Single Slice Spiral - Computed Tomography - Lecture Slides, Slides of Computed Tomography

Download Single Slice Spiral - Computed Tomography - Lecture Slides and more Slides Computed Tomography in PDF only on Docsity! Single Slice Spiral - Helical CT Oh no, not more physics… docsity.com Spiral CT  Incentives for development  Shorter study times  Improved 3D imaging  New technology required  Slip ring  Allows continuous gantry rotation docsity.com Non-spiral Intergroup Delay  Scans grouped for single breath hold  Inter-scan delay causes long study  Because of delay, studies may require >1 group  Reduced scanner throughput docsity.com Limitations of Conventional (non- spiral) Scanning  Long exam times  Inter-scan delays  Table motion  Inter-group delays  Breathing  Limitations for angiography  Few scans made during maximum contrast enhancement docsity.com Faked Image  Respiration variations from group to group can cause  Anatomy omissions  Slice-to-slice misregistration  Inaccurate 3D images  Step-line contours docsity.com Helical Reconstruction Complication  Patient moves as gantry rotates  No two fan beams at same z coordinate “z” direction docsity.com As Gantry Rotates, Fan Angles Repeat  Distance between repetitions is movement of table during one rotation “z” direction docsity.com Data Acquisition Challenges  Projection data not confined to single slice  Streak artifacts  appear with “standard” or “conventional” (non-spiral) reconstruction  caused by motion  special algorithms required Position at start of rotation Position at start of rotation Position of interest docsity.com Spiral Reconstruction Algorithms  Uses interpolation for  input projection data  output slice attenuation data  Slice can be calculated at any position from raw projection data = real data point coordinate of interest Interpolated data docsity.com Interpolation  Estimates value of function using known values on either side When x = 50, y = 311 When x = 80, y = 500 What will be the value of y when x=58? ? y = 311 + (500-311)* (58-50) / (80-50) (x1,y1) (x2,y2) (x,y) y = x1+ (y2 – y1)* (x – x1) / (x2 – x1) 58 80 50 500 311 docsity.com Disadvantage of Interpolation  Can increase effective slice thickness  Calculation averages data measured at many z values “z” direction docsity.com Redundant Data  Can reduce slice thickness averaging substantially by using only 180o worth of data 180o rotation 360o rotation * docsity.com 180o Reconstruction for Spiral Scanning  Substantially reduces effective slice thickness  Better z-axis resolution  Increases image noise  Image based on only 180o instead of 360o of data  Redundant data reduces noise docsity.com Spiral CT Challenges  Requires special interpolation reconstruction  More computing-intensive docsity.com Spiral CT Advantages  Shorter acquisition times  no inter-scan delays  shorter study times  entire organs / volumes scanned together  Better throughput  BUT: Larger demands on tube  Much less cooling time docsity.com Spiral CT Advantages  No gaps in data acquisition  slice can be reconstructed for any axial position  Patient motion artifacts reduced docsity.com Spiral CT = Faster Scanning: Advantages  Less potential for motion  Less effect of varying respiration  spiral scan done in single breath hold  Less effect of shifting anatomy between slices  Improved contrast protocols possible  faster scanning; less dilution  more uniform contrast concentration  Greater accuracy for multiplanar & 3D images docsity.com Single-Slice Detectors  Many detectors rotate around patient  Single row in z-direction  Slice thickness determined by collimation Z-Axis docsity.com Single Slice CT: Changing Slice Thickness Z-Axis Thin Slice Z-Axis Thick Slice docsity.com Pitch = 1 table motion during one rotation Slice Pitch = --------------------------------------- slice thickness  Pitch = 1 means slices abut one another docsity.com Spiral vs. Conventional CT & Patient Dose  Dose is strongly dependent on pitch Please explain. Inquiring minds wanna know docsity.com Pitch = 1 © equivalent dose to non-spiral oD docsity.com Pitch >1  lower dose for spiral if table increment per rotation > one slice thickness docsity.com