Multi detector ct cerebral angiography

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Multi detector ct cerebral angiography

  1. 1. Ehab Abou Elfotouh Elaryan. Lecture Of Radio-diagnosis. Al-Azhar University.
  2. 2.  In recent years, rapid advances in computed tomographic (CT) technology and image postprocessing software have been made.  CT angiography was improved substantially by increasing scan speed and decreasing section thickness and emerged as a powerful tool in neurovascular imaging.  Nowadays, spiral CT systems with acquisition capabilities of up to 64 sections per gantry rotation are introduced in clinical practice.  Assessment of vascular studies based on axial images alone is not straightforward; two-dimensional (2D) and three-dimensional (3D) visualization methods are routinely used to obtained images comparable to those acquired with catheter angiography.
  3. 3.  Optimal image quality depends on two factors: A- CT angiography technique (scan protocol, contrast material injection protocol, image reconstruction methods). B- Data visualization technique (image post- processing).
  4. 4.  A- Technique:  Scan speed :  For evaluation of the basal intracranial arteries, a scan range of approximately 100 mm needs to be covered.  Examination of the whole length of the carotid arteries from the aortic arch to the circle of Willis requires a scan range of approximately 250 mm.  The arterio-venous transit time in cerebral bed equal about 5 second.
  5. 5.  A- Technique:  Scan speed :  With four–detector row CT at a collimated section width of 1 mm, a pitch of 1.5, and a gantry rotation time of 0.5 second, the volume of cerebral artery can be covered in about 9 seconds. This is not fast enough to avoid venous overlay.  With 16–detector row CT at a collimated section width of 0.75 mm, a pitch of 1.5, and a rotation time of 0.5 second, the same range can be covered in 3 seconds, well beyond the arterio-venous transit time.
  6. 6.  A- Technique:  Scan speed:  At examination of the whole length of the carotid arteries from the aortic arch to the circle of Willis:  The scan time would be 21 seconds for four– detector row CT.  7 seconds for 16–detector row CT.  4 seconds for 64–detector row CT (64 × 0.6 mm, pitch of 1.3, 0.33-second rotation time).
  7. 7.  Spatial Resolution:  The smallest distance between two points in the object that can be differentiated as separate details in the image, generally indicated as a length or a number of black and white line pairs per mm (lp/mm).  The small caliber of cervical and intracranial vessels requires the highest spatial resolution in all three dimensions.  In-plane spatial resolution is predominantly determined by detector geometry and the convolution kernel.  It is not substantially improved in scanners with increasing detector row numbers.
  8. 8.  Spatial Resolution:  The major advantage of more detector rows is higher through-plane resolution by reducing the width of a single detector row from 1–1.25 mm (four–detector row CT) to 0.5–0.6 mm (64–detector row CT).  Typical in-plane resolution with application of a CT angiography protocol:  16 × 0.75-mm detector configuration, 120 kV, 100 mAs field of view of 120 mm, medium sharp convolution kernel is 0.6 mm and through-plane resolution is 0.7 mm.  64 × 0.6-mm detector configuration, 120 kV, 140 mAs field of view of 120 mm, medium sharp convolution kernel is 0.6–0.7 mm and through-plane resolution is 0.5 mm.
  9. 9.  Contrast Material Injection:  In order, to obtain high-quality CTA images, high concentration of contrast in the vessels is necessary.  Short scan times require short contrast material injection.  Technique-related factor: To deliver an appropriate amount of iodine, injection rates of 4–5 mL/sec and highly concentrated contrast medium (iodine, 350– 370 mmol/mL) are preferable.  Type of injection and volume of contrast material may also effect Ct contrast enhancement.
  10. 10.  TYPE OF INJECTION:  I- Intra-venous contrast agent administration, including three methods: A- Fixed scan delay technique (15-45s). B- Test bolus injection technique. C- Automated bolus-tracking technique (Smart Prep, CARE Bolus, and Sure Start). *Individual timing of contrast material injection (bolus tracking or test bolus injection) is mandatory to take advantage of phase-resolved image acquisition.
  11. 11.  TYPE OF INJECTION:  I- Intra-arterial contrast agent administration:  Invasive method.  Performed with a combined angiography and CT unite.  High concentration of contrast material can be obtained in intra-cranial arteries without consideration the appropriate timing of injection.  Need small amount of contrast material.
  12. 12.  Image Reconstruction:  To reduce image noise, images may be reconstructed slightly thicker than the detector collimation, for example with a 0.75-mm section thickness from a data set acquired with 0.6-mm detector collimation.  Overlapping image reconstruction should always be performed to improve 3D post-processing.  The reconstruction algorithm influences the spatial resolution in plane.  The ideal algorithm would combine low image noise and sharp edge definition, maintaining good low-contrast resolution.
  13. 13.  Image Reconstruction:  Soft algorithm reduce image noise and allow smooth surfaces with rendering techniques, improving the visualization of aneurysms and vascular malformations.  Sharper algorithm improve edge definition and reduce blooming effects from calcifications, necessary for stenosis measurements, at the expense of higher image noise.
  14. 14.  Image Post-processing Techniques:  Several image processing techniques for CT angiography are currently being used clinically.  Image processing involves traditional operations such as: A- Multi-planar reformation (MPR) . B- Maximum intensity projection (MIP). C- Surface and volume rendering associated with bone subtraction.
  15. 15.  Multi-planar Reformation ( MPR ):  MPR creates views in different planes without loss of original CT information.  Only 2D views can be generated.  If the CT data meet the requirements of isotropy, spatial resolution is similar to the original source images.  Both diameter reduction and area reduction can be measured, and no information is suppressed in the final image.
  16. 16.  Multi-planar Reformation ( MPR ):  A variant of MPR is curved planar reformation.  Curved planar reformation provides a 2D image that is created by sampling CT volume data along a predefined curved plane.  This technique is employed to display tortuous structures.
  17. 17.  Maximum Intensity Projection ( MIP ):  MIP images are created by displaying only the highest attenuation value.  The depth information along the projection ray is lost to visualize the spatial relationship of various structures.  The volume has to be rotated and viewed from different angles.
  18. 18.  Volume Rendering:  is a visualization technique that creates a 3D impression and provides densitometric information.  Visualization of CT angiography data with volume rendering is based on transfer functions that map measured intensities to colors and opacities.  Opacity values on a spectrum from 0% to 100% (total transparency to total opacity) are assigned along artificial rays that pass through the data.
  19. 19.  Volume Rendering:  Separation of different tissue types (ie, bone, contrast- enhanced vessels, soft tissue) can be performed and can be color encoded.

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