RADIOLOGICAL DESCRIPTIVE TERMS

Reporting on diagnostic imaging studies revolves around being able to accurately describe what we are seeing on the images. There are a number of terms that we commonly use in our reports that you should understand, and that you should be able to use when asked to describe an image.

Radiographs

The formation of an image in radiography is dependent on tissues of different density blocking a different proportion of the incoming xrays from passing through them and reaching the digital sensor on the other side of the patient.

Areas that are less dense, such as something containing gas, will allow more xrays through, appear darker on the radiograph, and are termed ‘radiolucent’, ‘lucent’, or ‘of increased lucency’. Denser structures block more xrays, appear whiter on the image, and are termed ‘radio-opaque’, ‘opaque’ or ‘of increased opacity’. When a structure is of very high density, we might use a different descriptive term. If similar to bone, we often refer to a ‘calcified density’; examples would include renal calculi or vascular calcification. Surgical clips, staples, screws, pins, wires, plates and joint prostheses may be referred to as being of ‘metallic density’ and appear very white on the image.

CT

Although the images produced are obviously very different, the principle behind CT is the same as for radiography: tissues of different density block a different proportion of the xrays passing through them from reaching a row of detectors on the opposite side of the patient. For that reason, we also use the term ‘density’ when describing the appearance of tissues or lesions on CT. The denser a structure is, the brighter it will be on the CT image.

Lesions may be ‘high density’ or ‘hyperdense’, or ‘low density’ or ‘hypodense’. Occasionally, when something is of very similar density to something else (for example, an adjacent structure or surrounding tissue), we describe it as being ‘isodense’. Another descriptive term that we commonly use in CT reports refers to the attenuation of tissues, which is basically the same thing as density – dense tissues will ‘attenuate’ the xray beam more than low density tissues. We can therefore describe things as being ‘high attenuation’ or ‘low attenuation’.

Ultrasound

The formation of an image in ultrasound is due to the ultrasound waves reaching a tissue being reflected back to the ultrasound probe – the strength of this ‘echo’ determines how that tissue will appear in the image. When a large proportion of the ultrasound waves striking a structure are reflected back to the probe, the structure is described as being ‘echogenic’, ‘increased echogenicity’ or ‘hyperechoic’ and will appear bright on the image.

When a structure reflects relatively few ultrasound waves, it is ‘hypoechoic’ or ‘decreased echogenicity’ and appears relatively dark. Fluid-containing structures reflect few if any of the ultrasound waves and are described as being ‘anechoic’ – these will appear completely black on the image. Some lesions may be of almost identical echogenicity to surrounding tissue (for example, some liver tumours) and can be described as ‘isoechoic’. When a structure reflects all of the ultrasound waves reaching it (for example, bone or calcium), a very bright line is produced on the image, deep to which everything appears black (as no ultrasound waves have reached these deeper tissues to generate an echo). This appearance is called ‘acoustic shadowing’ and is helpful when searching for gallstones or renal calculi.

MRI

Although it works in a very different way to ultrasound, this modality also uses ‘echoes’ to form images. The ‘echo’ emitted by the part of the body being assessed produces a signal in the radiofrequency receiver, and the intensity of this signal determines how bright this area will be on the image. Therefore we describe MR images based on the ‘intensity’ of the tissue or lesion in question. Structures can be ‘hyperintense’, ‘isointense’ or ‘hypointense’; alternatively, we can describe them as being ‘high signal’ or ‘low signal’.

Nuclear Medicine

In this imaging modality, a radionuclide is injected into the patient attached to a radiopharmaceutical that is designed to accumulate in a specific tissue or abnormality (for example, bone or thyroid). The radiopharmaceutical then emits radiation (usually gamma rays), which is detected by a camera that then forms an image that is based on the pattern of ‘uptake’ of the radiopharmaceutical in the body. Therefore we describe nuclear medicine images on the basis of ‘increased uptake’ or, occasionally, ‘decreased uptake’.

Enhancement

This term refers to structures or lesions becoming brighter on imaging after an intravenous contrast medium has been injected, and can be used when contrast has been injected for a CT, MRI or ultrasound.

Summary

Modality

Bright on Image

Dark on image

Radiography

Opacity / Density

Lucency

CT

Hyperdense / High attenuation

Hypodense / Low attenuation

MRI

Hyperintense / High signal

Hypointense / Low signal

Ultrasound

Hyperechoic / Echogenic

Hypoechoic / Anechoic (fluid)

Nuclear Medicine*

Increased uptake / or FDG avid (when describing a PET study)

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