Ciip Chapter 1: Medical Imaging Modalities And Digital Images

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Medical Imaging Hierarchy

Back 1

Patient – Examination (Study) – Series (Sequence) – Image

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Artifact

Back 2

Any component of the image that is extraneous to the representation of tissue structures; can be caused by a technique, technology, hardware, or software error

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modalities using ionizing radiation

Back 3

radiography, fluoroscopy, mammography, CT, and nuclear medicine

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modalities using non-ionizing radiation

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ultrasound and magnetic resonance imaging (MRI)

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Ionizing Radiation

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Radiation capable of producing energetic charged particles that move through space from one object to another where the energy is absorbed; may be hazardous if used improperly

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X-Ray Attenuation

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Attenuation of an X-ray beam is largely a function of tissue radiodensity. Bone, for example, has a higher attenuation coefficient than soft tissue. In a radiograph of the chest, bony structures highly attenuate (or absorb) X-rays, passing less signal through the body to the detector; whereas soft tissues are less attenuating, passing more signal through to the detector. Air is least attenuating, and thus high signal hits the detector and is represented as black in most images; no signal hitting the detector is usually represented as white.
In a chest radiograph, the air spaces in the lungs appear black, soft tissues are lighter gray, and the bony ribs and spine are white.

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Projection Radiography Source

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X-rays; ionizing radiation; part of the electromagnetic spectrum emitted as a result of bombardment of a tungsten anode by free electrons from a cathode.

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Projection Radiography Analog detector

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fluorescent screen and radiographic film.

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Projection Radiography Digital Detector

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computed radiography (CR) uses a photostimulable or storage phosphor imaging plate; direct digital radiography
(DR) devices convert X-ray energy to electron–hole pairs in an amorphous selenium photoconductor, which are read out by a thin-film transistor (TFT) array of amorphous
silicon (Am-Si). For indirect DR devices, light is generated using an X-ray sensitive phosphor and converted to a proportional charge in a photodiode (e.g., cesium iodide scintillator) and read out by a charge-coupled device (CCD) or flat panel Am-Si TFT array.

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Projection Radiography Image Attributes

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variations in the grayscale of the image represent the X-ray
attenuation or density of tissues; bone absorbs large amounts of radiation allowing less signal to reach the detector, resulting in white or bright areas of the image; air has the least attenuation causing maximum signal to reach the detector, resulting in black or dark areas of the image.

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Projection Radiography Advantages

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fast and easy to perform; equipment is relatively inexpensive
and widely available; low amounts of radiation; high spatial
resolution capability. Particularly useful for assessing the parts of the body that have inherently high contrast resolution but require fine detail such as for imaging the chest or skeletal system.

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Projection Radiography Disadvantages

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poor differentiation of low contrast objects; superposition
of structures makes image interpretation difficult; uses ionizing radiation.

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Fluorography Source

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continuous low-power X-ray beam; ionizing radiation.

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Fluorography Detector

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X-ray image intensifier amplifies the output image.

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Fluorography Image attributes

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continuous acquisition of a sequence of X-ray images over
time results in a real-time X-ray movie.

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Fluorography Advantages

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Can image anatomic motion and provide real-time image feedback during procedures. Useful for monitoring and carrying out barium studies of the gastrointestinal tract, arteriography, and interventional procedures such as positioning catheters.

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Fluorgraphy Disadvantages

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Lower quality moving projection radiograph.

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Computed Tomography Source

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collimated X-ray beam; X-ray tube rotates around the patient.

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Computed Tomography Detector

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early sensors were scintillation detectors with photomultiplier
tubes excited by sodium iodide (NaI) crystals; modern detectors are solidstate scintillators coupled to photodiodes or are filled with low-pressure xenon gas. An image is obtained by computer processing of the digital readings of the detectors.

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Computed Tomography Image Attributes

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thin transverse sections of the body are acquired representing an absorption pattern or X-ray attenuation of each tissue. Absorption values are expressed as Hounsfield Units.

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Computed Tomography Advantages

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good contrast resolution allowing differentiation of tissues with similar physical densities; tomographic acquisition eliminates the superposition of images of overlapping structures; advanced scanners can produce images that can be viewed in multiple planes or as volumes. Any region of the body can be scanned; has become diagnostic modality of choice for a large number of disease entities; useful for tumor staging.

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Computed Tomography Disadvantages

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high cost of equipment and procedure; high dose of ionizing
radiation per examination; artifacts from high contrast objects in the body such as bone or devices.

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Magnetic Resonance Imaging Source

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high-intensity magnetic field; typically, helium-cooled superconducting magnets are used today; non-ionizing; gradient coils turn radiofrequency (RF) pulses on/off.