Principles of Imaging

Question 1

CT (Helical) Image Production

Answer 1

CT imaging is an x-ray based modality by which a patient is placed in the gantry of a machine which has an x-ray source and an array of detector devices which move around the patient as the patient travels through the aperture.

Question 2

Planar Viewing (Tomography)

Answer 2

Planar viewing describes each individual 2D slice that makes up the whole 3D image.

Question 3

RED

Answer 3

The four tissue types, bone, soft tissue, fat and air are assigned a relative electron density. Water is the main constituent in the composition of the body and is given a value of 1 to quantify its electron density. Other tissues are given a value relative to that of water. Lungs and air filler spaces have RED of 0-0.2, Soft tissue 0.8-1, Bone 1.4-1.7

Question 4

Hounsfield Scale

Answer 4

Hounsfield units are the unit of measurement for the representation of tissues on medical images. Each tissue type is assigned a value on the HU scale. Their position on the scale is relative to its radiodensity. The scale typically ranges from -1000HU to +3000HU. The range of values is represented by a grayscale, so like plain x-ray images, more dense structures will appear a lighter shade of gray, or white. Less dense structures appear darker shades of gray, or black.

Question 5

Pitch

Answer 5

Pitch affects the resolution of CT images obtained via helical scanning. The pitch is determined by the rate that the couch moves through the gantry aperture. The slower (low pitch) the couch moves through the aperture, the more revolutions around the patient by the tube and detector thus, more data may be obtained. If the couch is moved faster (high pitch) rate, then less data is collected and therefore low quality image. Patients may be exposed to a higher dose of radiation if travelling through the bore slower so it is a matter of weighing up risks and benefits.

Question 6

Windowing

Answer 6

Windowing is a filter used to focus on certain types of tissue types by which a segment of the HU scale is selected and only pixels within that segment are displayed over the full range from black to white. Windowing is an example of the use of HU or variations in radiodensity to manipulate images. It is important to note that the use of windowing is not changing the RED just how we view it.

Question 7

Contrast

Answer 7

Tissue is divided into four groups: bone, water/soft tissue, fat, and air. The shade of which each of these appear on medical imaging is known as contrast. Bone will show as white, soft tissue as lighter shades of gray, fat as darker shades of gray, and air as black.

Question 8

Production of Images

Answer 8

Radiographic images are produced as a result of x-rays being directed at a patient, and tissues of different densities attenuate various amounts of those rays. The photons comprising the x-ray beam will emerge from the patient to interact with a cassette on the other side of them. The cassette contains a fluorescent intensifying screen followed by a layer of emulsion, film base, another layer of emulsion and finally another layer of intensifying screen. When exposed to x-rays the intensifying screen will emit ordinary light photons. It is these photons that the photographic film will react to produce the image.