1 Single Slice Spiral - Helical CTOh no, not more physics…
2 Spiral CT Incentives for development New technology requiredShorter study times Improved 3D imaging New technology required Slip ring Allows continuous gantry rotation
3 Conventional (Non-spiral) CTTube rotates once around patient Table stationary data for one slice collected Table increments one slice thickness Repeat Tube rotates opposite direction
4 Volume Scanning Data collected continuously Table moves continuouslyFocal spot traces spiral path with respect to patient
5 Helical Reconstruction ComplicationPatient moves as gantry rotates No two fan beams at same z coordinate “z” direction
6 As Gantry Rotates, Fan Angles RepeatDistance between repetitions is movement of table during one rotation “z” direction
7 Data Acquisition ChallengesProjection data not confined to single slice Streak artifacts caused by motion special algorithms required Position at start of rotation Position at start of rotation Position of interest
8 Calculating Fan Beams at Odd Locations using InterpolationNo complete data set for any single z location Use 2 closest beams in correct orientation Calculate beam attenuation by interpolating between adjacent beams “z” direction
9 Spiral Reconstruction AlgorithmsUses 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
10 Disadvantage of InterpolationTrust me, interpolation is a guess Can increase effective slice thickness Calculation averages data measured at many z values “z” direction
11 Data Acquisition ChallengesNo single slice defined by acquisition geometry slice localization more difficult Different slice volume geometry conventional: cylinder spiral: wafer with radial crack Slight increase in effective slice thickness slice thickness influenced by fan beam thickness speed of table motion
12 Table Moves During Helical Scanningtable increment during one rotation Slice Pitch = slice thickness Slice thickness Table Increment
13 Table Moves During Helical ScanningSlice thickness determined by collimation Table motion per revolution determined by table speed table motion during one rotation Slice Pitch = slice thickness Slice thickness Table Increment
14 Single-Slice DetectorsMany detectors rotate around patient Single row in z-direction Slice thickness determined by collimation Slice Thickness Z-Axis
15 Beam positions when tube directly above patientPitch = 1 Pitch = 1 means patient moves exactly one slice thickness per revolution of tube table motion during one tube rotation Slice Pitch = slice thickness Beam positions when tube directly above patient
16 Beam positions when tube directly above patientPitch <1 Pitch < 1 means patient moves less than a slice thickness during one tube rotation Can improve visualization of objects table motion during one tube rotation Slice Pitch = slice thickness Beam positions when tube directly above patient
17 Beam positions when tube directly above patientPitch >1 Pitch > 1 means patient moves further than slice thickness during one tube rotation table motion during one tube rotation Slice Pitch = slice thickness Beam positions when tube directly above patient
18 Beam position when tube directly below patientPitch >1 No gap in image coverage when viewing study table motion during one tube rotation Slice Pitch = slice thickness No Gap Beam position when tube directly below patient
19 Spiral vs. Conventional CT & Patient DoseDose is strongly dependent on pitch Please explain. Inquiring minds wanna know
20 Pitch = 1 equivalent dose to non-spiral
21 Pitch >1 lower dose for spiral Table moves fasterTable increment per tube rotation > one slice thickness
22 Pitch <1 higher dose for spiral Table moves slowertable increment per tube rotation < one slice thickness
23 Pitch & Dose Dose inversely proportional to pitchPitch = 0.5 => Dose doubles Pitch = 2 => Dose cut in half