physics week 2

8/6/2019 Physics Week 2

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PHYSICS WEEK 2 LECTURE 2

Limitations of conventional radiographic images

y Processing time time for chemical reactions to occur, table to viewing box etc. => more exposurethan they need

y Delayy Hard copy image enhancement not possibley Physical Storagey Image noisey Other difficulties

o Difficult to image both soft tissue and bone structures in the same image (bones attenuatemore than soft tissues)

o Difficult to differentiate between the subtle differences of soft tissues (shades of grey longexposure, low contrast) => hard to distinguish soft tissue, unless giving the patient longer

exposure

o Unable to gather quantitative info about attenuation characteristics of anatomyo Image is processed to permanent as is cant do anything e.g. enhance

Digital Imaging Techniques applied to (technology always improve and prices always go down)

y Computed Tomography (CT)y Ultrasonographyy Nuclear Mediciney Magnetic Resonance Imaging (MRI)y Digital radiography (DR)y Digital fluoroscopy (DF)

Detectors

y Linear array of detectors or

y Photostimulable phosphor (area detector)Image characteristics

y Analogue imageso Continuous level of brightness

y Digital images a matrix of pixelso Pixels = picture element

Each contains a number to represent the shadeo Characteristics of matrix determine resolution of image o For computer system: (FOV = field of view)


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Data representation in computers

Computers deal fundamentally with binary date i.e. data has only 2 possible values 0, 1

y These binary digits are called BITs and are used to represent numeric valueso E.g.

o Common subunit is a BYTE = 8 bits, WORDS consist of 2 or more bytes (we dont use WORDSanymore)

o Need many bits to represent large numbers (dynamic range) e.g. 8 bits can represent 2^8 =256 distinct values (0 through 255)

Dynamic Range

y Dynamic range is described as the number of shades of grey that can be represented


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CT Number

y In CT imaging numeric value of each pixel is a CT number ofHounsfield unit (HU) y A normalised value of the calculated X-ray absorption coefficient of a pixel in a computed tomogram,

expressed in HounsfieldUnits, where the CT number of air is -1000, and that of water is zero

Window level and width (range)


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Digital radiography (DR)

y No film is used (although we can print for patients)y Uses radiation detector (e.g. photostimulable phosphor, scintillation detector array commonly use)

whose electrical output is proportional to the radiation intensity

y Photostimulated luminescence (PSL) is the release of stored energy within a phosphor by stimulationwith visible light, to produce a luminescent signal

y Scintillation detector array incorporates Scintillation crystal photodiode assemblyScan projection radiography (SPR)

y Early implementation ofDRy X-ray projection radiograph by scanning with a fan beam & a linear array detectory The spatial resolution along the fan beam is determined by the detector aperture, geometric

magnification, and focal spot size

y The spatial resolution in the scanning direction is determined by the fan beam thicknessy Advantages reduced scatter radiation, better image qualityy SPR requires high heat capacity X-ray tubey 500 to 2000 mAs tube current requiredy Can be performed on a computed tomography system by translating the patient through the CT

gantry aperture while transmission measurements are obtained with a fixed X-ray tube position

y SPR uses a linear scintillation detector arrayy The most widely used scintillation crystals is Nal(TI) (Sodium iodide doped with thallium). Other

inorganic alkali halide crystals are: CsI(Tl), Csl(Na), (Csl)(pure). Some non -alkali crystals include:

CaF2(Eu) andCdWO4

y Scanning time is the limitations of SPR > image blur due to patient movement

Computed Radiography

y Use an area beam and photostimulable phosphory The phosphors used to coat the screen are europium-activated barium fluorohalide crystals

(BaFX:Eu2+ where X is a halogen of either iodine or bromine)

y Charge coupled devices (CCDs) are photosensitive silicon chips and are also used more increasinglyuse nowadays


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y LP Layers Luminous PhosphorLayerso Protective protects phosphoro Phosphor/active PSP-barium fluorohalideo Reflective sends light forward when release in the readero Conductive absorbs/reduces static electricityo Colour Absorb stimulating light (laser)/reflects emitted lighto Back support protects the back of cassetteo Barcode Match image with patient

Digital radiography creates latent image in photostimulable phosphor

y Reading the IPo Red laser light scans in horizontal scanning pattern at 2eVo Laser scans multiple times as IPo Light produced detected by photomultiplier

y Digitizing device that convert to digital signalo Phosphor storage center is scannedo Released electrons enter digitizer divides the analog signal into squares (matrix)o Each square is assigned a number based on the brightness of the squareo Square is called a pixel (picture element)


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Spatial Resolution determine how fine we can detect features

y Film screen = 10 line pairs per mmy CR (computed radiography) = 2.55 to 51 line pairs per mm (lp/mm)y Less detail in CR but more tissue densities seen given the appearance of better detaily Wider dynamic recording range (improving due to technology always improving e.g. pixels)

Speed how much we need to expose film to produce image

y Film determined by size and layers of crystals and phosphorsy CR amount of photostimulable luminescence given off = 100 film speed screen (approx.) good

standard film speed

y Increase > in low lighty Decrease > in bright light

Similarities between CR and film radiography

y Same x-ray tube and generatory Still select optimum kVp and mAs y Accurate positioningy Use cassette or image receptor (different things in cassette now) y There is still a latent image which can be processed into a manifest image (using laser beam)

Differences between CR and film radiography

y Imaging plate rather than intensifying screen/film y Photostimulable phosphor-europium activated barium fluorohalide phosphory 200 screen speed equivalenty Phosphors absorb photosy Capable of wider latitudes (dynamic range) = better visualisation of soft tissues and bone in the same

image

y Film made of minute strands of black metallic silvery Digital image = rows and columns called a matrix


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QUESTIONS

Define the following

y Matrix

y Pixel

y Voxel

y Dynamic range

y FOV

Define windowing, and explain window level and window width. Give their typical values for soft tissue,

bone and lung

Your holiday picture was taken using a 4 mega pixel digital camera. What is the pixel size of this picture if

the print size was 30cm X 30cm? What would be the pixel size if you wanted to make a poster of 1sqaure

meter of the same p icture?


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List some of the disadvantages of conventional radiography and explain how these difficulties are overcome

using digital radiography (DR)

A dedicated DR chest imaging system has a 12 bit dynamic range. How many shades of grey can it render?

Describe the process of reading the IP

Describe the differences and similarities between CR and film radiography

physics week 2

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