Basic Principles of CT Scanning

CT CT - Computed Tomography CAT Scan - Computerized Axial Tomography

The basics of CT CT imaging chain System components Acquisition methods Image quality Applications

X-ray: The beginning X-Rays founded in 1895 by Wilhelm Conrad Roentgen

CT: The beginning CT founded in 1970 by Sir Godfrey Hounsfield –Engineer with EMI, LTD. –first applications were in neuroradiology

CT Scanner X-Ray modality used to view the body in cross section Used to determine –extent of trauma –location and type of tumors –status of blood vessels –pre surgical planning

CT System

Basic CT scanner components Gantry X-Ray Tube Detector Control Console

Gantry CT X-ray tube High voltage generator Detector array Data acquistion system Slip ring

The CT X-ray Tube Anode heat capacity –3.5 MHU up to 28 MHU Determines maximum mAs Determines volume length Dictates generator size

Detector Elements Capture energy that has not been attenuated by the patient

Control console Set scan parameters –kVp, mA, scan time, reconstruction filter, etc. Set scan mode –Surview, Axial or Spiral IRS (Image reconstruction System) Review and archive images Post-processing

Scanning methods Surview –AP,Lat –Surview, Scanogram, Topogram…. Conventional CT –Axial Start/stop Volumetric CT –Helical or spiral CT Continuous acquisition

Digital Projection X-ray tube and detector remain stationary Patient table moves continuously –With X-rays “on” Produces an image covering a range of anatomy –Similar to a conventional X-ray image, e.g. flat plate of the abdomen Image used to determine scan location

Axial CT X-ray tube and detector rotate 360° Patient table is stationary –With X-ray’s “on” Produces one cross-sectional image Once this is complete patient is moved to next position –Process starts again at the beginning

Volume CT X-ray tube and detector rotate 360° Patient table moves continuously –With X-ray’s “on” Produces a helix of image information –This is reconstructed into 30 to 1000 images

Volume Scanning: Pitch 1:12:1

Advantages of Volume CT More coverage in a breath-hold –Chest, Vascular studies, trauma Reduced misregistration of slices –Improved MPR, 3D and MIP images Potentially less IV contrast required Gapless coverage Arbitrary slice positioning

Fundamentals of Multislice CT

Multislice Fundamentals Everything is better (R)esolution –Z-axis, spatial, low contrast (S)peed –Temporal - bolus capture, stopped motion (V)olume –Thin slice - – organ-specific coverage (P)ower –Enough photons - uncompromising image quality

Single Slice =One10mm slice per rotation Dual Slice =Two5mm slices per rotation Quad Slice =Four2.5mm slices per rotation Multislice Effectiveness Everything is better –Resolution2x4x-8x –Speedsamesame –Volumesamesame –Powersamesame Dual Quad

.. single detector arcdual detector arc pre-patient collimation post-patient collimation x-ray tube focal spot ___ Mx8000 Dual Slice Dual Slice Detector Optimized for 2 Slice Acquisition

Approximately 10% more efficient than matrix detectors Variable Wide Area Detector Asymmetrix™ Variable detector length Fixed detector length Quad Detector Technology variable wide area detector Variable slice thickness –4 x 1mm –4 x 5mm –4 x 2.5mm –2 x 0.5mm –2 x 8mm –2 x 10mm

8 Element 2-D array 4 Slices Quad Technology How it works

CT CT attenuation information CT image quality

Attenuation X-ray beam passes through patient Each structure attenuates X-ray beam differently –According to individual densities Radiation received by detector varies according to these densities

Density information Transferred from detector to CT computer (A to D converter) Reconstructed by computer into a cross-sectional image –Displayed on screen –Each pixel displayed on monitor has varying brightness The greater the attenuation, the brighter the pixel The less attenuation, the darker the pixel

Density information Density values correspond to a range of numbers –Hounsfield scale

Window settings Window width –Determines range of CT numbers displayed on an image Values above this range = white Values below this range = black –Window level Sets the center CT number displayed on the monitor Determines the location on the Hounsfield scale about which the window width will be centered

CT image quality Spatial resolution –Ability to resolve small objects in an image –Measured in lp/cm

CT image quality Spatial resolution –Ability to resolve small objects in an image –Measured in lp/cm

Isotropic Imaging True 0.5mm Isotropic imaging

CT image quality Contrast resolution –Ability to differentiate small density differences in an image

Post Processing Options Visualization of vasculature in relation to pathology –Show course of vessels –Show stent placement –Define vascular stricture

Cervical Spine Spiral Acquisition Rotation – 0.75 sec Coverage – 160 mm Pitch – Acq. Time – 36 sec FOV – 250 mm ST – 1.0 mm Rec. Incr. – 0.6 mm Std Res. – 8 lp/cm 120 kV, 200 mAs CTDI 100 w – 39 mGy CTDI FDA w – 17 mGy Thin-Slice Spiral Neck

Neuro-Angiography Circle of Willis

Thin-Slice Spiral Lungs

MIP 4D-Angio MasterCut Panoramic View Renal Arteries

Ext. Spiral Acq. Spiral Acquisition UltraFast – 0.5 sec Coverage – 1400mm Pitch – 1.75 Acq. Time – 41.5 sec FOV – 420 mm ST –2.5 mm Rec. Incr. – 1.6 mm Std Res. – 8 lp/cm 120 kV, 96 mAs CTDI100w – 7.3 mGy CTDIFDAw – 4.9 mGy Extended Spiral Acquisition