Radiographic Noise Definition & Measurement PDF

Topic 7: Definition & Radiographic Noise  Objectives describe the features of a good quality radiograph differentiate between "objective" and "subjective" definition list the main factors affecting definition define the main terminology relating to image quality...

Topic 7: Definition & Radiographic Noise  Objectives describe the features of a good quality radiograph differentiate between "objective" and "subjective" definition list the main factors affecting definition define the main terminology relating to image quality measure unsharpness, IC and resolving power describe the relationship between the angle of the anode, the SID and the maximum field size identify unsharpness and evaluate its causes and remedies differentiate between the "Actual" and the "Effective" focal spots, and list the factors affecting them 2  describe the "Line Focus Principle“ solve Ug problems describe the various factors affecting Ug, Um, Up and Ua evaluate the factors that contribute the most to total unsharpness (Ut) from a given a case study list some applications where Um is used as an advantage state the advantages of LSF & MTF over Resolving Power evaluate and compare MTF graphs of various recording systems analyse the factors affecting subjective definition To define image noise, artefacts and image mottle. To identify, name and classify artefacts and mottle on images, and state their causes To define the various types of image mottle and list their contributing factors. 3  Definition What is the definition of “Definition”? Visibility of small structures Image Detail Spatial resolution Sharpness Objective definition Subjective definition 4  Umbra and Penumbra Umbra: The sharp area on the image Penumbra o determines unsharpness or blur o It is the width of the border on the edges of a structure 5 Sources of Unsharpness Geometric (Ug) Motion (Um) Photographic (Up) Absorption (Ua) 6  Geometric Unsharpness (Ug) It is due to the geometry of shadow formation Factors Affecting Ug: o FOCAL SPOT SIZE: ▪ Actual FS: Area bombarded by é on target ▪ Effective FS: Actual FS seen from the Central Ray (CR) ▪ Ug is proportional to the actual and effective focal spot o SID o SOD o OID 7  Actual Focal Spot It is the area covered by the electron beam on the target when looking at it at an angle of 90o with the surface of the anode SOD In theory a “point source” as the actual focal spot is needed for minimal Ug OID In practice an “area source” is the actual focal spot and it is tolerated because of the heat generated (99%) in x-ray production 8  Actual Focal Spot Size Large FS size Small FS size Ug is inversely proportional the FS size The x-ray tube load is proportional to the FS size  Effective FS Effective FS: It is the focal spot when viewed from the CR Also called the Nominal FS 10 Factors Affecting the Size of the Effective FS 11 Actual FS α Effective FS 12  Effective FS α Angle of the Anode A smaller Angle of the Anode causes a: o ↓ Effective FS o ↓ Ug o But also DECREASES Field Coverage  13  Filament size α Effective FS ↓ Filament Size, ↓ Actual Focus, ↓ Effective Focus, ↓ Ug 14  mA and kVp vs. Actual and effective FS In theory: o ↑ mA & ↓ kVp, ↑ é dispersion, ↑ Actual FS, ↑ Eff. FS, ↑ Ug In practice: o mA and kVp do not affect focal spot size because the voltage applied on the focussing cup corrects it; therefore, Ug is inversely proportional to the voltage applied to the focussing cup 15  EFS size α heat capacity of target Percentage of heat generated versus percentage of x- rays produced is 99% heat to 1 % x-rays How can a heavy thermal load be maintained on the target while using the small FS? o↓ Angle of anode ▪ same AFS with a smaller EFS ▪ Disadvantage: Limited by field coverage o↑ Cooling of anode efficiency ▪ Faster rotating anode – higher cooling 16  Rotation of Anode Vs. FS Size Faster rotation of the anode allows the use of a smaller actual focal spot because cooling of the anode improves 17  Variations in Effective FS Size & Anode Angle Variations in Angle of Anode Small Focal Spot sizes Large Focal Spot sizes At 40” SID a minimum of 13o angle (fine focus) (broad focus) of the anode is needed to cover a 0.1 mm (microfocus) 0.6 mm 17” x 17” field of exposure 0.3 mm 1.0 mm 0.6 mm 1.2 mm At 48” SID 12o angle of the anode is 1.0 mm 2.0 mm sufficient 1.2 mm 2.5 mm Most x-ray tubes have anodes with 1.5 mm Most x-ray tubes have Small Focal Spot sizes angles between 13 to 15 degrees between 0.6 mm and 1.0 mm Large Focal Spot sizes between 1.2 mm 6o 10o 12o 13o 15o 17o 20o and 2.0 mm 18  In Summary the Factors Affecting Effective FS are Angle of the anode (α) Size of the filament (α) Size of the Actual Focal Spot (α) mA (α) kVp (1/α) Voltage of the Focusing cup (1/α) Rotation of the anode (indirect effect) (1/α) 19  Line Focus Principle (or Goetz Effect) Ug is greater on Cathode side than Anode side ∆ in Effective FS size along long axis of tube At central ray: EFS is square On anode side: FS “appears” smaller On cathode side: FS “appears” larger Ug is greater on Cathode side than Anode side 20  Effect of OID on Ug To ↓ Ug, ↓ OID oExamples: ▪ Orbits Ug: PA < AP, ▪ Chest (heart) Ug: PA < AP 21  Effect of SOD on Ug SOD is inversely proportional to Ug If SOD is short, use the Small Focal Spot size to ↓ Ug When OID cannot be ↓, the SID is ↑ to ↓ Ug o Example: Chest & Lat. C-spine ∆ in SOD do not affect much Ug when OID is close to nil 22 23 Extension cones and cylinders can ↓ Ug FS Ug 24  keeping Ug to a minimum small AFS therefore small EFS ↑ SID & ↑ SOD (but will require ↑ tube load) ↓ OID (preferred because it will not require an ↑ in tube load) Use HS rotor because it may allow Small Focal spot size on greater mA setting (higher Cooling of x-ray tube) 25  Image Magnification Technique in Mammography Micro focal spot size is < 0.3 ↑ OID detector 26 Calculation of Ug OID Ug = F X ---------- SOD Ug α F & OID Ug α 1 / SOD 27 Math Problems with Ug: 1. Calculate the width of the penumbra (Ug) that an object will project if it is placed at 20" from the IR and exposed at 40" SID with a 2 mm focal spot (1" = 25.4 mm). Include the formula and calculations. F = 2 mm F = 2.0 mm OID = 20” SOD = 20” SOD = SID – OID SOD = 40” - 20” = 20” OID = 20” Ug = F X OID / SOD Ug = 2.0 mm X 20” / 20” Ug = 2.0 mm Ug = ? 28  Motion Unsharpness (Um) Um is due to movement of the patient or the equipment 29  Motion Unsharpness (Um) Um Depends on: o The rate of movement (how often) ▪ depends on whether the patient motion is continuous throughout the whole exposure time o The extent of movement (direction & length) o The duration of the movement 30  Motion Unsharpness (Um) The extent of movement depends on the direction of movement: o Vertical movement causes only small variations in magnification, so the increase in Um is small o Horizontal movement causes a large ↑ in Um which is proportional to the length of the movement 31  Motion Unsharpness (Um) If the exposure time is long, this may ↑ the rate of involuntary movement and ↑ the chance of voluntary movement. How to ↓ Um: Choose ↓ exposure time and ↑ mA combination to maintain comparable mAs Keep OID to a minimum and ↑ SID to minimize horizontal motion Use a ↑ kVp and ↓ mAs combination technique 32  In Some Applications Um is an Advantage Normal Tomography Autotomography IR 33 Lat. Thoracic Spine: Breathing Technique vs. non breathing Technique Stop Breathing during exposure Breathing during exposure (< 0.5 sec) (> 2.0 sec) 34 Breathing Stop Breathing (> 2.0 sec) (< 0.5 sec) 35 Stop Breathing Breathing (< 0.5 sec) (> 2.0 sec) 36  Photographic Unsharpness (Up) also called intrinsic unsharpness, it is the unsharpness generated by the imaging system – will be discussed in the CR/DR topic 37  Absorption Unsharpness (Ua) Ua is the blur due to the oval shape of some anatomic organs (e.g., kidneys) 38 INTRAVENOUS UROGRAM (IVU) 39  Ua is visible with certain destructive diseases such as arthritis Arthritis - ↑ Ua Normal - ↓ Ua 40  Total Unsharpness (Ut) Ut is the sum of all causes of unsharpness. It is defined as follows: the individual causes of unsharpness cannot be separated and as a result we only see Ut. Trying to reduce one type of blur can cause for another type of blur to ↑. o Example: ↑ SID will ↓ Ug but may ↑ Um because of the need to ↑ exposure time 41  Um is often the greatest contributor to Ut Efforts to ↓ Um include: o ↓ Exposure Time while maintaining comparable exposure can be done by: ▪ ↑ mA ▪ ↑ kVp ▪ ↓ SID o Patient factors are also very important: ▪ ↓ breathing ▪ good verbal & non-verbal communication ▪ Effective immobilization 42  Objective Measurement of Image Definition or Sharpness or Detail Methods: Resolution test pattern Line Spread Function (LSF) Modulation Transfer Function (MTF) 43 Objective Measurement of Definition or spatial resolution using the resolution test pattern tool (RTP) 44  The resolution test pattern tool What is a resolution test pattern tool? o A quality control test tool used to measure spatial resolution o It contains radiopaque lines (Pb) and radiotransparent lines (plastic) that are paired together in a millimeter o They are line pairs per mm (Lp/mm) o Each line represents an object on a radiographic image 45 46 47  Spatial Frequency The number of line pairs visible in a mm is called spatial frequency or spatial resolution Spatial Frequency or Spatial Resolution is the amount of detail that can be seen on the image Each line (black or white) on the RTP image represents a single object on an image 1 1 Minimum Object Size = X Spatial Frequency 2 The higher the Spatial Frequency, the smaller the objects shown on the image Example: Solution: 𝟏 𝟏 What is the smallest object that Object Size = 𝐗 𝐒𝐩𝐚𝐭𝐢𝐚𝐥 𝐅𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲 𝟐 can be resolved by a CR imaging system with a spatial frequency 𝟏 𝟏 Object Size = 𝐗 = 𝟎. 𝟏𝟐𝟓 𝐦𝐦 of 4.0 Lp/mm? 𝟐 𝟒.𝟎 𝐋𝐩/𝐦𝐦 48  Spatial Frequency The higher the SF the... greater the number of Lp resolved in a mm smaller the line pairs become smaller the object visible on the image therefore higher resolving power of the imaging system 49  Measuring resolution with a RTP is influenced subjectively by: o Visual acuity of observer: ▪ subjective sharpness ▪ subjective contrast ▪ fatigue o Viewing conditions: ▪ monitors ▪ ambient lights 50  Disadvantages of using the RTP to measure resolution Subjective to the viewer only measures the limits of the imaging system It does not evaluate definition reproduction of the complex variations in body tissues does not record information lost by the imaging system 51 Objective Measure of spatial resolution Using the Line Spread Function (LSF) IR 1 IR 2 Micro meter (not Motion Unsharpness) 52  Line Spread Function (LSF) also known as Edge Response Function (ERF) measures more the Up than spatial resolution IR 2 measures the blackness on the image (Optical IR 1 Densities) at the edges of the structures LSF Method steps: o Two Metal sheets with narrow slit openings are placed in perfect contact with the IR o The IR is exposed o The resulting image is a fine black line o A hard copy on film is printed o the width of the edges Is measured with a microdensitometer device (measures minute optical densities or blackness differences on a film) o A graph of OD vs. Up (in μm) is created using the data obtained 53  Objective Measure of spatial resolution Using the Modulation Transfer Function (MTF) Measures the ability of the IR to faithfully image objects The object has 100% of the available information and the image will always contain less than 100% of the information, therefore MTF is always less than 1 54  Modulation Transfer Function (MTF) blurring on the image is measured as a function of spatial frequency (RTP) Blur Modulation of the bar pattern is plotted against spatial frequency The Fourier Function is used to analyze the data The data analysis is used to produce an MTF curve which is used to determine the degree of blur produced by a given imaging system Limiting spatial frequency of an IR is measured @ 10% MTF 55 56  MTF Problem: A linear resolution test pattern is exposed to assess the MTF of a CR system compared to a DR system. The MTF graphs obtained from the resultant images of the test tool are shown below. 3. Label each curve as CR and DR? ✓ A = CR & B = DR B = DR 3. What is the MTF of the DR system, if the spatial resolution of the recorded ROI is 5 Lp/mm? ✓ Answer: 0.7 A = CR 3. What is the limiting spatial frequency of the CR system? ✓ Answer: 5.7 Lp/mm (measured at 10%) 57  Subjective Image Definition or Distinctness It is the ability of the viewer to see fine detail structures as separate entities The amount of distinctness obtained should depend on the minimum Diagnostic Value required minimum DV should be determined before the examination begins by: ✓ reading the requisition ✓ Asking pertinent questions 58  Subjective Image Definition or Distinctness How to improve distinctness if: sharpness is low but contrast is high ↑ viewing distance IC is low but sharpness is high ↓ viewing distance & ↑ optical magnification When it is not possible to ↓ unsharpness, an ↑ in image contrast will produce an image which appears to have less unsharpness Conclusion on Distinctness: Both sharpness and IC are needed Important to remember that objectively IC DOES NOT affect sharpness and vice-versa 59 RADIOGRAPHIC NOISE 60 61  image noise is an unwanted fluctuation in the image which tends to obscure the visibility of structures Noise is the stochastic (random) component in the image The principal source of noise on an image is SCATTER 62  Image Mottle 3 types of image Mottle: Quantum Electronic Structured 63  Quantum Noise (quantum mottle) In Quantum noise or quantum mottle, the amount of noise is determined by the variation in photon concentration from point to point within a small image area due to the random spatial distribution of X-ray photons on the image It has a dottiness, grainy, sandy appearance. % of Random Photon Fluctuation =± √N Where N is equal to the # of x-ray photons / area If N ↑, QM (fluctuation) ↓: o N= 100, fluctuation = √100 = 10 → ± 10% o N= 10,000, fluctuation = √10,000 = 100 → ± 1% QM α 1 / Exposure dose to IR 64 QM is the greatest contributor to Image Mottle QM is higher with sensitive IR QM is a main limiting factor of image quality 65  Image Mottle Type 1: Quantum Noise (quantum mottle) Visibility of QM is ↑ when: o Low exposures o Low IC o high sharpness o Low Brightness 66  Imaging systems cannot compensate for excessive noise caused by QM Therefore IMAGE HAS TO BE RE-TAKEN If QM is present and the thickest structures in the ROI are not all distinguishable, then an increase in kVp is needed. Why kVp and not mAs? ✓ MORE PENETRATION NEEDED TO INCREASE X RAY TRANSMISSION THROUGH THE THICK AREAS OF THE OBJECT If QM is present and all the structures in the ROI are distinguishable, then an increase in mAs is needed. Why mAs and not kVp? ✓ X RAYS ARE ENERGETIC ENOUGH TO BE TRANSMITTED THROUGH THE THICK AREA THEREFORE A HIGHER QUANTITY OF PHOTONS ARE NEEDED 67 68 69  Electronic Mottle (Noise) Caused by fluctuations in the electrical signal, a characteristic of all electronic circuits in the Image Receptor Also known as Dark Noise or System Noise should generally be much less than Quantum Noise It can be present even though no exposure of the IR occurs DR receptors eliminate dark noise before each exposure is made 70  Structured Noise In Computed Radiography (CR) o it is due to fluctuations in the IR response to exposure and o In CR is caused by the non-uniformity in the structure of the material (phosphor crystals) in the image receptor due to: ▪ non-uniform coating (rarely) ▪ physical imperfections Corrected with re-calibration QC procedure (Flat Field Calibration) 71