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Multi detector ct cerebral angiography
This document discusses techniques for computed tomography (CT) angiography. It covers advances in CT technology that have improved angiography, including faster scan speeds and thinner slices. Optimal CT angiography depends on scan technique factors like protocol and contrast injection, as well as image post-processing techniques. Newer multi-detector CT machines allow covering volumes more quickly and with higher resolution. Methods like multi-planar reformation and volume rendering help visualize vascular structures from CT image data.
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Multi detector ct cerebral angiography
- 1. Ehab Abou ElfotouhElaryan. Lecture Of Radio-diagnosis. Al-Azhar University.
- 2. In recentyears, 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. Optimal imagequality 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. 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. 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. 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. 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. 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. Contrast MaterialInjection: 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. TYPE OFINJECTION: 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.
- 13. TYPE OFINJECTION: 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.
- 14. 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.
- 15. 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.
- 16. Image Post-processingTechniques: 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.
- 17. 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.
- 18. 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.
- 19. 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.
- 21. 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.
- 22. Volume Rendering: Separation of different tissue types (ie, bone, contrast- enhanced vessels, soft tissue) can be performed and can be color encoded.