RT can override the APR and the RT retains responsibility for patient dose and image quality.  Magnification Radiography  Magnification Radiography enhances the visualization of small structures.  Primarily used in interventional radiography and mammography.  Normally we strive to reduce OID to minimize magnification. This technique deliberately increases OID.  Magnification Images on the radiograph are larger than the objects they represent Magnification Factor = Image Size ÷ Object Size Magnification Factor = SID ÷ SOD Image Size SID Magnification Factor = Object Size = SOD Terminology Review  Minimizing Magnification Large SID: Use as much SID as possible Small OID: Place the object as close to the IR as possible  Patient Factors Body habitus Hypersthenic, sthenic, hyposthenic, asthenic Part thickness affects: Beam attenuation Exposure reaching image receptor Scatter production and image contrast Pediatric patients Small size may require a reduction in exposure. Quicker exposure times may be necessary. Minimizing Patient Exposure Use higher kVp and lower mAs values. Restrict the size of the x-ray beam. Use grids only when needed and use lower grid ratio. Careful selection of digital exposure techniques is necessary. Technique Modifications Projections and positions Casts and splints Pathology Soft tissue imaging Contrast media Density  Density Primary controlling factor is mAs mAs and density are directly proportional Window level changes density/brightness  Good radiograph produced using: 15 mAs (300 mA/.05 second), 80 kVp,.6 mm focal spot, 72” SID, 2” OID, 8:1 grid ratio, and full 14x17 IR Factor Changed Effect on Density 400 mA Increased.02 sec Decreased 30 mAs Increased 70 kVp Decreased 40” SID Increased 8” OID Decreased 2.0 mm focal spot No change 12:1 grid ratio Decreased 10 x 12 IR Decreased Contrast Contrast Primary controlling factor is kVp kVp and contrast are inversely proportional Window width changes contrast  Good radiograph produced using: 133 mAs (400 mA/.33 second), 80 kVp, 1.5 mm focal spot, 56” SID, 2” OID, 8:1 grid ratio, and full 14x17 IR Factor Changed Effect on Contrast 200 mA No change.5 second No change 50 mAs No change 70 kVp Increased 50” SID No change 8” OID Increased 1.2 mm focal spot No change 6:1 grid ratio Decreased 10x12 IR Increased Detail  Detail Controlling Factors: SID OID Focal Spot Motion  Good radiograph produced using: 20 mAs (100 mA/.2 second), 80 kVp, 1.5 mm focal spot, 72” SID, 2” OID, 6:1 grid ratio, and full 10x12 IR Factor Changed Effect on Detail 300 mA No change.5 second No change 10 mAs No change 70 kVp No change 50” SID Decreased 6” OID Decreased.6 mm focal spot Increased 8:1 grid ratio No change 14x17 IR No change Distortion Distortion Distance affects magnification/distortion Central ray alignment with object and IR  Good radiograph produced using: 4 mAs (200 mA/2 msec), 75 kVp, 2.0 mm focal spot, 48” SID, 2” OID, 16:1 grid ratio, and full 14x17 IR Factor Changed Effect on Distortion 100 mA No change 30 msec No change 2 mAs No change 85 kVp No change 60” SID Decreased 6” OID Increased.3 mm focal spot No change no grid No change 8x10 IR No change  Distance Changes If you would use 10 mAs at 40” SID, but need to use 72” SID, what mAs should you use? New mAs = old mAs x new distance2 old distance2 New mAs = 10 mAs x 72” 2 40”2 New mAs = 10 mAs x 5184 1600 New mAs = 51,840 1600 New mAs = 32.4  15% Rule If the kVp is increased by 15%, the mAs must be cut in half to maintain density. If the kVp is decreased by 15%, the mAs must be doubled to maintain the density. Original factors: 15 mAs, 70 kVp Increase Decrease 70 x 1.15 = 80.5 kVp 70 x.85 = 59.5 kVp 15/2 = 7.5 mAs 15 x 2 = 30 mAs  Grid Ratio Grid Ratio Multiplier No grid 1 5:1 2 6:1 3 8:1 4 12:1 5 16:1 6 Original Factors: 10 mAs, 75 kVp, 6:1 grid Convert technique for 12:1 ratio grid 5 10 x 1.66= 16.7 mAs 3  Collimation Field Size IR/Field Size Multiplier 14 x 17 1 10 x 12 1.25 8 x 10 1.4 Original Factors: 5 mAs, 80 kVp, 14 x 17 IR Convert technique for 10 x 12 IR 1.25 1.25 x 5 mAs = 6.25 mAs 1  Generator Generator Multiplier Three phase 1 Single phase 2 Original Factors: 15 mAs (300 mA x.05 sec), 70 kVp, on single phase equipment. Convert technique for three phase equipment. 1.5 x 15 mAs = 7.5 mAs 2  Review: Grid Ratio Grid Ratio Multiplier No grid 1 5:1 2 6:1 3 8:1 4 12:1 5 16:1 6 Original Factors: 10 mAs, 75 kVp, 6:1 grid Convert technique for 12:1 ratio grid 5 10 x 1.66= 16.7 mAs 3  Review: Collimation Field Size IR/Field Size Multiplier 14 x 17 1 10 x 12 1.25 8 x 10 1.4 Original Factors: 5 mAs, 80 kVp, 14 x 17 IR Convert technique for 10 x 12 IR 1.25 1.25 x 5 mAs = 6.25 mAs 1 The End?