SPECIAL IMAGING/ADVANCED IMAGING INTRODUCTION TO DIGITAL RADIOGRAPHY Tomography- Chapter 15, pg 265- Bushong CT –Chapter 23 MRI – Chapter 24, Bontrager CR/DR- Chapters 1,4, 7-Carter Chapter 12 - Fauber

FACTS ABOUT TOMOGRAPHY Conventional radiography-structures are superimposed Before widespread use of CT and MRI, tomography was the procedure of choice TOMOGRAPHY isolates and visualizes a particular section of the body. Blurs out structures above and below the area of interest

DO THE “TOMOMOTION” WITH ME! X-RAY TUBE IS ATTACHED TO THE IMAGE RECEPTOR (BUCKY). TUBE MOVES IN ONE DIRECTION, BUCKY IN ANOTHER

FULCRUM (POINT OF PIVOT) FACTS ABOUT THE FULCRUM LIES IN OBJECT PLANE OBJECTS ABOVE AND BELOW THE FULCRUM ARE PROJECTED TO VARIOUS LOCATIONS ON IMAGE RECEPTOR FULCRUM IS USUALLY ADJUSTABLE DETERMINES WHAT SECTION OF THE BODY IS NEEDED TO BE VISUALIZED

TOMOGRAPHY ANGLES Determines the thickness of the “cut” More Angle = thinner cut

THE ARC SHOULD BE LONG ENOUGH TO ACCOMMODATE EXPOSURE TIME

WORDS TO AVOID CUT! SLICE! SECTION IS THE BEST DESCRIPTION!

TYPES OF TOMOGRAPHY UNITS Conventional Movement Linear Circular Elliptical Hypocycloidal Trispiral

TYPES OF TOMOGRAPHY UNITS Zonography Panoramic Tomography All tomography exams increase patient dose. A 16 film tomographic exam can equal patient dose of several rad. Stereoradiography Magnification Radiography

CT Conventional tomography produces coronal and sagital images CT produces transaxial images

CT SIMPLIFIED Rotating x-ray source Fan shaped beam Multiple stationary detectors Tube rotates around body-translation Body attenuates x-ray beam The attenuated beam (pixel) is assigned a CT number (Hounsfield unit) Computer calculates attenuation of the individual voxels- three dimensional tissue volumes (height, width, depth)-pg 732-Bontrager

CT HISTORY (KEY WORDS) Godfrey Hounsfield EMI scanner 1st generation 2nd generation 3rdgeneration 4th generation 5th generation

COMPONENTS OF A CT SCANNER GANTRY Detectors, track for x-ray tube Scintillation, gas filled, High frequency circuit (Low frequency circuit is located in CT room) X-ray tube (8,000,000+ HU) Collimation Two collimators prepatient Determines dose profile and patient dose and predetector Determines sensitivity profile and slice thickness

HIGH VOLTAGE GENERATOR ACCOMMODATES HIGHER ROTOR SPEEDS POWER SURGES OF PULSED SYSTEMS IN THE GANTRY

Patient support table Table indexes ( moves at a preset distance when the exam begins) Movement must be reproducible within 1 mm In spiral CT table moves continuously Weight limit of 450 lbs, made with low atomic number

$$$$COMPUTER SYSTEM$$$$ 1/3 the cost of the entire system The “brains” of the CT unit May calculate up to 250,000 mathematical equations simultaneously Reconstruction time=end of scanning to image appearance

Operating Consoles Dual Monitors Operator-turns CT scanner on and off Selects and can control the protocol which is Predetermined Includes kVp, mAs, pitch, FOV,slice thickness, table indexing, reconstruction, algorithms and display windows KVP and mAs preselected as is focal spot size kVp usually in excess of 120 kVp Usual mA station is 100 mA in continuous beam and several hundred mA in pulsed beam Physicians viewing console

SPATIAL RESOLUTION DEPENDENT ON: Focal spot size (not operator controlled: pre determined) Beam collimation Detector size Matrix and pixel size(Larger matrices with smaller pixels= better spatial resolution)

MRI Magnetize the atomic nuclei in hydrogen atoms Bombard these atoms with radiofrequency waves Hydrogen atoms absorb RT and re-emit back as radiowaves. Signals are sent to computer to construct an image

DIGITAL RADIOGRAPHY CR AND DR REFERENCES: Digital Radiography and PACS, Carter Rad. Imaging and Exposure, Fauber

CR VS DR CASSETTE BASED DR CR: -Imaging plate composed of photostimulable phosphor -Barium fluorohalide crystals doped with europium -Do not fluoresce when hit by x-ray photons, store energy instead. -laser releases the stored energy in a form of light -Collected by photomultiplier tube and converted to digital data. CASSETTE-LESS SYSTEM INDIRECT CAPTURE X-RAYS CONVERTED TO LIGHT LIGHT DETECTED BY AN AREA CCD OR TFT (THIN FILM TRANSISTOR) CONVERTED TO ELECTRICAL SIGNAL DIRECT CAPTURE/CONVERTS X-RAY INTO AN ELECTRICAL SIGNAL DETECTOR REPLACES THE CONVENTIONAL BUCKY

PROCESSING Film – image produced by interaction of the chemicals with the exposed silver halide crystals CR- Computer near the reader (digitizer) DR –computer next to the console

TECHNIQUE FILM – NON-LINEAR RESPONSE (THINK OF CHARACTERISTIC CURVE) CR/DR – kVP influences subject contrast but radiographic contrast is controlled by the LUT CR/DR –mAs affects pt. exposure and image noise but density is controlled by image processing algorithms (with LUT) CR/DR – more sensitive to scatter

LUT???? LOOK-UP TABLE SEE PAGE 115-116 (Carter et al) REMEMBER EACH PIXEL HAS IT’S OWN GRAY VALUE pg 73 (Carter et al) Pixel is a picture element Contains bits of information Make up the matrix http://photo.net/equipment/digital/basics/pixels.jpg To be continued in LAB on March 31/April 4