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Fundamentals of CT Imaging
Fundamentals of CT Imaging
1) Mechanical Principles:
1) Mechanical Principles:
◦ An x-ray tube moves around the body, continuously projecting a thin fan of x-rays.
◦ An x-ray tube moves around the body, continuously projecting a thin fan of x-rays.
◦ Electronic detectors opposite the tube monitor the number of x-rays and the beam angle.
◦ Electronic detectors opposite the tube monitor the number of x-rays and the beam angle.
◦ The resulting image is an attenuation map displayed as a cross-sectional "slice".
◦ The resulting image is an attenuation map displayed as a cross-sectional "slice".
◦ Voxel (Volume Element): Refers to individual attenuation points in the image.
◦ Voxel (Volume Element): Refers to individual attenuation points in the image.
2) The Hounsfield Unit (HU) Scale:
2) The Hounsfield Unit (HU) Scale:
◦ Density is mathematically determined by a computer and measured in HU.
◦ Density is mathematically determined by a computer and measured in HU.
◦ The spectrum includes 4,000 levels, ranging from –1,000 to +3,000.
◦ The spectrum includes 4,000 levels, ranging from –1,000 to +3,000.
◦ Reference Points: Air = –1,000; Water = 0; Lead or heavy metals = +3,000.
◦ Reference Points: Air = –1,000; Water = 0; Lead or heavy metals = +3,000.
3) Primary Advantages:
3) Primary Advantages:
◦ Eliminates superimposition of structures by imaging the entire volume of interest sequentially.
◦ Eliminates superimposition of structures by imaging the entire volume of interest sequentially.
◦ Offers significantly better contrast resolution than standard planar x-rays.
◦ Offers significantly better contrast resolution than standard planar x-rays.
Technological Advancements
Technological Advancements
• Modern Multislice Scanners:
• Modern Multislice Scanners:
◦ Capable of helical or spiral scanning (continuous rotation while the patient moves through the gantry).
◦ Capable of helical or spiral scanning (continuous rotation while the patient moves through the gantry).
◦ Can acquire thousands of images in seconds; a small animal thorax or abdomen can be scanned in < 10 seconds.
◦ Can acquire thousands of images in seconds; a small animal thorax or abdomen can be scanned in < 10 seconds.
◦ Modern 64-slice scanners can generate over 5,000 images in a single 10-minute study.
◦ Modern 64-slice scanners can generate over 5,000 images in a single 10-minute study.
• Dynamic Imaging:
• Dynamic Imaging:
◦ ECG-based control allows for CT images of a beating heart during all portions of the cardiac cycle.
◦ ECG-based control allows for CT images of a beating heart during all portions of the cardiac cycle.
◦ High-speed systems evaluate dynamic physiological processes like blood flow, intestinal dynamics, and respiratory volume changes.
◦ High-speed systems evaluate dynamic physiological processes like blood flow, intestinal dynamics, and respiratory volume changes.
Patient Management and Procedures
• Immobilization:
• Immobilization:
◦ Veterinary patients typically require anesthesia and immobilization to ensure controlled positioning and prevent movement artifacts.
◦ Veterinary patients typically require anesthesia and immobilization to ensure controlled positioning and prevent movement artifacts.
◦ Scans on awake or minimally sedated animals are difficult to interpret and reserved for severely obtunded patients or specific drug intervention studies.
◦ Scans on awake or minimally sedated animals are difficult to interpret and reserved for severely obtunded patients or specific drug intervention studies.
• Contrast Enhancement:
• Contrast Enhancement:
◦ Nonionic contrast agents improve diagnostic accuracy and characterize lesions.
◦ Nonionic contrast agents improve diagnostic accuracy and characterize lesions.
◦ Unlike human medicine, nearly all veterinary CT studies of soft tissues (brain, abdominal organs) are contrast-enhanced.
◦ Unlike human medicine, nearly all veterinary CT studies of soft tissues (brain, abdominal organs) are contrast-enhanced.
◦ Comparing nonenhanced and contrast-enhanced images provides insights into hemodynamic and physiological changes.
◦ Comparing nonenhanced and contrast-enhanced images provides insights into hemodynamic and physiological changes.
Clinical Applications
Clinical Applications
• Small Animal Specialty Uses:
• Small Animal Specialty Uses:
◦ Skull Imaging: Far more diagnostic than radiographs for complex skull anatomy; however, intraoral radiographs remain superior for dental imaging.
◦ Skull Imaging: Far more diagnostic than radiographs for complex skull anatomy; however, intraoral radiographs remain superior for dental imaging.
◦ Spine: Replaced myelography for evaluating intervertebral disc disease due to speed and safety.
◦ Spine: Replaced myelography for evaluating intervertebral disc disease due to speed and safety.
◦ Metastasis Screening: Far more sensitive than radiographs at detecting small lung nodules.
◦ Metastasis Screening: Far more sensitive than radiographs at detecting small lung nodules.
• Equine Applications:
• Equine Applications:
◦ Primarily used for the skull, cervical vertebral column, and appendicular skeleton (superior to radiographs for bone and soft tissue lesions).
◦ Primarily used for the skull, cervical vertebral column, and appendicular skeleton (superior to radiographs for bone and soft tissue lesions).
◦ Standing Systems: Sedated horse is placed in a restraint while the CT gantry moves.
◦ Standing Systems: Sedated horse is placed in a restraint while the CT gantry moves.
◦ Recumbent Systems: Necessitate general anesthesia.
◦ Recumbent Systems: Necessitate general anesthesia.
• Interventional CT:
• Interventional CT:
◦ Guides biopsy and aspirate collection in areas unreachable by ultrasound (e.g., lungs, spine, brain).
◦ Guides biopsy and aspirate collection in areas unreachable by ultrasound (e.g., lungs, spine, brain).
◦ Used for planning and performing radiation therapy.
◦ Used for planning and performing radiation therapy.
V. Image Reconstruction and Expertise
V. Image Reconstruction and Expertise
• Reformatting Capabilities:
• Reformatting Capabilities:
◦ Algorithms generate images in orthogonal planes (transverse, sagittal, dorsal) or oblique planes.
◦ Algorithms generate images in orthogonal planes (transverse, sagittal, dorsal) or oblique planes.
◦ 3D Reconstruction: Allows depiction of specific densities, such as bones without soft tissue or rotating images of vascular structures.
◦ 3D Reconstruction: Allows depiction of specific densities, such as bones without soft tissue or rotating images of vascular structures.
• Professional Requirements:
• Professional Requirements:
◦ Radiologists require firm knowledge of anatomy, physiology, and artifacts to interpret scans.
◦ Radiologists require firm knowledge of anatomy, physiology, and artifacts to interpret scans.
◦ Technologists should be specifically trained to ensure the highest-quality studies for the specific machine used.
◦ Technologists should be specifically trained to ensure the highest-quality studies for the specific machine used.
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