MDCT PHYSICS Dr Varun Bansal Dept of Radio-Diagnosis IGMC, Shimla TOPICS TO BE COVERED  Basic Principles CT concept  Generation of CT image Scan/ data acquisition Reconstruction Display  Image Quality Quantitative measurements Image artifacts Basic principle  CT image is a display of the anatomy of a thin slice of the body developed from multiple x-ray absorption measurements made around the body’s periphery  Image in conventional tomography blurring out the information from unwanted regions  In CT  constructed using data arising only from section of interest  Routine CT generated axial image  image is reformatted to coronal / sagittal  CONCEPT:  Internal structure of an object can be reconstucted from multiple projections of the object Generation of CT image Scan / data acquisition: Components: Scan frame: early Ct scanners – rotating frames and recoiling system cable  step and shoot methods Current systems  slip rings (transmitting electrical energy across rotating) X ray Generators: operating frequencies to 50 kilohertz power  15 to 60 kW 80 to 140 kV and 30 to 500 mA Computer modulated generators: ability to obtain high x-ray flux when needed provision for tube heat management ability to change the current as needed to maintain high image quality within the context of ALARA  X-ray Tubes: rotating anodes, unique cooling methods MHU with 300kHU/min  MHU with MHU/min enhance system’s ability to cover large areas of anatomy at diagnostic levels of x-ray output  Double tube designs: increase x-ray source, faster acquisition time spectral absorption differences of tissues characterising tissues and abnormalities – fat content, stones  Data acquisition system (heart of CT system): Detector system Analogue to digital conversion Data processing  Detectors: Scintillation crystals: produce light when ionizing radiation reacts with them Scintillation detector 1st gen used Thallium-activated NaI Disadv- hygroscopic and very long afterglow Replaced by silicon photodiodes (a/c solid-state) CeI, gadolinium oxysulfide and Cd tungstate ( no afterglow) Xenon Gas Ionization Chambers: limited to use in rotate-rotate type scanners Should have- anode and cathode, inert gas, voltage, walls separate, window disadv- inefficiency xenon (heaviest), compressing to – 10 atm, long chamber size FIXED ARRAY DETECTORS ( equal width) ADAPTIVE ARRAY DETECTORS ( unequal width) MultiDetector CT MDCT sequential (axial) scanning  Using sequential (axial) scanning, the scan volume is covered by subsequent axial scans in a ‘‘step-and-shoot’’ technique  In between the individual axial scans the table is moved to the next zposition  With the advent of MDCT, axial ‘‘step-and-shoot’’ scanning has remained in use for only a few clinical applications, such as - head scanning, - high-resolution lung scanning, - perfusion CT - interventional applications MIP (Maximum Intensity Projection)  Unlike three-dimensional shaded-surface and VR displays, no preprocessing is required  The rays are cast throughout the volume, and depending on whether it is maximum intensity projection or minimum intensity projection, maximum or minimum values along the rays are used in the final image display  Using maximum intensity projection (MIP) for visualization permits easy viewing of vascular structures or air-filled cavities  MIP enables easy viewing of an entire vessel in one image  This is because voxels representing the contrast-filled vessels are most likely to be the ones with the highest values along the ray (assuming no bone along the ray)  Along the same lines, minimum intensity projection can be used to demonstrate air-filled cavities Image Quality Quantitative Measurements  Spatial Resolution: measured by the ability of a CT system to distinguish two small, high-contrast objects located very close to each other under noise-free conditions  required for evaluating high-contrast areas of anatomy,  such as the inner ear, orbits, sinuses, and bone in general, because of their complicated shapes  Spatial resolution can be specified by spatial frequencies, which indicate how efficiently the CT scanner represents different frequencies Modulation transfer function (MTF) describes this property Filter effects on resolution  the major role of the convolution filter is to remove the image blurring created by the back-projection process  Various filters control the amount of image blurring created by accentuating high-frequency components found in the data  For a crisp image, the high spatial frequencies are accentuated, and this has the effect of sharpening the edges and improving resolution  One pays for a crisp picture with a decrease in density resolution Similarly, by increasing density resolution, one pays by loss of some spatial resolution and image crispness Opening size of Detector Aperture  The detector aperture MTF curve depends on the magnification factor of the system and the physical size of the detector  If the object being viewed is smaller than the width of the data ring, it will be difficult to resolve because it occupies only a fraction of the space seen by the detector  Typical detector apertures of CT systems today range from less than mm to 1.5 mm, with center-to-center detector spacing of approximately mm Factors Affecting Spatial Resolution  Focal Spot Smaller focal spot, SR improves  Detector width  Number of Projections More projections, SR improves  Slice thickness  Pitch  Pixel Size Smaller Pixel Sixe, SR improves  FOVDecreasing FOV(everything else constant), SR improves  Patient Motion Smaller Detector Width, SR improves Smaller ST, SR improves Lower pitch, SR improves Decreased Patient motion, SR improves Pixel Size  the spatial resolution can be no greater than the size represented by the pixel length  In reality, pixel size should be 1.5 to times smaller than the desired resolution  Unless a matrix element exactly coincides with an object, the object representation will be averaged over two or more pixels and thus may not be visualized  It must be realized that the pixel size refers to the FOV (or body), not the viewing screen or film Contrast Resolution  ability to differentiate the attenuation coefficients of adjacent areas of tissue  In the computation of any single pixel value, there is error in the form of statistical variation; it is this variation that limits the ultimate contrast resolution  This variation (called image noise) is manifested as a grainy background, or mottle  The parameter used to evaluate this variation is the standard deviation (SD) NOISE can be reduced by  Increasing tube voltage, tube current, scan time, FOV &Slice thickness  Using reconstruction filters Factors Affecting Contrast Resolution  mAs More mAs, CR improves  Pixel Size FOV and pixel size increase, CR improves  Slice thickness  Reconstruction filter Using Soft tissue improves CR  Patient Size ST increases, CR improves For larger patients, at same technique, more attenuation, detected photons decreases, CR degrades Temporal resolution  refers to the ability of a CT scanner to capture objects that change shape or position over time and depends primarily on the gantry rotation speed and the reconstruction method used  depends on: gantry rotation speed spiral interpolating algorithm used during reconstruction Ring Artifacts  usually the result of difficulty with the detector  each detector is associated with a data ring A malfunction of any one detector incorrectly back-projects along the data ring to produce the ring artifact  If a detector is not matched or is not intercalibrated accurately, the backprojection for each data ring will be slightly different, causing multiple rings  Detectors in the center of the detector arc are most sensitive Metal and Bone Artifacts  The presence of objects having an exceptionally high or low attenuation can create artifacts by forcing the detector to operate in a nonlinear response region  Because this incorrect response occurs at specific directions of the beam through the object, incomplete cancellation of the backprojected rays during reconstruction occurs and yields streaking artifacts Beam-Hardening artifacts  result from the preferential absorption of low-energy photons from the beam  average beam photon energy is progressively increased  toward the end of the x-ray path, the attenuation is less than at the beginning because the attenuation coefficient is smaller with higher energy  The reconstruction program, however, assumes a monochromatic beam and attributes any change in beam intensity to a change in tissue composition rather than to the result of a shift in average photon energy  The assigned attenuation coefficients are thus in error, and the densities seen on the image are in error  The effect is most pronounced in regions of large attenuation, such as bone Stair-Stepping Artifact  occur when in one direction the pixel of the reformatted image has the same length as the axial image but in the other direction the pixel length is the same as the slice thickness  Because pixel length in most scans is considerably smaller than slice thickness, the reformatted scan has an unusual appearance  Uncommon in modern CT Thank You References : CT and MRI of the Whole Body: 5th edition; HAAGA  Christensen’s Physics of Diagnostic Radiology  Recent Advances: AIIMS, PGI, MAMC Series  Volume CT: State-of-the-Art Reporting AJR 2007; 189:528–534  Three-dimensional volume rendering of spiral CT data: theory and methods Radiographics 1999;745-764  Developments in CT Imaging, 18 (2006), 45–61 ... content, stones  Data acquisition system (heart of CT system): Detector system Analogue to digital conversion Data processing  Detectors: Scintillation crystals: produce light when ionizing radiation