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Computed Tomography: Pioneering Work and Technical Overview

• PRIOR READING ASSIGNMENT
• Challenge Questions
• IDENTIFYING AREAS TO STUDY

PRIOR READING ASSIGNMENT

Before attempting to answer these review questions, read the following brief summary notes on this topic.

In radiography, images are usually referred to as planar images. These images have the following limitations: superimposition of all structures on the image receptor (film-screen detector) and the qualitative nature of radiographic imaging. The latter simply means that it is difficult to distinguish between a homogeneous object (one tissue type) of non-uniform thickness and a heterogeneous object (bone, soft tissue, and air) of uniform thickness. Furthermore, the beam used in radiography is an open beam (wide beam), and this creates more scattered rays that reach the image and essentially destroy the image contrast.

Computed tomography (CT) overcomes these limitations by removing the superimposition of structures, improving image contrast, and imaging very small differences in tissue contrast, using a more sensitive detector. In particular, CT produces cross-sectional images of patient anatomy, referred to as transverse axial images. These sections are perpendicular to the long axis of the patient. The invention of the CT scanner is credited to two individuals: Godfrey Hounsfield (who worked at EMI [Electric and Musical Industries] in England) and Allan Cormack, who shared the Nobel Prize in Medicine in 1979 for their contributions to the development of the scanner. Both of these pioneers worked out the mathematical solutions to the problem in CT, but Hounsfield is credited with the development of the first useful clinical CT scanner.

The technical evolution of the CT scanner is marked by the development of more efficient data collection methods leading to multislice CT imaging (as opposed to single-slice CT imaging), faster image reconstruction algorithms resulting in low-dose CT scanning, and improved image postprocessing methods such as three-dimensional images.

The major components of a CT scanner include the data acquisition system, the computer system, and the image display, storage, and communications system. The data acquisition system contains imaging system components designed to collect radiation attenuation values from the patient using an x-ray tube and special detectors coupled to detector electronics. These components are housed in what is referred to as the CT gantry. The computer is a central and integral component in CT. The primary role of the computer is image reconstruction and image postprocessing. A graphics processing unit (GPU) is now used to reduce the processing requirements of the computer's central processing unit (CPU). Image reconstruction uses special algorithms to create the image using the attenuation values collected from the patient. Today these algorithms are iterative reconstruction algorithms and are much faster than the previous algorithm (the filtered back-projection algorithm).

CT examinations generate large amounts of data; hence large storage space on the order of gigabytes (GB) is required. Storage devices for CT include magnetic tape and disks, digital videotape, optical disks, and optical tape. Communications refer to electronic networking or connectivity by using a local area network or wide area network (LAN or WAN). Connectivity ensures the transfer of data and images from multivendor and multimodality equipment according to a defined standard. A popular standard for medical images is the Digital Imaging and Communications in Medicine (DICOM) standard. CT scanners are now connected to Picture Archiving and Communications Systems (PACS).

In general, the software used in CT includes image reconstruction software, preprocessing software, and image postprocessing software. Image reconstruction software uses algorithms to build up the image from the raw data collected from the detectors. Preprocessing software performs corrections (such as correcting a bad detector reading, for example) on the data collected from the detectors before the data is sent to the computer. Image postprocessing software operates on reconstructed images displayed for viewing and interpretation and typically includes visualization and analysis software.

Self-Assessment Questions will be based on the following Keywords and Concepts

• Major differences between CT and radiography
• Godfrey Hounsfield
• Allan Cormack
• Major components of a CT scanner
• Technical evolution overview
• The imaging system
• The computer system
• Storage capacity
• Connectivity
• CT software

Challenge Questions

Answer the following questions to check your understanding of the materials studied.

True (T)/False (F)

1. CT is a planar radiographic imaging modality.
2. CT was developed by EMI.
3. Godfrey Hounsfield was awarded the Nobel Prize in Medicine for his contributions to the development of the first useful CT scanner.
4. Allan Cormack did not share the Nobel Prize in Medicine with Hounsfield.
5. Radiographic imaging produces planar images of the patient's body.
6. CT can show very small differences in tissue attenuation compared to radiography.
7. CT shows soft tissue contrast much better than radiography.
8. CT uses rendering algorithms to create three-dimensional images to enhance diagnostic interpretation.
9. Images produced by the CT scanner are generally referred to as planar images.
10. Two notable technical innovations for CT scanners are the development of multislice detectors and iterative reconstruction algorithms.
11. Data acquisition in CT refers to the process of converting attenuation data to electrical signals that are subsequently converted into digital data.
12. GPUs are now used in CT to reduce the processing requirements of the CPU.

Multiple Choice

1. Which company pioneered the development of the CT scanner?
   1. General Electric Healthcare
   2. Siemens Healthineers
   3. Philips Healthcare
   4. EMI (now called Thorn EMI)
2. Which type of image does radiography produce?
   1. Planar static image
   2. Cross-sectional image
   3. Three-dimensional image
   4. Both A and C are correct.
3. A significant difference between CT and radiography is that:
   1. CT shows very small differences in soft tissue contrast compared to radiography.
   2. CT shows better image sharpness compared to radiography.
   3. CT images show better image contrast than radiographic images.
   4. A and C are correct.
4. Which of the following problems of radiography are overcome by CT?
   1. Superimposition of all structures on the image receptor
   2. The open-beam geometry of radiography, which causes more scattered radiation to reach the image receptor
   3. The qualitative nature of radiography
   4. The radiographic image receptor sensitivity to radiation
   1. 1 only
   2. 1 and 2
   3. 1, 2, and 3
   4. 1, 2, 3, and 4
5. The major system component of the CT scanner responsible for image reconstruction is the:
   1. Data acquisition components
   2. CT detector
   3. Computer system
   4. Picture archiving and communication system (PACS)
6. Which of the following data sets is used by the CT image reconstruction algorithm to create CT images?
   1. Attenuation data from the patient
   2. Demographic data sets about the patient
   3. Exposure factors (kV and mAs)
   4. The computer storage capacity
7. The anatomical section that is perpendicular to the longitudinal axis of the patient is called the:
   1. Planar section
   2. Transverse axial section
   3. Cross section
   4. B and C are correct.
8. Who developed the first clinically useful CT scanner?
   1. Dr. Euclid Seeram
   2. Dr. Godfrey Hounsfield
   3. Dr. Allan Cormack
   4. B and C are correct.
9. The first CT scanner was limited to scanning only the:
   1. Brain
   2. Chest
   3. Abdomen
   4. Whole body
10. Which of the following recent technical innovations in CT are intended to improve image quality and reduce radiation dose?
    1. Multislice detectors
    2. Iterative algorithms
    3. Dose optimization tools
    4. Quality control tests
    1. 1 only
    2. 2 and 3
    3. 3 and 4
    4. 1, 2, 3, and 4
11. The purpose of the data acquisition system components in CT is to:
    1. Produce x-rays
    2. Shape and filter the x-ray beam falling upon the patient
    3. Detect the radiation passing through the patient
    4. Convert the transmitted x-ray photons into digital information
    1. 1 only
    2. 2 and 3
    3. 3 and 4
    4. 1, 2, 3, and 4
12. A term used to describe the transfer of data and images from multivendor and multimodality equipment according to a defined standard is:
    1. Digital Imaging...