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Automatic Exposure Control for C-arm CT Using Histogram Analysis

IP.com Disclosure Number: IPCOM000207919D
Original Publication Date: 2011-Jun-16
Included in the Prior Art Database: 2011-Jun-16
Document File: 2 page(s) / 80K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

X-ray image quality parameters such as image noise and artifact content are strongly related to the exposure setting. This is especially important in computer tomography (CT) applications. C-arms performing angiographic C-arm CT are typically equipped with automatic exposure control systems, which adapt to the attenuation of the object in the field of measurement adjusting the settings of the X-ray tube. It adapts the current and exposure time during the rotation of the C-arm using a prospective estimation of the attenuation, typically after the assessment of the last 2-3 frames. Typical systems just calculate an average dose in a region of interest at the detector. If the region selected is not representative for the object of interest, then over- or underexposure can occur that results in artifacts. Especially crucial is underexposure, which results in photon starvation at the detector and thus in streak artifacts in the reconstructed images.

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Automatic Exposure Control for C-arm CT Using Histogram Analysis

Idea: Dr. Yiannis Kyriakou, DE-Forchheim

X-ray image quality parameters such as image noise and artifact content are strongly related to the

exposure setting. This is especially important in computer tomography (CT) applications. C-arms

performing angiographic C-arm CT are typically equipped with automatic exposure control systems,

which adapt to the attenuation of the object in the field of measurement adjusting the settings of the X- ray tube. It adapts the current and exposure time during the rotation of the C-arm using a prospective

estimation of the attenuation, typically after the assessment of the last 2-3 frames. Typical systems

just calculate an average dose in a region of interest at the detector. If the region selected is not

representative for the object of interest, then over- or underexposure can occur that results in artifacts.

Especially crucial is underexposure, which results in photon starvation at the detector and thus in

streak artifacts in the reconstructed images.

The standard angiographic C-arm CT provides a full field acquisition of the object to provide a three-

dimensional (3D) volume of the patient. The standard procedure measures an average dose value in a

region of interest (ROI) at the detector. The automatic exposure system is set to adjust to a certain

target average dose value in this region. If the object is very dense it increases the X-ray exposure,

and if the object is less dense it increases the exposure accordingly. Those systems work with a

latency of about 1-3 detector frames. One shortcoming of this standard approach is that it just

calculates an average value without investigation of the contents and spatial distribution of the dose

values in the region of interest.

Another shortcoming is that if a large amount of X-ray passing through air is included in the region of

interest, this influences the automatic exposure control as follows: A very high dose is measured due

to the low attenuation of air. Hence, the AEC (Automatic Exposure Control) reduces the exposure to

reduce the average dose value in the ROI without checking on the real X-ray density of the object.

This results in underexposure. An additional shortcoming is that the image quality in the image highly

depends on the positioning of the object through the examiner, due to the fixed position of the ROI. Furthermore, if very dense objects, e.g. metals, are part of the object and are not simultaneously part

of the ROI, a large amount of photon starvation is expected. Typically each digital detector has a lower

limit of gray value detectable (LO). In case of photon starvation the measured values are below this

level, e.g. electronic noise. In addition, neither prior-information nor simultaneous multiple ROIs are

allowed.

To overcome the above mentioned sh...