Items tagged “physics”

Medical ultrasound is based on the use of high-frequency sound to aid in the diagnosis and treatment of patients. Ultrasound frequencies range from 2 to approximately 15 MHz, although even higher frequencies may be used in some situations. The ultrasound beam originates from mechanical oscillat...

The inverse square law describes the principle of dose reduction as the distance from the source increases. This assumes a point source. The source can be considered as a "point" source if the distance is more than 7 times the dimensions of the source 4. The radiation will spread equally in all...

Noise, variability that is not part of a desired signal, is present in all electronic systems, and originates from a number of sources including electronic interference. It appears as an irregular granular pattern in all images and degrades image information. It may be inapparent or render image...

Nuclear magnetic resonance (NMR) is a powerful technique which allows the study of the magnetic properties of an atom's nucleus 1. It involves placing nuclei within an external magnetic field enabling thus them to undergo precession 2. The 'resonance' part of the names implies the fact that a se...

Gibbs artifact, also known as truncation artifact or ringing artifact, is a type of MRI artifact. It refers to a series of lines in the MR image parallel to abrupt and intense changes in the object at this location, such as the CSF-spinal cord and the skull-brain interface. The MR image is reco...

Gradient coils are used to produce deliberate variations in the main magnetic field (B0). There are three sets of gradient coils, one for each direction. The variation in the magnetic field permits localization of image slices as well as phase encoding and frequency encoding. The set of gradient...

Radiofrequency coils (RF coils) are the "antennae" of the MRI system, broadcasting the RF signal to the patient and/or receiving the return signal. RF coils can be receive-only, in which case the body coil is used as a transmitter; or transmit and receive (transceiver). Surface coils are the si...

Paramagnetic substances include oxygen and ions of various metals like iron, magnesium and gadolinium. These ions have unpaired electrons, resulting in a positive magnetic susceptibility. The magnitude of this susceptibility is less than 0.1% of that of ferromagnetic materials. The effect on MR...

Superparamagnetic materials consist of individual domains of elements that have ferromagnetic properties in bulk. Their magnetic susceptibility is between that of ferromagnetic and paramagnetic materials.  The figure illustrates the effect of a superparamagnetic material (grey circle) on the ma...

Diastolic pseudogating appears as periodic bright and dark signal in arteries such as the aorta as one progresses through a series of images. Synchronisation of the cardiac cycle and the pulse sequence results in high signal in the artery during diastole when blood is relatively stationary and l...

Spiral scanning on MRI is unlike spiral scanning on CT where the x-ray tube is continuously rotating and data is continuously being acquired. On MRI the word "spiral" refers to the pattern of sampling k-space. On conventional imaging sequences including spin echo and gradient echo and on fast im...

Mirror image artifact in sonography is seen when there is a highly reflective surface (e.g. diaphragm) in the path of the primary beam. The primary beam reflects from such a surface (e.g. diaphragm) but instead of directly being received by the transducer, it encounters another structure (e.g. ...