Presentation on theme: "FRCR: Physics Lectures Diagnostic Radiology"— Presentation transcript:
1 FRCR: Physics Lectures Diagnostic Radiology Digital detectors, tomography and dual energy imagingDr Tim WoodClinical Scientist
2 Overview What is a digital image? Computed Radiography (CR) The advantages of digitalThe disadvantagesComputed Radiography (CR)The theory of CRThe advantages and disadvantagesDigital Radiography (DR)The theory of DRDR vs CRTomographyDual energy imaging
3 The story so far…We know how X-rays are made in the X-ray tube and how they interact with the patientWe know how we control the quality and intensity of the X-ray beam, and hence patient dose, with kVp, mAs, filtration and distanceWe discussed the main descriptors of image qualityContrastSpatial ResolutionNoiseDiscussed ways to improve contrast by minimising scatter and using contrast agentsRemember, there is always a balance between patient dose and image quality – fit for the clinical task!Film is a dying medium for X-ray imaging…
5 What is a digital image?A digital image can be thought of as an array of pixels (or voxels in 3D imaging) that each take a discrete valueThe value assigned is dependent on the X-ray intensity striking itDepending on its value, each pixel is assigned a shade of greyPixel size may determine the limiting spatial resolution of the system
7 Why bother with digital? Film has been used since the beginning, so why are we changing to digital techniques?Increased latitude and dynamic rangeImages can be accessed simultaneously at multiple workstationsViewing stations can be set up in any locationUses digital archives rather than film librariesImages quicker to retrieve and less likely to be lostPost processingSoftcopy reporting – lower cost if do not printNo need for dangerous processing chemicals
8 Disadvantages of digital Initial costProblems with interconnectivityLack of information and set up of automatic exposure control (AEC)Lack of link between exposure and brightnessPotential for dose creep (see following slides)Generally poorer limiting spatial resolution when compared with film
9 Dynamic Range - FilmWith conventional film, too low a dose will results in a ‘thin’ filmToo high a dose results in a very dark filmFixed and limited dynamic range – must match exposure parameters to the film being usedGives a measure of control over patient dose!
10 Dynamic Range - Digital With digital, too low a dose will still produce a recognisable image (just a bit noisy!)Similarly, too high a dose will produce a recognisable image (but with very little noise!)Consequences:Less retakes = GOODDose creep = BAD – must pay special attention to digital imaging to ensure doses are optimised
11 Detector Dose Indicators (DDI) The restricted latitude of film gives a clear indication of doseToo dark a film = overexposureToo light a film = underexposureFilm has ‘in-built’ quality control of exposureDigital images will be presented with the greyscale optimised no matter what dose is givenWill always see a recognisable image, but the noise will varyCan result in dose creepIncreased dose (lower noise) is not punished by the detection medium, so tendency to go for slowly increasing image quality – NOT ACCEPTABLE!
12 DDIThe DDI has been introduced for digital imaging as an indication of the level exposure on a broad region of the detectorAnalogous to the OD of filmThe definition of DDI is manufacturer specific!Some manufacturers have high DDI = underexposureSome the other way roundSome are a function of the log of doseSome are linear…Manufacturers will provide an indication of acceptable range of DDI, but local departments must validate these – DRLs and OPTIMISATIONOperators should monitor DDI of patient exposures to ensure doses remain acceptable
13 Digital Imaging Techniques Computed Tomography (CT)Radionuclide imagingFilm scannerComputed radiography (CR)Direct digital radiography (DR)Flat panel fluoroscopyElectronic portal imaging device (EPID)
15 Computed Radiography (CR) The most common technique for producing digital imagesWas the first digital technique available commerciallyExploits storage phosphors which emit light that is proportional to the intensity of the X-rays that hit it, when they are stimulated by a laser beamPrimary reason for being the most common technique is that it is the cheapest (at least in the short term)Old X-ray sets used for film-screen radiography can be used, provided exposure factors and AECs are adjusted for the new type of detector
17 CR Stage 1: Image Capture Image receptor is a laser stimulable phosphor, known as an image plate (IP)Capture image by irradiating an IP in the same way as conventional filmDoes not need a new X-ray system when replacing film-screen (just make sure automatic exposure controls are re-calibrated)Typically ~40% of X ray photons are absorbedIPs retain majority of absorbed X-ray energy as a pattern of electrons in meta-stable energy statesThe spatial distribution of stored electrons is equivalent to the pattern of absorbed x rays – latent image
18 Electron Trapping Photon is absorbed by an electron Electron can move through conduction bandThey can then be trapped in Colour Centres which forms our latent image
19 CR Stage 2: Image Read Out Electrons are actively stimulated to release their stored energyThis is done by scanning the IP with an intense laser beam
20 CR Stage 2: Image Read Out A red Laser is used as this matches the energy gap between Colour Centre and conduction bandLight in the blue end of the visible spectrum is emittedHence, optical separation of input and output light photonsMeans a colour filter can be used to prevent laser photons contaminating the output signalBlue light photons are collected via a photomultiplier tube and digital image is produced
21 Read OutStimulation of IPs with laser causes trapped electrons to transfer to conduction bandThese then relax to the ground state, emitting blue light photons
22 How Does CR work? Conduction band Electron traps X Rays are absorbed Valence band
23 Photo stimulated luminescence Conduction bandRed laser lightBlue lightValence band
25 The read/erasure cycle The image plate is removed from the cassette inside the CR readerScanning or laser achieved with a rotating mirrorThe light guide (with optical filter) directs the emitted blue light to a photomultiplier tube, which measures the intensity of the light (proportional to the number of X-rays absorbed)Whilst repeatedly scanning the plate, it is moved through the laser beamOnce scanned, the residual signal is removed by exposing the plate to a very bright light source (erasure cycle)Takes about s to read and erase an image plate
27 Image quality Pixel size limits spatial resolution in CR For small plates (detail required) ~5.5 lp/mmLarge plates (detail not essential) ~3.5 lp/mm(Film-screen ~8-12 lp/mm)Other limits to resolution in CR;Scattering of laser light in the phosphor layer results in detected light from a larger area than expectedDivergence of light emitted before detectionIncreases with thickness of phosphorSome phosphors have needle-like structure to guide light (like an optical fibre), but quite brittle so not for general use
29 Image qualityImage processing (e.g. edge enhancement) may improve visibility of fine detailContrast is determined by the image processing and LUT that is applied (and the window/level the user decides upon)
31 Digital RadiographyDirectly acquire the data in digital format (no separate read-out phase like with CR)Improves throughput of X-ray systems – could be important in chest clinic, mammo, etcMost expensive method, as it requires complete dedicated X-ray systemMain technologies:Phosphor coupled to a read out device – Indirect conversiona-Se/TFT array – Direct conversion flat panels
32 Detective Quantum Efficiency DQE reflects the efficiency of photon detection and the noise addedEvery photon detected and no noise added, DQE = 100%DQE for DR ~65%DQE for CR and film-screen ~30%In principle, DR could be used with lower patient doses as it is more efficient at using what is available!
33 Indirect conversionIndirect conversion involves converting the X-rays into visible light (in a phosphor), and detecting the resulting light photons (akin to film-screen radiography!)Either amorphous silicon (a-Si) photodiode TFT array, or CCD for readoutSharpness limited by both pixel pitch of readout array, and spread of light in phosphorUsually CsI(Tl) needle phosphors to focus light down to the detector (like mini-fibre optics to minimise spread)Needle phosphors can be thicker (more efficient)
35 Indirect flat panels Phosphor => X-ray to light photons Light photons detected in photodiode array => light photons to electrical chargeRead out by the amorphous silicon TFT array (discussed after direct conversion)Can be manufactured as a single panel up 45 x 45 cm2, but in practice tend to be made up of four smaller detectors ‘stitched’ togetherTiled detectorsRequires image processing and interpolation to cover the join between panels
37 CCD detectorsCCD light detectors (like in a camera) can only be manufactured in relatively small sizesUsually need multiple CCDs to cover image area (‘tiled detector’), or slot scanning techniqueAlso thicker than flat panels due to the optics between the phosphor and detector
38 Direct conversion flat panels Amorphous Selenium (a-Se) is a photoconductorConverts X-rays directly to electronsDeposited directly onto amorphous silicon TFT arrayNo phosphor, hence no light spreadResolution governed by effective pixel pitch
39 Flat Panel Physics X ray photons reach the panel These are converted to an electrical chargeEither in the phosphor/photodiode arrangement, or photoconductor layerCharge read-out by the TFT arrayAn image is instantaneously produced on the computer screenGrey levels depend on the charge from each sensor and the LUT/window/level settings
40 The TFT array Amorphous Silicon thin-film transistor array Transistors amplify electrical signalsElectrical charge is stored in the TFT array until release by applying a high potentialEach row of detectors is connected to the same activating potential (gate-line control), and each column to a charge measuring device (read-out electronics)The activating potential is applied row-by-row, so the timing of the detected signal determines the position of the pixel from which it originatesEach pixel ~100 μm
43 Advantages of DR over CR Image displayed immediately to operator in roomFasterGreater throughput of patients as no intermediate read-out phaseSlightly better resolution (CR limited by laser spot size and scatter)Harder wearing imaging device (as long as you don’t drop it!)Or at least that’s the theory…
44 Disadvantages of DR over CR Much more expensiveNeed to refurbish X-ray roomLess flexible?Originally, DR with fixed detectors, in carefully controlled ambient conditions, wired directly into the systemNow have mobile DR units with wireless detectorsBUT need to make sure you can get the images off to PACS – secure wireless network? Fixed connection points?
45 And now for something a bit different… Tomography and Dual Energy Imaging
46 TomographyConventional radiography superimposes structures on the film/detectorResults in;Inability to determine the depth of structuresLimited ability to resolve the shape of structuresReduction in contrastCan resolve depth by using orthogonal projectionsIn other situations conventional or mechanical tomography may be useful (or going full 3D with CT, MRI, etc)
47 TomographyIn tomography, only structures in a selected plane of the patient, parallel to the film, are imaged sharply – everything else above and below appears blurred to the point they become unrecognisableBlurring is produced by simultaneous movement of at least two of;The tube,The film and/orThe patientActively exploit motion unsharpness that we normally wish to minimise!
48 Linear tomographyThe tube and film/detector are linked by an extensible rod, hinged about a pivotDuring exposure, the film/detector moves in a straight line (e.g. right-to-left) along rails, whilst the tube moves the other wayWhilst moving, the tube is rotated so that the central ray always points toward the pivot pointStructures that are in the same plane as pivot point (focal plane) will not move when projected onto the film, producing a sharp imageThe projections of structures outside the focal plane will move around on the film, and hence are blurred in the final image
49 Linear tomographyStructures in focal plane maintain same position on the film = sharp image
50 Linear tomographyStructures above and below the focal plane will move from one end of the film to the other = blurred image
51 Linear tomographyThe further the object is from the focal plane, the greater the blurringStructures lying within a plane of thickness t will be sufficiently sharp to recogniseEverything outside this will be too blurred and low contrastCut-height (the focal plane position) is adjusted by lowering or raising the pivott is controlled by the tomographic angle and height of pivot (higher = thinner thickness of cut)Narrower angles reduces the degree of blurring, and hence increase t (and vice-versa)0° = conventional projection radiography = everything in focus!
52 Linear tomography Typical angle = 40°, equivalent to t~3 mm However, thin slices and the spread of off-focus anatomy over the whole film reduces contrastUse low kV (consistent with penetrating the patient)Most useful for high contrast structures e.g. bony structures in the earPatient dose is higher compared with projection radiographyLinear tomography less effective for linear structures lying in the plane of movementUse alternative, more complex movements (e.g. circular, elliptical, etc)Tomography equipment becoming less common with the availability of CT
55 Dual energy imagingA subtraction technique where images are taken at high and low kV in rapid successionUsed in general radiography (e.g. chests), CT, fluoroLow kV = high contrast between bone and soft tissue (photoelectric effect)High kV = image contrast determined by tissue density rather than atomic number (Compton scatter)Subtracting low kV image from high kV minimises visibility of bone and improves soft-tissue contrastRemove ribs in chest radiography!Conversely, subtract high kV from low kV image displays bony anatomy in greater detail
56 Dual energy imagingImages (shown here) and good video of how it works can be found at:
58 Dual energy technology CR and DR based solutions available for general radiographyCR;Use two image plates with a Cu plate in betweenAcquire two images at the same time (no artefacts due to motion)Cu plate filters the beam that leaves the first plate (‘low energy’) to give ‘high energy’ imageLittle energy separation using this technique – results in relatively low SNR
60 Dual energy technology DR;Low energy image (60 kVp), read-out, high-energy image, read-outLarge energy separation = higher SNRRelatively long read-out times mean motion artefacts are common (involuntary and voluntary)
66 Dual Energy CT Technological challenges Commercial solutions Constant data free from motion and contrast changesNeed largest practical energy separation and detector optimisationCommercial solutionsMultiple rotations at different kVpSingle spiral with alternating kVpDual x-ray sourceRapid switching kVpEnergy sensitive detectors (not double exposure)
67 Dual Energy CT Applications Virtual non-contrast imagingBone segmentation and removalTissue typing?Mono-energetic imaging – PET/SPECT apps?Specialist breast CT?Not particularly proven yet – a technology looking for an application?
68 Dual Energy CT – Manufacturers SiemensDual sourceGERapid kVp switchingToshibaSingle spiral with alternating kVpPhilipsAcquire two lots of data at high and low kVp, register and use dual energy softwareBUT, they are developing dual-energy detectors (dual layer and photon-counting)