Computed Tomography Computed Tomography is the most significant development in radiology in the past 40 years. MRI and Ultrasound are also significant developments but they do not use x-ray to produce the image. The x-ray tube spins around the patient.

Basic CT Principles Conventional tomography has the image parallel to the long axis of the body. This is referred to as Axial Tomography.

Basic CT Principles Computed Tomography has the x-ray tube move across the axis so the image is called a transverse image or one perpendicular to the long axis of the body.

First Generation CT Scanner Pencil Beam Translate-Rotate Design 180 one degree images or translations. One or two detectors. 5 minutes scan time

Second Generation CT Scanner Translate-Rotate Fan beam collimation so there is more scatter radiation. 5 to 30 detectors 10 degrees /translation 18 per scan. 30 second scan times Faster scan time

Third Generation CT Scanner Rotate-Rotate Fan shaped beam of 30 to 60° for full patient coverage. Constant Source to detector distance due to curvilinear detector array.

Third Generation CT Scanner If one detector fails, a ring artifact appears. 1 second scan times Superior reconstruction and resolution.

Fourth Generation CT Scanner The tube rotates around a stationary ring of detectors. Fan beam Variable slice thickness with pre and post patient collimation.

Fourth Generation CT Scanner As many as 8000 detectors. 1 second scan time. Auto-detector calibration so no ring artifact. High radiation dose compared to earlier scanners.

Fifth Generation CT Scanner This is the latest generation of CT. Allows for continuous rotation of the tube for spiral CT. 5th Generation also includes two novel designs:

Fifth Generation CT Scanner Toshiba maintains the same SID by wobbling the detectors. Heartscan by Imatron used an electron beam instead of x-ray tube and 50 ms scan times.

Fifth Generation CT Scanner Spiral CT scanners allow for contiguous or even overlapping data acquisition. As the tube spins, the table moves. On earlier units, the table moved between scans.

Spiral CT Scanner Spiral CT is made possible by slip-ring technology. The tube can continuously rotate 360 degrees with continuous movement of the coach within the gantry, where it must stop after each rotation with conventional CT.

Spiral CT Scanner The detector array may contain as many as 14,600 detectors that are 1.25mm wide. This allows multiple slice to be made with one scan and more tissue volume to be imaged.

Benefits of Spiral CT Less motion artifacts Improve lesion detection because the reconstructed image can be at arbitrary intervals. Reduced partial volume because of overlapping reconstruction intervals. Reduced scan time.

Benefits of Spiral CT Advances in computer processing allows for multi-planar reconstruction and even 3D reconstruction.

Basic CT Scanner Components Gantry includes: the Pedestal or table X-ray Tube, Collimators, Detectors & High Voltage Generator Mechanical Supports Operators Console Computer

Two Collimators in CT Pre-patient collimator determines slice thickness Pre-detector (post-patient) collimators reduce scatter radiation to improve contrast.

CT Image Characteristic Image matrix: Original EMI format was 80 x 80 so there were 6400 cells of information called pixels. Today the format is 512 x 512 resulting in 262,144 pixels. The numerical number in each pixel is a CT number or Hounsfeld Number.

CT Image Characteristic CT number or Hounsfeld Number represents the tissue volume in the pixel. Field of View (FOV)is the diameter of the reconstructed image. As the FOV increases, the size of the pixel increases. Voxel: is the square of the matrix times the thickness of the slice.

Hounsfeld or CT Number The precise CT number is related to the attenuation of the tissue contained in the voxel. Bone = +1000 Muscle= +50 Water= 0 Fat=-100 Lung= -200 Air = -1000

Image Quality Spatial Resolution: The motion of CT tends to blur the image compared to the actual object. (The ability to discriminate objects) The ability of the scanner to reproduce high contrast or sharp edges (edge response function) is measured as Modulation Transfer Function (MTF). (MTF: is a measure of the ability of the scanner to reproduce high contrast or sharp edges.) (Image contrast: the intensity of image produced)

Image Quality Contrast Resolution: The ability to distinguish one soft tissue from another is contrast resolution. This is where CT excels. The absorption or attenuation characteristics is affected by the atomic number and the mass density of the tissue.

Contrast Resolution Conventional Radiography has relatively poor contrast resolution. CT can amplify the tissue characteristics to provide superior contrast resolution.

Contrast Resolution The Contrast Resolution is improved because of the predetector collimation. The contrast resolution for low contrast tissues is limited by the size and uniformity of the object and the noise in the system. Noise is determined by the number of x-rays used by the detector to make the image.

Computed Tomography Problems CT scans require significantly higher doses of radiation compared to conventional radiography. Therefore the risks of the radiation and the benefits of the information gained by the scan must be factored when determining the need for Computed Tomography.

Computed Tomography Problems If a chest x-ray is equal to the amount of radiation received in 10 days from our natural environment, a CT of the brain is equal to 8 months exposure and CT abdomen, chest or lumbar spine is equal to 3 years each. Do they mention this when they advertise total body CT scanning?

Computed Tomography Problems Computed Tomography equipment are expensive and have high service costs. Computed Tomography is expensive for the patient or insurance. As much as $1,000 per exam. HMO’s require preauthorization