CT Scan PDF

COMPUTED TOMOGRAPHY JOSE MICHAEL T. BORJA, RRT, MSRT  COMPUTED TOMOGRAPHY The early history of conventional tomography reflects the level of poor communication of that era between scientists of the same discipline living in separate countries, shortcoming that explains why t...

COMPUTED TOMOGRAPHY JOSE MICHAEL T. BORJA, RRT, MSRT  COMPUTED TOMOGRAPHY The early history of conventional tomography reflects the level of poor communication of that era between scientists of the same discipline living in separate countries, shortcoming that explains why the basic principle of body-section radiography evolved independently among several researchers in different settings.  TOMOGRAPHY Tomography – a type of radiographic examination that served as the most promising method to separate "confusing shadows", so that specific anatomic detail could be seen without interference from overlying structures (principle of body-section radiography)  X-RAY PRODUCES SHADOWGRAM WHERE 3D INFORMATION IS FLATTENED INTO 2D IMAGE  TOMOGRAPHY In this principle, a layer of tissue from within the body is imaged as an isolated section with overlying structures outside of this section excluded from the picture. A series of consecutive sections are imaged and viewed in sequence to examine the entire body part under investigation. TOMOGRAPHY  TOMOGRAPHY This image is a simple representation of the tomographic principle. Here are the points to take note: All points of the object located in the section plane (A) will be projected to the same points on the film (A1, A2) during the exposure and hence will be recorded in sharp focus on the film. Points of the object outside of the section plane (B) will be continuously projected to different positions on the film during the exposure (B1, B2) and, therefore, will be blurred or effaced from the final image. Changing the section plane can be accomplished either by moving the point of rotation (fulcrum) between the tube and film or by moving the object. The degree of blurring of body structures outside of the section plane will depend on: the distance, either above or below the section plane the degree of angular displacement of the tube the spatial configuration of the tube-film movement  CONVENTIONAL TOMOGRAPHY Conventional Tomography – this is the earliest method of tomography which recorded the image on X-ray film. To accomplish body-section radiography in a non-computed, mechanical system, two of three elements (tube, patient, and film) must move synchronously during the X-ray exposure. Many techniques for moving elements have been explored, but the most popular and enduring method depends on the synchronous movement of the X-ray tube and film in opposite directions and the patient remaining stationary during the X-ray exposure. CT SCAN History ► Godfrey N. Hounsfield (1970-71) Invented the first CT-Scan machine for EMI Ltd.(Electric and Musical Industries) Profession -Electrical engineer History ► William Oldendorf (1961) and Allan Cormack (1963) ► Primitive applications of 3D-Recon and initial investigations in use to medical imaging. Professions -Neurologist -Physicist History ► Johann Radon (1917) Austrian mathematician that proves that an image of 3 dimensional object could be produced from its mathematical projection.  History 1930’S – ALLESANDRO VALLEBONA ∙ Proposed a method to represent a single slice of the body on the radiographic film (TOPOGRAPHY) History ► Both Hounsfield and Cormack share the 1979 Nobel Prize for medicine. History CAT Scanner- Computed Axial Tomography, the old term for CT Scan, because it can only produce Axial Images. -The first CAT Scanner can produce a single sectional image in 9 days  COMPUTED TOMOGRAPHY ► Other Names: ► Computed Axial Tomography (CAT) ► Computed Transaxial Tomography (CTAT) ► Computed Reconstruction Tomography (CRT) ► Digital Axial Tomography (DAT) ► Body Section Roentgenography ► Greek Word: “Tomos” = slice/section; “Graphia” = describing COMPUTED TOMOGRAPHY ► CT SCANNER Consists of an x-ray source emitting finely collimated x-ray beam and a single detector both moving synchronously in a translate or rotate mode or a combination of both.  The Main Parts of the CT Scan 1. Gantry Assembly 2. Patient Table/Couch 3. X-ray Tube 4. Detectors 5. Computer 6. Display Console 7. Speakers 8. Mic  Gantry Assembly The most prominent part of a CT scanner is the gantry – a circular, rotating frame with an X-ray tube mounted on one side and a detector on the opposite side. aka “doughnut shape equipment”  Gantry aperture Aperture: 50-80 cm (20”-34”)general diagnostic 100 cm (39.3”) for dedicated machines (radtheraphy). Limits: Tilt (+/- 30 deg) Patient Couch/Table Curved (Gen Radiography), or Flat (Radtherapy) Low Z material (Carbon Fiber Graphite) Weight Limit (manufacturer’s specs)  X-ray tube CT ►for 80x80 matrix display: Stationary anode with 2mmx16mm focal spot. 120 KVp, 30mA. ►for 512x512 matrix display: Rotating anode with 0.6mmx1.2mm focal spot. KVp selectable (80, 120, 140), 1000mA. ►0.5-5 milliom Heat Units thermal capacity.  CT Collimator ► Prepatient Collimator ► Limits the area of the patient that intercepts the useful beam ► Mounted on the x-ray tube housing or adjacent to it ► Purpose: to decrease patient dose ► Determines: ► Dose profile ► Patient dose ► Predetector/Post Patient Collimator ► Restricts the x-ray beam viewed by the detector array ► Purpose:  CT Detectors Detectors ►Receive and convert xray photons to digital signal ►Source to Detector Distance: 44” (110cm)  st 1 Generation CT Scan First Generation: ►Head only ►Rotate-Translate ►Xray Collimator: 3mmx26mm ►Scan Arc: 180deg ►4.5-5min scan per section. ►single ray pencil beam ►single detector (Solid State: NaI-PM tube) ►Display: Matrix (80x80), Voxel (3mmx3mmx13mm)  nd 2 Generation CT Scan Second Generation: ►Whole Body ►Rotate-Translate ►Xray Collimator: 3mmx13mm (30septums) ►Scan Arc: 180deg ►10-90 sec per section scanning time ►Fan shaped xray beam (narrow) ►30 detectors (linear) of Solid State or Gas-Filled ►Display:Matrix (Selectable up to 320x320), Voxel(Selectable)  rd 3 Generation CT Scan Third Generation: ►Whole Body with dynamic scanning ►Single Projection exposure ►Scan Arc: Selectable (220 deg or 360 deg) ►2-10 sec scanning time per section ►Fan shaped xray beam (wide) ►250-750detectors (Curvilinear) of Solid State or Gas- Filled. Prone to Ring Artifacts. ►Display:Matrix (selectable up to 526x526), Voxel (selectable)  th 4 Generation CT Scan Fourth Generation: ►Whole body with dynamic scanning ►Single Projection Exposure ►Scan Arc: Selectable (180-360+deg) ►2-10 sec scanning time per section ►Fan shaped xray beam (wide) ►600-2000 detectors (fixed circular) Solid State. ►Display: Matrix (Selectable up to 512x512), Voxel (Selectable) 5th Generation CT Scan/Electron beam computed tomography (EBCT) ► This 5th generation is uses a flying electron beam, steered electromagnetically and to hit one of the anode strips that encircle the patient. No moving parts, therefore very fast (about 50 ms). It developed specifically for cardiac tomographic imaging ► The advantage of this 5th generation ct scan are extremely fast and capable of imaging the beating heart. ► The disadvantage of this 5th generation ct scan are high costs and difficult to calibrate, therefore it is not clinically used ► Electron Gun- produces a focused electron beam that generates a rotating x-ray fan beam after being steered along tungsten target Gantry assembly ► Dynamic Spatial Reconstructor- a computed tomography system for high-speed simultaneous scanning of multiple cross sections of the heart.  th 6 Generation CT Scan/Helical/Spiral CT scan Helical CT ►Whole Body with dynamic scanning ►Multiple Projection Exposure ►Scan Arc: 360 deg ►Scanning time as a function of Pitch Ratio ►Fan shape xray beam (wide) ►600-2000 detectors (fixed circular) Solid State. ►Display:Matrix (Selectable up to 512x512), Voxel (Selectable) ►Excels in 3D Multi-planar Reformation (MPR) ►Uses slip ring technology  th 6 Generation CT SCAN ► Double Helix CT – uses dual focus xray beam. Can cut scanning time by half and increase image resolution by x2. ► Nutating CT – uses a wobbling detector assembly. ► nutating slice CT image reconstruction apparatus and method generate a set of projection data using helical cone-beam scanning. ► Spiral rings- are electromechanical device that conduct electricity & electric signals through rings & brushes across a rotating surface onto a fixed surface.  th 7 Generation CT Scan (64/128/256 multi-slice CT) ►Whole Body with dynamic scanning ►Multiple Projection Exposure ►Scan Arc: 360 deg ►Multiple Detector Array ►Fan shape xray beam (wide) ►Fast scanning CT Scan ►Can produce High Definition topography ►Excels MPR and 3D reconstruction Pitch Ratio ► PITCH/SPIRAL PITCH ► The relationship between patient couch movement and x-ray beam width ► Spiral Pitch Ratio: ► PITCH = Couch movement each 360o ÷ Beam width ► MSCT Pitch: >1 ► Rationale: ► Multiple slices are obtained ► Z-axis location and reconstruction width can be selected after imaging ► CTA Pitch: 1:1: ► Decreases Z-axis resolution ► Rationale: a wide section sensitivity profile  Body planes ► Transverse/Axial Plane- A plane that divides the body into inferior and superior ► Sagittal- A plane passes vertically through the midline of the body dividing the body into right and left parts. ► Coronal- A plane that divides the entire body or a body part into anterior and posterior segments. Instrumentation: Detectors Detectors ►Receive and convert xray photons to digital signal ►Source to Detector Distance: 44” (110cm)  Detectors Detector Parameters: a. Capture Efficiency – Efficiency to receive photons. Controlled by detector size and interspacing. b. Absorption Efficiency – Efficiency to convert photons to light or ions. Controlled by detector material, size and thickness. c. Conversion Efficiency – Efficiency to Convert light or ions to digital signal.  Detectors Detector Parameters: d. Detector Dose Efficiency – Overall efficiency ( DDE= CapE + AE + ConE) e. Stability- the ability to maintain in a quality calibrated state. Fixed detector arrays are the most stable. f. Response Time- the speed of the detector to react/recognize incoming xray photon and recover for the next input. g.The Dynamic Range- describes the range of x-ray intensities a detector can differentiate. A high dynamic range provides the discrimination between small differences in x-ray attenuation.  Detectors Types of Detectors: a. Solid State Detectors Consist of Scintillation Crystals (materials capable of releasing light photons when struct by xray) and Photomultiplier tube. DDE = >50%  Detectors : Solid State ►Near 100% Absorption Efficiency ►Cannot be packed tightly (interspacing), Capture Efficiency is around 50%. ►UsesScintillation materials like Sodium Iodide [NaI] (100% AE, but has phosphorescence, decrease in Response Time.) ►Modern solid state detectors uses Calcium Flouride [CaF2], Bismuth Germinate [Bi4Ge3O12], Cesium Iodide [CsI], Gadolinium Ceramics [Gd], and Calcium Tungstate [CaWO4]. 90% Absoption Efficiency, but has no afterglow, increase in Response Time.  Detectors Types of Detectors: b. Gas-Filled Detectors Consist of pressurized Gas-Filled (Inert/Noble Gases) Ionization chambers and tungsten electrode plate (1.5mm apart). inert gas or noble gas, any of the elements in Group 18 of the periodic table. In order of increasing atomic number they are: helium, neon, argon, krypton, xenon, and radon. DDE= 90%. ►Fast Response Time ►Highly Directional and must be set in a fixed position oriented to the xray source. ►Cannot be used in 4th,5th,7th and Helical Scanners Computer Computer ►Controlsdata acquisition system(DAS), process and display, and storage. ►Access to software programs. CT SCAN DATA ACQUISITION CT SCAN ROOM LOAF OF BREAD ANALOGY A: POSITIONING OF THE PATIENT DIFFERENT PATIENT POSITION *HEAD FIRST SUPINE *HEAD FIRST PRONE DIFFERENT PATIENT POSITION *FEET FIRST SUPINE *FEET FIRST PRONE EXTREMITY POSITIONS: HEAD FIRST OR FEET FIRST CONSOLE ON GANTRY TABLE “DOWN” TABLE “UP” fast movement LASER GUIDE TABLE “IN” TABLE “OUT” TABLE “IN” fast movement TABLE “OUT” fast movement TABLE “DOWN” fast movement TABLE “DOWN” Console beside AUDIO (VOICE computer SPEAKER ACTIVATION) EMERGENCY STOP AUDIO (INSIDE CT ROOM) EXPOSURE TABLE MOVEMENTS  Instrumentation: Display Console Display Console – responsible for processing and display program controls.  DISPLAY CONSOLE ON COMPUTER IMAGE DISPLAY FOR FULL SCAN IMAGES CURRENT EXAM TAB IMAGE DISPLAY POST SCAN FOR SCANOGRAM VIEWING TAB FILMING TAB PATIENT REGISTRATION TAB 3D RECONSTRUCTION TAB TABLE POSITION AND HEIGHT TABLE POSITION AND HEIGHT B: PATIENT REGISTRATION  SELECTING EXAM PROTOCOL START EXAMINATION  SELECTING EXAM PROTOCOL PATIENT TYPE (ADULT OR CHILD) EXAM PROTOCOLS PATIENT POSITION SELECTION OF BODY PART UNDERSTUDY  SELECTING EXAM PROTOCOL SET OF PROTOCOLS MADE BY THE APPLICATION SPECIALIST OR RADTECH  SETTING UP OF PARAMETERS FOR SCANOGRAM TOPOGRAM OR SCANOGRAM TECHNICAL FACTOR FOR SCANOGRAM SCANOGRAM IF ALL SET SELECT SLICE THICKNESS “OK” SCANOGRAM LENGTH TUBE STARTING COMMENTS/ANNOTATION POSITION SCAN TYPE (CRANIO CAUDAL OR CAUDOCRANIAL AFTER SELECTING “OK”, SELECT “LOAD”, this means the table will be moved on pre exposure position TABLE LOAD  TABLE AFTER SELECTING MOVEMENTS “LOAD”on computer the “LOAD” button on After the table console will blink and movement is indication to start the done the table movement. exposure button will blink and indication to start the scanogram exposure. EXPOSURE  SCANOGRAM WINDOW WIDTH AND LEVEL OF IMAGE PATIENT DETAILS SCAN IMAGE SUSPEND BUTTON IF PATIENT SUDDENLY MOVE DURING SCAN Scanogram – Localization Image Scanogram ► A radiographic technique used for showing true dimensions by moving a narrow orthogonal beam of x-rays along the length of the structure being measured. ► Where you could also apply the particular planning for specific structures. ► The first image you will see in the CT Images. SCANOGRAM ► The scanogram is an image comparable to a conventional radiograph, but obtained using a CT scanner. ► Applications of the scanogram include patient preparation and positioning for CT scanning, archival of slice positions, and radiation therapy planning. The scanogram can also be used as a diagnostic tool. PLANNING PHASE The pink box indicates the field of view to be scanned, this will be manipulated by the radtech to achieve the images of the part understudy. SCAN SEQUENCE SETTING OF SCAN PARAMETERS RECONSTRUCTION TABS  Scanning Procedures ► Scanning protocols includes the following parameters: 1. Section Interval and Section Thickness 2. Scan Arc 3. Exposure Factors 4. Algorithm 5. Scan Field Size (FOV/ROI) Scanning Procedures 1. Section Interval and Thickness -Section Interval is the distance between scan sections. -Section thickness is the width of the volume of the tissue being examined (Voxel).  Scanning Procedures Scan Arc: -The arcs of xray exposure. Travel of xray tube in the gantry. -halfscan (180), Fullscan (360), Overscan (>360). Scanning Procedures Exposure Factors: -120 KVp – most common -KVp selectable (80, 120, 140) -30mA – 1000mA (manufacturer’s specs) Scanning Procedures Algorithms -Reconstruction Algorithms for reformatting and targeting. -Reconstruction Algorithms for windowing and Filters.  Scanning Procedures Scan Field Size -also known as Field of View (FOV) or Region of Interest (ROI). -must be set to accommodate the size of the part under examination. -Common Field Sizes: Head (25cm). Small bodies (35cm). Large bodies (48cm)  SCANNING PHASE 1. Before scanning, select load 2. After selecting load button, press table movement on console the table will move to the original position before the scanogram. TABLE MOVEMENTS  TABLE SCANNING PHASE MOVEMENTS After the table movement is done the exposure button will blink and indication to start the full scan. EXPOSURE  SCANNING PHASE In scanning process you will see the images being retrieved to the computer in a slice manner.  VIEWING After scanning and the images are completely retrieved, select viewing tab. In viewing tab all the images will be displayed with different settings that will be used for checking and viewing of the physician. Selection of the patient Selection of the images with different reconstructed image Image display per folder VIEWING  VIEWING Grid Application- localizing the target area Cursor – measurements, outlining area of interest, marker of area. i. Density Contouring- HU selection ii. Radiation Therapy Planning- integration with dosimetric data. Reverse Display – directional reverse or Negative reverse.  CT Numbers/Hounsfield Unit ►Tissuedensity values that represents the linear attenuation coefficient equivalent of various tissues. CT NUMBER FOR VARIOUS TISSUES TISSUES APPROXIMATE CT NUMBER Dense bone 3000HU Bone 1000HU Liver 40-60HU Muscle 50HU White matter 45HU Gray matter 40HU Kidney 30HU Blood 20HU CSF 15HU Water 0H Fat -100HU Lungs -200HU Air -1000HU Grid Application Cursor  VIEWING Magnification – up to x3 without distortion. Suppression – deletion of unwanted parts from reconstructed image. Annotation – labeling and text adding Histograms – HU analization by use of a bar graph. 3D Imaging  FILMING If the images are requested to be printed, Print film go to filming tab, here the film size, number of Print status images in a film and orientation can be manipulated. Print parameters  3D RECONSTRUCTION In this tab, the raw data collected can be made into different data, different parameters such as slice thickness and image distance can be manipulated.  Image Reconstruction -Reformatting – reconstruction of images from the information from a series of sectional images to produce an image of a section not actually scanned. -Targeting – magnification reformatting of the selected area with minimal distortion. 3D RECONSTRUCTION CT number/Hounsfield ► CT number/Hounsfield unit: the numeric information contained in each pixel ► The Hounsfield scale or CT numbers, named after Sir Godfrey Hounsfield, is a quantitative scale for describing Radiodensity.  CT Numbers/Hounsfield Unit ►Tissuedensity values that represents the linear attenuation coefficient equivalent of various tissues. CT NUMBER FOR VARIOUS TISSUES TISSUES APPROXIMATE CT NUMBER Dense bone 3000HU Bone 1000HU Liver 40-60HU Muscle 50HU White matter 45HU Gray matter 40HU Kidney 30HU Blood 20HU CSF 15HU Water 0H Fat -100HU Lungs -200HU Air -1000HU Image Quality - CT image quality is primarily controlled by resolution and noise. - Density and Contrast can be manipulated by the RT.  Image Quality ► Factors Affecting Image Quality 1. Density and Contrast (window width) 2. Resolution 3. Interface Artifacts 4. Noise 5. Motion (Temporal Resolution)  Image Quality 1. Density and Contrast – controlled primarily by window level and window width. Must demonstrate 0.4% contrast difference. a. Window width – range of scale of contrast. Head (70-200HU), Abdomen (250-450HU), Lungs (1200-1400HU), Inner Ear (2000-4000) Image Quality Image Quality Resolution – controls primarily by the matrix size, pixel and voxel sizes. High contrast objects must use smaller pixel sizes, while low contrast objects is better to be image using larger pixel sizes. Increase Resolution, Increases Image Quality  Image Quality Resolution: ► A Pixel: a picture element ► Each cell of information ► Two-dimensional B. Voxel: a volume element ► The tissue volume C. Matrix Size – Total number of pixel displayed (DRT). ► rows and columns of pixels displayed on a digital image  Image Quality ► Field of view (FOV): the diameter of image reconstruction ► FOV increased, fixed matrix size ► Result: increase/larger pixel size ► Fixed FOV, increase matrix size ► Result: decrease/smaller pixel size  FOV AND MATRIX SIZE FOV increased, fixed matrix size Fixed FOV, increase matrix size Result: increase/larger pixel Result: decrease/smaller size pixel size MATRIX PIXEL MATRIX PIXEL  Image Quality: Artifacts Interface Artifacts: ►occurs when the two objects have more than 60% in difference in attenuation coefficient. ►it can be undershoot or overshoot ►produces a star pattern artifacts ► Increase in Interface Artifacts, Decreases Image Quality  Image Quality Noise Image graininess from quantum mottle and statistical fluctuation in the information detected. standard normal noise values are 3-5% of the image. ruled out by water phantom test Increase in Noise, Decrease in Image Quality  Image Quality Motion (Temporal Resolution) – image blurring resulting from voluntary and involuntary movements. Increase in Motion, Decrease in Image Quality Unwanted aberration on CT images: CT artifacts 1. Motion 2. Metal/Star 3. Beam Hardening 4. Partial Volume Effect 5. Ring Artifacts THANK YOU