Transcript alternative imaging procedures

SPECIAL IMAGING/ADVANCED IMAGING
INTRODUCTION TO DIGITAL
RADIOGRAPHY
Tomography- Chapter 15, pg 265Bushong
CT –Chapter 23
MRI – Chapter 24, Bontrager
CR/DR- Chapters 1,4, 7-Carter
Chapter 12 - Fauber
FACTS ABOUT TOMOGRAPHY
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Conventional radiographystructures 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!
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X-RAY TUBE IS ATTACHED TO THE
IMAGE RECEPTOR (BUCKY).
TUBE MOVES IN ONE DIRECTION,
BUCKY IN ANOTHER
FULCRUM (POINT OF PIVOT)
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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
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Determines the
thickness of the
“cut”
More Angle =
thinner cut
THE ARC
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SHOULD BE LONG ENOUGH TO
ACCOMMODATE EXPOSURE TIME
WORDS TO AVOID
 CUT!
 SLICE!
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SECTION IS
THE BEST
DESCRIPTION!
CT
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Conventional tomography produces
coronal and sagital images
CT produces transaxial images
CT SIMPLIFIED
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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)
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Godfrey Hounsfield
EMI scanner
1st generation
2nd generation
3rdgeneration
4th generation
5th generation
COMPONENTS OF A CT SCANNER
GANTRY
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Detectors, track for x-ray tube
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Scintillation,solid state detectors
High frequency circuit (Low frequency circuit is
located in CT room)
X-ray tube (8,000,000+ HU)
Collimation
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Two collimators
prepatient
 Determines dose profile and patient dose
and predetector
 Determines sensitivity profile and slice
thickness
HIGH VOLTAGE GENERATOR
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ACCOMMODATES HIGHER ROTOR
SPEEDS
POWER SURGES OF PULSED
SYSTEMS
IN THE GANTRY
Patient support table
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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
$$$$COMPUTER SYSTEM$$$$
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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
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Dual Monitors
Operator-turns CT scanner on and off
Selects and can control the protocol which is
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KVP and mAs preselected as is focal spot size
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Predetermined
Includes kVp, mAs, pitch, FOV,slice thickness, table
indexing, reconstruction, algorithms and display
windows
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
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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
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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
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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.
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CASSETTE-LESS SYSTEM
INDIRECT CAPTURE
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X-RAYS CONVERTED TO
LIGHT
LIGHT DETECTED BY AN
AREA CCD OR TFT (THIN
FILM TRANSISTOR)
CONVERTED TO
ELECTRICAL SIGNAL
DIRECT
CAPTURE/CONVERTS XRAY INTO AN ELECTRICAL
SIGNAL
DETECTOR REPLACES THE
CONVENTIONAL BUCKY
PROCESSING
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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
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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????
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LOOK-UP TABLE
SEE PAGE 115-116 (Carter et al)
REMEMBER EACH PIXEL HAS IT’S OWN
GRAY VALUE pg 73 (Carter et al)
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Pixel is a picture element
Contains bits of information
Make up the matrix
http://photo.net/equipment/digital/basics/pixel
s.jpg