Computed Tomography Basics

Computed Tomography (CT)

• CT is a type of cross-sectional imaging that eliminates unwanted planes or layers of the body.
• A thin beam is transmitted through a specific cross-section, striking special detectors opposite the x-ray tube.

Goal of CT

• Overcome the limitations of radiography and conventional tomography.
• Minimize superimposition.
• Improve image contrast.
• Record very small differences in tissue contrast.

Limitations of Radiography

• Superimposition of structures in one film, making it difficult to distinguish particular details.
• Radiography is qualitative rather than quantitative, making it difficult to distinguish between homogeneous objects of non-uniform thickness and heterogeneous objects.

CT Defined

• Images are cross-sectional, showing only parts of the anatomy imaged at a particular level.
• Each cross-sectional slice represents a specific plane in the patient.
• Slice thickness is referred to as the z-axis.

Cross-Sectional Images

• Data that form the CT slice are further sectioned into elements called pixels.
• Width is represented by x, and height is represented by y.

Pixels and Voxels

• A pixel is a 2D square shade of gray.
• A voxel is a 3D volume of gray.

Attenuation and Hounsfield Units

• Attenuation is the absorption of x-rays by different tissues.
• Hounsfield units (CT numbers) represent the percent difference between the x-ray attenuation coefficient for a voxel and that of water, multiplied by 1000.

Advantages of CT

• Excellent low-contrast resolution due to highly collimated x-ray beams and special detectors.
• Ability to change window width and level to suit the observer's needs.
• Spiral CT includes volume data acquisition in a single breath, improving 3D imaging and multiplane image reformatting.
• Facilitates diagnosis using various techniques, such as xenon CT and determination of bone mineral content.

Limitations of CT

• Difficulty imaging soft tissue surrounding bone, such as the posterior fossa and spinal cord.
• Presence of metallic objects produces streak artifacts.

CT Reconstruction

• MPR (Multi-Planar Reconstruction) reconstruction using spiral scanning.
• Curved reformatting reconstruction using spiral scanning.
• SI joint sagittal image used to reconstruct coronal image.

CT-Guided Biopsy

• Allows for accurate placement of catheters and biopsies under CT guidance.
• Enables drainage of fluid collections and pus.

CT Angiography

• Visualizes blood vessels and detects vascular diseases.
• Used for brain, renal, and pulmonary angiography.

CT Reconstruction Applications

• Bone reconstruction.
• Lung reconstruction.

Computed Tomography (CT)

• CT is a type of cross-sectional imaging that eliminates unwanted planes or layers of the body.
• A thin beam is transmitted through a specific cross-section, striking special detectors opposite the x-ray tube.

Goal of CT

• Overcome the limitations of radiography and conventional tomography.
• Minimize superimposition.
• Improve image contrast.
• Record very small differences in tissue contrast.

Limitations of Radiography

• Superimposition of structures in one film, making it difficult to distinguish particular details.
• Radiography is qualitative rather than quantitative, making it difficult to distinguish between homogeneous objects of non-uniform thickness and heterogeneous objects.

CT Defined

• Images are cross-sectional, showing only parts of the anatomy imaged at a particular level.
• Each cross-sectional slice represents a specific plane in the patient.
• Slice thickness is referred to as the z-axis.

Cross-Sectional Images

• Data that form the CT slice are further sectioned into elements called pixels.
• Width is represented by x, and height is represented by y.

Pixels and Voxels

• A pixel is a 2D square shade of gray.
• A voxel is a 3D volume of gray.

Attenuation and Hounsfield Units

• Attenuation is the absorption of x-rays by different tissues.
• Hounsfield units (CT numbers) represent the percent difference between the x-ray attenuation coefficient for a voxel and that of water, multiplied by 1000.

Advantages of CT

• Excellent low-contrast resolution due to highly collimated x-ray beams and special detectors.
• Ability to change window width and level to suit the observer's needs.
• Spiral CT includes volume data acquisition in a single breath, improving 3D imaging and multiplane image reformatting.
• Facilitates diagnosis using various techniques, such as xenon CT and determination of bone mineral content.

Limitations of CT

• Difficulty imaging soft tissue surrounding bone, such as the posterior fossa and spinal cord.
• Presence of metallic objects produces streak artifacts.

CT Reconstruction

• MPR (Multi-Planar Reconstruction) reconstruction using spiral scanning.
• Curved reformatting reconstruction using spiral scanning.
• SI joint sagittal image used to reconstruct coronal image.

CT-Guided Biopsy

• Allows for accurate placement of catheters and biopsies under CT guidance.
• Enables drainage of fluid collections and pus.

CT Angiography

• Visualizes blood vessels and detects vascular diseases.
• Used for brain, renal, and pulmonary angiography.

CT Reconstruction Applications

• Bone reconstruction.
• Lung reconstruction.