What is attenuation on CT images?

Attenuation is the measurement of energy absorbed and deflected as it passes through a medium. In simpler terms, attenuation is how much stopping power a material has on energy.

How is the image formed in a CT scan?

The term “computed tomography”, or CT, refers to a computerized x-ray imaging procedure in which a narrow beam of x-rays is aimed at a patient and quickly rotated around the body, producing signals that are processed by the machine’s computer to generate cross-sectional images—or “slices”—of the body.

How is CT number calculated in CT imaging?

The CT numbers are calculated from the x-ray linear attenuation coefficient values for each individual tissue voxel. It is the attenuation coefficient that is first calculated by the reconstruction process and then used to calculate the CT number values.

What are the three steps involved in the formation of CT?

CT is accomplished in three steps—scanning the patient (data acquisition), processing the data (image reconstruction), and displaying the image.

What determines attenuation?

The attenuation coefficient is dependent upon the type of material and the energy of the radiation. Generally, for electromagnetic radiation, the higher the energy of the incident photons and the less dense the material in question, the lower the corresponding attenuation coefficient will be.

What is kV and mA in CT?

mA and kV. • Tube milliamperage (mA) –rate of x-ray production. • Kilovoltage (kV) –number of x-rays produced.

What is Hounsfield number in CT scan?

After image reconstruction, each pixel in the CT image is assigned a number, referred to as the Hounsfield or CT number, which is a rescaled normalized function of the linear attenuation coefficient [1, 2].

What is CT number in radiology?

Computed tomography (CT) number is a calculated value reflecting the X-ray attenuation coefficient in an image voxel, generally expressed in Hounsfield units (HU), where the CT number of water is 0 HU.

What is CT number and how it is measured?

The intensity of the attenuated X-ray beam is expressed as a CT number (also known as the linear attenuation coefficient, or attenuation value). This number is a measure of attenuation relative to air and water expressed in Hounsfield units (HU): CT number of bone = +1000 HU.

What is geometric efficiency CT?

Reduced z-Axis geometric efficiency (GE), defined as the. percentage of the x-ray beam width in the z-direction that is ‘seen’ by the detectors, is one of the main. factors that contribute to the increased dose in MDCT scanners [2].

What does attenuation mean in radiology?

Attenuation is the reduction of the intensity of an x-ray beam as it traverses matter.

How do you correct attenuation in a transmission scan?

In most regions of the body, appropriate attenuation correction can be done only when the attenuation of the body is measured directly with a transmission scan. X-ray CT images that are aligned with the SPECT data can be used for attenuation correction if appropriate adjustments are made to the attenuation values.

Can X-ray CT images be used for attenuation correction in SPECT?

X-ray CT images that are aligned with the SPECT data can be used for attenuation correction if appropriate adjustments are made to the attenuation values. In addition, the anatomic information that can be deduced from radiotracer distributions is sometimes insufficient for accurate localization of SPECT lesions.

How are CT numbers calculated during image reconstruction?

During the image reconstruction process, the slice of tissue is divided into a matrix of voxels (volume elements). As we will see later, a CT number is calculated and displayed in each pixel of the image. The value of the CT number is calculated from the x-ray attenuation properties of the corresponding tissue voxel.

What happens to X-ray attenuation when photon energy increases?

As the photon energy increases, x-ray attenuation is reduced. The attenuation coefficients in a CT image are expressed in Hounsfield Unit (HU), which quantifies the amount of attenuation of any specified tissue relative to the attenuation of water. In general, changes in x-ray photon energy will also result in a change in the tissue HU value.