MRS Physics - Lecture notes 2-14 PDF

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PHYS1250 taught by Lachlan Rogers ...

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MRS Physics & Radiation protection Basis of medical instruments General components of a medical instrument

1. Source (e.g x rays) 2. Medium (sample) 3. Filter (exclude info that is not relevant) 4. Detector (radiation from source detected as energy) 5. Signal processing (conversion of data to be readable) Voltage is an energy difference between 2 points (for a charge) Current is the amount of charge (measured in Amp) Intrinsic efficiency (sensitivity)

Detector efficiency factors - Type of crystal - Thickness of crystal - Energy of photons High sensitivity allows for faster image, reduced dose Dead time (resolving time) - Time it takes for detector to record an event and ‘reset’ ready to respond to the next one - If a second photon arrives in this time it is not recorded- detector is ‘dead’ - If photon reset dead time Yes (para - ‘Dead time’  Saturation of detected

Energy discrimination ( E resolution) - Ability of a detector to distinguish between photons of different energy - No detector can perfectly detect the exact energy of photons - Measured by Full Width at Half Maximum (FWHM) - Narrower peaks are better, wider makes it hard to separate!! The diagnostic and superficial x-ray spectrum

Source of MRS x-rays is an x-ray tube Characteristic x-ray - Incoming high energy electron knock inner shell electron. - Vacancy filled by higher energy shell electron - Lost energy released as x ray!! Bremsstrahlung x-ray - Incoming energy gets deflected from nucleus - Slows negative electrons and loses energy when it deviates from path.

Detectors in DR Four main steps of digital imaging - Generation (Acquire)  Detector exposed to x-rays, energy absorbed and transformed, charges recorded into grey scale and represents xray deposited -

Processing  Organising raw data into clinically meaningful image

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Archiving  Images sent for STORAGE!, file containing patient demographic information

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Presentation of image  Images are viewed digitally, images can be manipulated (e.g contrast, zoom, invert etc)

Digital radiography and the types - Computed Radiography (CT)  Trapped electrons exposed to light known as photostimulable luminescence (PSL), light emitted generates image -

Indirect Digital Radiography  Light emitted when the electrons fall from conduction band to valence band to produce image

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Direct Digital Radiography  Electrons in conduction band are collected and are used for image production. Crystals used in process include amorphous selenium.

Digital radiography; flat-panel thin film transistor digital detectors - Designed as matric of detector elements which are regarded as pixel. - Matrix has rows and columns which are sensing areas from pixel - Each pixel contains a TFT (switch), storage capacitor and sensing area= photodetector (indirect detector) and photoconductor (direct detector) Detection and conversion of x-rays into electrical charges - Indirect flat panel TFT detectors Light sensitive photodetector to produce charge - Direct flat panel TFT detector Amorphous- Se photoconductor to convert x-rays directly to electrical charges

Indirect digital systems; flat panel TFT digital detectors (SILICON) 1. X-rays converted to light using scintillator 2. Light is converted to an electronic signal using amorphous silicon (a-Si) photodiode (photodetector) 3. Charge is stored in a capacitor and readout by TFT 4. Charge is converted to a digital signal by ADC Cesium iodide= structured phosphor Gadolinium oxisulphide= unstructured phosphor

-Structured needle like structure -Unstructured produced large -Increases number of x-ray photon and reduces amount of light= spatial resolution lateral scattering of photons -Absorption is 4x higher Amorphous silicon - Phot etector is array of a-SI photodiodes - Purp is to convert light produced by scintilla ctronic char (proportional) Capacitor - Acts as a storage and collection point for electrical charge produced in a-Si photodiode array. Direct Digital Systems; flat-panel TFT digital detectors (SELENIUM) - Uses a photoconductive material as x-ray converter (layer of a-Se) Fill factor - Percentage of a pixel area which is sensitive to the image signal - Affects both spatial resolution and contrast Exposure latitude (dynamic range) - Respond of detector to radiation falling upon it. - As radiation dose increases= image quality improves Spatial resolution - How clear/sharp details are in object - Depends on spacing and size of pixels - Best done with a-Se detector Detective Quantum Efficiency - How efficient a detector Is at converting radiation falling upon it into useful signal - The DQE for perfect digital detector is 1 or 100% (no loss of info) - At 70 kVp 67% for Direct detector and 77% for indirect detector - At 120kVp  37% for direct detector and 52% for indirect detector

What is the energy band theory? Is a theory of valence electron moving in a periodic potential field of crystalline lattice. Single atoms have a discrete energy spectrum, which means they only supply discrete energy - Ranges of energy that electron can have within crystal is ENERGY BANDS - Ranges of energy electron may not have within crystal is FORBIDDEN BANDS (band gap) Valence Band  outer-shell electrons considered to occupy valence band Conduction band  When outer-shell electrons leave atom are considered to move to conduction band

Image quality in DR Radiographic contrast - Describes the differences in shades of grey Subject contrast Detector contrast Differences in terms of x-ray intensity from Differences recorded and displayed patient (e.g thickness of tissue, density, - Detector contrast= capabilities of det contrast agents, atomic number) to produce image contrast - Current time Product= no real - Dynamic range= Expresses rang effect on subject contrast as long as input signals over which image mAs is high enough to produce receptor is sensitive sufficient number of x-ray - Applied potential (kVp)= directly increasing kVp resulting in reduced contras photoe scatter

Radiographic detail Refers to clarity and sharpness of image (smaller pixels give better image quality) - More lp/mm observed= better spatial resolution of system - Factors affecting  pixel size, crystal material, FSS, distance, voluntary or involuntary motion and applied potential - As focal spot increases, area which x-rays are produced also increases= penumbra (blurring) - Focus Detector Distance = Resolution increases as FDD increases - Object Detector Distance= Patient and detector to be as close together as possible to maintain normal image production. (e.g think of moving lamp closer/further away from object, what does it do to the object?) Motion - Voluntary movement that is done with conscious effort, Infants and elderly move a lot , Immobilisation devices are option to reduce movement - Involuntary movement is movement that cannot be controlled, physiological activities such as heart beating, respiratory. Radiographic distortion Misrepresentation of image Shape distortion Refers to a non-true representation of an image - Caused by angulation!! - X-ray beam must be centred and perpendicular to both

Size distortion Occurs due to magnification - X-ray beam must be centred and perpendicular - Object-detector distance needs be small!! - Factors influencing inability to place anatomical part on detector, individual patient conditions

Scatter Scatter relates to the change in direction of photons after they have interacted with matter (Compton) Reducing scatter - Reduce the kVp - Reduce field size - Decrease tissue thickness - Use air gap technique - Use radiographic grid

Radiographic grids - Parallel  Strips are arranged parallel to each other and perpendicular to incident radiation beam - Focused  Strips are angled obliquely - Crossed  Uses two layers of parallel or focused strips at right angles - Moving  Grid is moved reciprocally to remove any shadows Air gap technique - Patient is deliberately moved away from detector about 10-15cm. - Improves image contrast resolution reducing amount of scattered radiation that reaches image detector

Computer Tomography (CT) Compare Computed Tomography to radiography in terms of: resolution, contrast and patient dose. DR -

NM Give 3D images of high contrast

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RT Combined with PET Give anatomical detail

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Used to provide images for treatment planning

ADVANTAGES OF CT: -

Reduces superimposition (cross sectional data acquisition) Improved subject contrast (differences between tissues that differ in density) Single breadth volume scans Multiplanar imaging (can be viewed in various planes)

DISADVANTAGES OF CT: -

Lower spatial resolution (small focus detector) High radiation dose (100-1000x more) Artifacts (streak)

Explain what is meant by the terms: ray, view and scan Ray  Part of the x-ray beam that falls on one single detector, gives info about attenuation that has occurred. View  Collection of rays= generate profile Scan  Collection of views from around patient, one scan produces one slice through body.

Explain and describe what is meant by CT numbers and be able to compare CT numbers of common tissues in the body based on density and atomic number CT numbers (produces data based on tissue ability to absorb x-ray beam): -

Related to linear attenuation coefficients of tissues in slice Each pixel is assigned a CT number Range of CT numbers known as the Hounsfield Scale  Higher atomic number and density is responsible for increase attenuation= higher CT values  Lung tissue and fat have negative values due to lower density. Typical range:

-1024HU to +3071 (metals) Discuss the significance of water in the calculation of CT numbers. -

CT numbers established with attenuation of water as a reference. Attenuation (density) > water= positive CT numbers

Discuss the three steps in the formation of CT images. 1. Data acquisition - Collection of info from patient to produce image.

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X-ray scanned around body and detector gives measure of amount of attenuation

2. Image reconstruction - Scan of particular body area is reconstructed into an axial image 3. Image display, manipulation, storage, recording and communication - Image is displayed on computer. - Image can be manipulated via adjusting contrast or saturation etc - Image details are shown and can be recorded

Describe the two elements in the data acquisition phase. Beam Geometry (Parallel, fan and spiral geometry) -

Size Shape Motion of the beam and its path

Components -

Physical device that shape and define beam Detectors measure x-ray transmission through patient Conversion of info into digital data for input into computer

Identify the different generations in CT scanning. First Generation scanners -

‘Rectilinear pencil beam scanning’ Uses parallel beam geometry Translate-rotate scanning motion highly collimated x-ray beam, repeats for 180 degrees.

Second generation scanners -

‘Rectilinear multiple pencil Fan beam geometry Translate rotate motion Linear detector array

Third generation scanner -

‘Continuously rotating fan beam’ Fan beam geometry

beam’

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Continuous 360 degree rotation

Fourth generation scanner -

Wide fan beam geometry Continuous 360 degree rotation Stationary circular detector array

Fifth generation scanners -

Electron beam CT scanner Produce high resolution images of moving organs free of artifacts No mechanical motion No x-ray tube used.

Sixth generation scanners -

Dual source CT scanner Two x-ray tubes Two sets of detectors Designed for cardiac CT (more resolution)

Seventh generation scanners -

Flat panel volume CT Detectors wide area flat panels

Discuss the advantages of spiral acquisition method over contiguous (slice by slice) CT scanning. Advantages of Spiral acquisition -

Very fast scanning times Acquire data while table is moving Address limitations of slice by slice

Explain the concept of pitch in spiral scanning. -

Describes how table moves in relation to scanning width

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Beam pitch of 1.0= no overlap or gap Beam pitch 1= interspersed acquisition

Describe the imaging system used in CT scanning. -

Used to produce x-rays, shape and filter x-ray beam, detect radiation passing through cross section, convert the transmitted photons into digital information Gantry houses the x-ray tube, collimators detectors etc Gantry aperture= opening, and gantry tilting (12-30 degrees) allows to accommodate for all patients CT generator has kVp setting of 80-140kVp, mA of 100-400

Discuss the important roles of filtration and collimation in CT scanning Filtration -

Removes low energy x-rays Shapes the energy distribution across radiation beam to produce uniform beam hardening

Collimation -

Shape beam and define slice thickness Restrict beam to cross section

Define the term ‘slice sensitivity profile’, discuss how its shape can be used to determine contrast, resolution and dose and describe how it changes with pitch. -

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SSP  Describe system resolving power in perpendicular direction with respect to reconstructed planes. Allows measurement of axial resolution of helical CT systems. Contrast  Highly attenuating ball is greater if bearing is in centre, contrast decreases as bearing moves towards edges Dose  Slice on body, receives the most radiation dose Pitch  Wide curve= spatial resolution bad, Curve low= reduced contrast, Area less= reduced dose

Describe the essential characteristics of detectors in CT Detectors capture into and convert into electrical signals, which are further converted into coded info Efficiency Capture efficiency

Absorption efficiency

Conversion efficiency

- Obtained photons transmitted from patient

- Number of photons absorbed by detector - Depends on atomic number, size etc

- Conversion of photons into a signal

Response time - Speed which detector can detect x-ray event and recover to detect again Dynamic range - Ratio of largest signal to smallest signal Afterglow - Persistence of image after radiation has been turned off. CT should be 100ms. Stability - Steadiness of detector response. Allows for artifact free images. Detectors used today in CT are solid-state scintillation (indirect) systems Describe the use of multi-slice detectors in CT including reference to matrix and adaptive arrays. - One detector with rows of detectors - Influences the thickness of slices. Matric array -

Known as ‘fixed’ and ‘isotropic’ Detectors are equal and uniform in size

Adaptive array -

Known as ‘anisotropic’ Detectors are of different sizes Each detector is 1.25mm wide, 8 detectors create 2.5mm thickness etc

Describe the four types of data used in CT.

Measurement (scan)

Raw data Image reconstructed

Convolved data

1. Measurement data= Comes from detectors. Subject to correction 2. Raw data= Pre-processed measurement data and subject to image reconstruction 3. Convolved data= Filtered and back projection. Improves quality through blur removal 4. Image reconstruction= Digital filters applied to supress noise and improve detail Four basic principles of reconstruction of image Algorithms - Solutions to mathematical problems in CT require computered algorithms to reconstruct image Fourier transform - Mathematical function that converts signal in spatial domain to a signal in frequency domain Convolution - Digital image processing technique to modify images through a filter function - Multiplication of overlapping portions of filter function. Interpolation - Estimate value of function from known values. Determination of slices in CT imaging Describe the process of windowing and the effects of changing window level and window width. Windowing is the process of remapping colours to improve certain aspects of an image. Window Width - Range of CT numbers to be displayed - 400-2000HU encompass tissues of differing attenuation (wide) Contrast easily - 50-350HU display soft tissue of similar densities (narrow) changed Window Level -

Midpoint of the range of CT numbers.

Narrow WW is very sharp contrast to point where lungs (black), bone (white), and liver (grey)

WL on lungs are displayed as white, WL on pelvis is black.

Describe the five image quality characteristics of a CT image. - Artifacts= Mimic pathology and can degrade image quality to nondiagnostic levels - High contrast resolution= Over contrast can create ‘noise’ in image - Low contrast resolution= Cannot distinguish between certain structures - Noise= Creates grainy appearance, reducing resolution on image - CT Number uniformity/accuracy= Absorption of x-rays by tissue - Temporal resolution= Duration of time for acquisition of single frame of process CT has significantly worse high contrast spatial resolution

Discuss factors affecting image quality in a CT image. Geometric factors Refers to data acquisition processes and devices - Focal spot size= Larger focal spot= worse resolution - Focus detector distance= 85cm, increased magnification= reduced resolution

- Object-detector distance= Increased magnification = reduced resolution Detectors - Use of smaller detectors improves spatial resolution - Slice thickness close to object size improves resolution, vice versa, this would be called ‘partial volume effect’ - As number of projections increased, more data is available for reconstruction - Greater number of pixels for field of view= better resolution - Greater pitch reduces resolution and increase in SSPW. Reconstruction algorithms - Photo flux= Depends on kVp, mAs and beam filtration and patient size. - Thicker slices use more photons and have better SNR, however, narrow slice reduces scattering and improves resolution - Faster gantry rotations result in reduced mAs= reducing contrast resolution - Increasing FOV increases SNR and contrast resolution.

Describe some common artifacts that can occur in CT. Is a discrepancy between the reconstructed CT numbers in image and true attenuation coefficient of object. Beam hardening - X-ray beam of different energy ranges passes through object, leaving selective attenuation of lower energy photons. - Streaking= Dark bands, polychromatic x-rays hardened at different rates - Cupping= Falsely bright appearance along periphery of object. Beam less attenuated in centred compared to edges. Partial volume averaging - CT number in each pixel is proportional to average linear attenuation coefficient of beam. - Use of thinner slices to enhance anatomical structures - Partial volume artefacts leads to misdiagnosis when presence of anatomic structures are not suspected. - E.g top of head, cranium shares number of voxels in brain tissue, causing details of brain to be lost because large coefficient of bone dominates Photon starvation - Parts of individual projection can be noisy due to insufficient amounts of photons passing through widest part of patient Ring artifacts - Occurs in third generation CT scanners, multiple bad detectors producing different signal outputs. - To correct, detectors can be recalibrated, and eliminated with software such as balancing algorithm. Helical artifacts - Occur as a result of pitch, circular objects appear elliptical - Artifacts increases= pitch increases

Discuss why patient dose reduction is particularly important in CT. - CT has high doses and increased absorption to...