Line focus principle Heal effect Ratings Tube failure

Slides:



Advertisements
Similar presentations
X-RAY EMISSION SPECTRUM
Advertisements

Producing an X-ray Exposure
Advanced Biomedical Imaging Lecture 3
EVERYTHING YOU WANTED TO KNOW BUT WERE AFRAID TO ASK!!!!
[Radiography] Technique - Exposure Factors
Chapter 9 The X-ray Machine
Mobile Radiographic Equipment. Introduction In-patients who could not leave their beds In-patients who could not leave their beds Surgeons who required.
Wilhelm Conrad Röntgen
X-ray tube.
X-RAY TUBE.
Chapter 11 Prime Factors.
Formation Characteristics
CHAPTER 6 THE X-RAY BEAM SPECIFIC OBJECTIVES NOTED IN THIS POWER POINT BEGIN ON PAGE
The Generation of X-ray:
Advanced Biomedical Imaging Lecture 4 Dr. Azza Helal A. Prof. of Medical Physics Faculty of Medicine Alexandria University.
The X-Ray Tube Bushong Ch 7.
Chapter 7 Radiographic Image Formation and Exposure Factors.
Topic 3 Selection of kV High kV scattered radiation is energetic. Angle of rays is same as those of primary beam and is not deflected very much. Follows.
X-ray tube and detection of X-rays Lecture 5. Reminder: The rough schematics of an X-ray tube filament cathod target anode photon flux e-e- electron kinetic.
MAMMO QC – covered in week 8
ACVR Artifacts Artifacts of Diagnostic Radiology
RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY
Analyzing the Image Density. Density Overall blackening of the image.
354 Chapt. 6 X-ray Tube TWO Primary components – cathode and anode Tube must be supported: Ceiling/floor mounted/C-arm, etc. – SID’s – Detents (center.
Resident Physics Lectures
Medical Imaging X-Rays I.
Dr. Mustafa Zuhair Mahmoud Mr.Ali B Alhailiy بسم الله الرحمن الرحيم 1.
BME 560 Medical Imaging: X-ray, CT, and Nuclear Methods X-ray Instrumentation Part 1.
8.1 PRODUCTION AND CHARACTERISTICS OF X-RAYS
Factors affecting the X-Ray output
X-Ray Production & Emission
X-Ray Production & Emission
COMPUTED TOMOGRAPHY - I RAD 365 CT - Scan
Quality Control Rad T 110.
Name each component of the x-ray circuit and
Quality Control.
Ch. 2 – Anatomy of the X-ray Machine
HABIS X-RAY PRODUCTION AND EXPOSURE FACTORS X-RAY PRODUCTION AND EXPOSURE FACTORS PREPARED BY PREPARED BY Dr fahad albadr radiology chairman radiology.
Resident Physics Lectures
02 X-ray Tube.
Chapter 2 The X-ray Beam.
Last time we defined a crystal as a solid containing translational symmetry. The directions of translation can be used to from a unit cell. A primitive.
CT Instrumentation and X-ray system
Elsevier items and derived items © 2009 by Mosby, Inc., an affiliate of Elsevier Inc. X-Ray Emission Chapter 9.
PRINCIPLES OF TECHNIQUE AND EXPOSURE
Radiographic Equipment
THE PHYSICS OF RADIOLOGY The Production and Properties of X Rays Part Two BME College Sherman sheen.
COMPUTED TOMOGRAPHY - I RAD 365 CT - Scan
Diagnostic X-Ray Production
S. Guilbaud Education Director School of Radiologic Technology
X-ray tube.
An early x-ray by Wilhem Rontgen
RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY
Exposure Factors or Prime Factors
Patient is placed between X-ray tube and silver halide film.
RAD 253 Chapter 7 The X-ray Tube Two primary components: cathode and anode.
Diagnostic Radiology II X-ray Tubes. Anode angle Anode angle defined as the angle of the target surface with respect to the central ray in the x-ray field.
Specifications for Rotating Anode X-ray Tube Ass’y Model : M125S20X Maximum X-ray Tube Voltage kV Focal Spot Size (Small/Large)
1 / Basic X Ray 2 / X-ray 简介 X 光球管简介 高压发生器简介 普通放射设备简介 透视摄影简介 血管造影设备简介 Topics.
Chapter 4: Diagnostic X-Ray Production
01 X-ray Tube.
Fundamentals of radiography.
X-RAY PRODUCTION AND EXPOSURE FACTORS
X-ray Production Sharif Qatarneh Medical Physics Division
Part No...., Module No....Lesson No
X-Radiation.
Factors Effecting the Production
Exposure Factors Chapter 4
Resident Physics Lectures
Generation of X-rays Q&A
Presentation transcript:

Line focus principle Heal effect Ratings Tube failure X-ray Tube -2 Line focus principle Heal effect Ratings Tube failure

Line-Focus Principle The Effective Focal Spot is the beam projected onto the patient. As the anode angle decreases, the effective focal spot decreases. Diagnostic tube target angles range from 5 to 15°.

Advantages & limitations of LFP The advantage of Line focus is it provides the sharpness of the small focal spot with the heat capacity of the large large focal spot. Smaller target angles will produce smaller effective focal spots and sharper images. Area covered by the beam reduces with target angle To cover a 17” the angle must be 12° To cover 36” the angle must be 14° Anode heel effect (next slide)

Anode Heel Effect Due to the absorption of x-rays by the anode heel the radiation intensity on the cathode side of the x- ray tube is higher than the anode side. This effect is increased when the target angle is reduced

Influence of Anode heel effect The difference in the intensity can vary by as much as 45%. If the center is 100% the anode side of the beam can as low as 75% and the cathode as much as 120%. The heel effect should be considered when positioning areas of the body with different thickness or density. The cathode side should be over the area of greatest density.

Anode Heel Effect on Focal spot size and Resolution The effective focal spot size and shape change across the projected field The sharpness of the image can be dependent upon which area of the beam coverage you are looking at. Similar to the shape distortion when the tube is not centered.

Off Focal Radiation The electrons can rebound and interact with other areas of the anode other than the focal area. These interations can produce x-rays too. This is called Off-Focal Radiation.

Control of Off Focal Radiation A diaphragm is placed between the tube and the collimator to reduce off focus rays. But the off focus radiation completely The percentage of off focus radiation increases with kV

X-ray Tube Rating Charts The that provide information on the safe exposures and safe operating conditions of x-ray tubes are called tube rating charts With careful use, the x-ray tube can provide long periods of service. Inconsiderate or careless operation can lead to shortened life or abrupt failure. X-ray tubes are very expensive. Cost varies from $2,000 to $20,000.

Types of rating charts Radiographic rating chart Anode cooling chart Housing cooling chart

Radiographic rating chart This is the most important of the three charts It conveys which radiographic exposures are safe and which are unsafe The chart shows a family of curves for different mA The two axes X & Y show scales of Time and kV respectively For a given mA, any combination of kVp and time that lies below the curve is safe Any combination that lies above the curve of desired mA is unsafe Modern x-ray systems have a microprocessor control that does not allow unsafe exposure to be made.

Use of Series of radiographic rating charts Important to use the correct rating chart e.g Rating charts for different filament sizes (focal spot sizes) For different anode rotation speeds For different anode angles For the type of high voltage rectification Radiographic rating charts can be used to check the proper operation of microprocessor control protection circuit

Question Radiographic examination of the abdomen with a tube that has a 0.6 mm focal spot and anode rotation of 10,000 rpm requires technique factors of 95 kVp, 150 mAs. Calculate using the correct rating chart, the shortest possible exposure time for this examination.

Anode cooling chart Anode cooling charts contain the information about the thermal capacity of an anode and its heat dissipation characteristics. It does not depend on the filament size and the speed of rotation Usually the cooling is rapid at first and slows as the anode cools In addition to knowing the maximum heat capacity the chart is used to determine the length of time required for complete cooling after any level of heat input.

Anode cooling chart

Anode thermal capacity, HU & Joule The maximum amount of heat that can be stored in the anode without thermal damage In x-ray applications, It is measured in Heat Units (HU) 1HU = 1kVp x 1 mA x 1s (for single phase) Actual heat energy is measured in Joules (J) 1 J = 1 volt x 1 amp x 1s = 1 kV x 1 mA x 1s (here the kV is the direct or the RMS voltage) For single phase kVp = 1.4 kV (rms) For 3 phase or high frequency kVp = kV (rms) Therefore for three phase/high frequency, HU=1.4 x kVp x mA x s

Questions Radiographic examination of the lateral lumber spine with a single-phase imaging system requires 98 kVp, 120 mAs. How many heat units are generated by this exposure? A fluoroscopic examination is performed with a single- phase imaging system at 76 kVp and 1.5 mA for 3.5 minutes. How many heat units are generate? Six sequential skull films are exposed with a three-phase generator operate at 82 kV, 120 mAs. What is the total heat generate?

Questions A particular examination results in delivery of 50,000 HU to the anoe in a matter of seconds. How long will it take the anode to cool completely? (use the anode cooling chart given in slide 16) How much heat energy (in joules) is produce during a single phase mammographic exposure of 25 kVp, 200 mAS?

Housing Cooling Chart The cooling chart for the housing of the x-ray tube has a similar shape as the anode cooling chart. The maximum heat capacity of the housing is in the range of several million heat units. Complete cooling after maximum heat capacity requires from 1 to 2 hours

X-ray tube failure All causes of tube failure relate to the thermal characteristics of the tube. When the temperature of the anode during a single exposure is excessive, localized melting and pitting occurs. These surface irregularities lead to variable and reduced radiation output. If the melting is severe, the tungsten vaporizes and can plate the port. This can cause added filtering or interference with the flow of electrons. If the temperature of the anode increases to rapidly, the anode can crack and then become unstable in rotation.

Maximum radiographic techniques must never be applied to a cold anode During long exposures (1 to 3 seconds) the anode may actually glow like a light bulb. The heat may cause a failure of the bearing for the anode or a crack in the glass envelope. Because of the high heat of the filament, the inside of the glass envelope. This will tungsten atoms are slowly vaporized and plate eventually lead to arcing and tube failure. Continuous high mA radiography will actually lead to the filament breakage.

Anode faults New Surface damage due to repeated over load Pitting due to slow rotation Surface damage due to Exceeding of heat capacity d C

Tube Warm-up Procedures By warming the anode through a series of exposures and increasing kVp settings, the anode will build up heat that is needed to avoid fracture of the anode. This process takes a little over one minute put will add to the life of the tube. Close shutters of collimator. Make exposure of 12 mAs @ 70 kVp Wait 15 seconds Make exposure of 12 mAs @ 80 kVp Make exposure of 12 mAs @ 90 kVp Tube warm up is now complete.

V.G.Wimalasena Principal School of radiography END V.G.Wimalasena Principal School of radiography