Items tagged “physics”

Doppler angle correction refers to an imaging post-processing method used to adjust for the effects of insonation angle on the Doppler shift. Measurement of flow velocity with Doppler imaging is dependent on the angle between the ultrasound beam and the target (insonation angle), with the maxim...

Direct digital radiography (abbreviated as DR, DDR, or DX) is a type of digital radiography in which the digital registration of the image takes place directly at the detector with no intermediate processing step such as cassette readout in computed radiography (CR). Terminology Direct digital...

The magnetic field describes the influence a magnet has on its surrounding area. Magnets create a magnetic field or line of force running from the magnetic north to the magnetic south pole of the magnet. Magnetic fields are the result of intrinsic magnetic moments and moving electric charges wit...

Magnetic dipoles are the magnetic equivalent of an electric dipole, where the two charges are positive and negative, with a flow of electric charge and surrounding electric field. Magnets are bipolar, having two poles: north and south. The term dipole means two charges.  In a magnetic dipole, th...

The magnetic dipole moment is a quantity that represents the strength and orientation of the magnetic dipoles. This can be represented by the torque that a material experiences when added to a magnetic field. The stronger the magnetic moment, the stronger the magnetic field and the stronger the ...

Nuclear magnetization refers to the magnetic moment of an atomic nucleus. Magnetic resonance imaging (MRI) makes use of nuclear magnetic resonance (NMR). Some nuclei may have nuclear magnetization depending on their nuclear charge distribution and the spin of their protons and neutrons. Nuclei w...

Magnetic susceptibility is the ability of external magnetic fields to affect the nuclei of an atom. This may also be thought of as the “magnetisability” of a material, or the extent to which a material becomes magnetized when placed in an external magnetic field. Magnetic susceptibility is rela...

The dependence of magnetism is based on proton density (PD), field strength and temperature. There is a frictional interchange of energy between the protons and the lattice (spin-lattice interaction), such that a balanced exchange occurs between the two energy states and the thermal equilibrium ...

Longitudinal magnetism and transverse magnetism are components of the net magnetism vector. Longitudinal magnetism Longitudinal magnetization is the component of the net magnetization vector parallel to the magnetic field (z-axis). This is due to a difference in the number of spins in parallel...

Electromagnetic induction is the induction of electric current via changing magnetic fields. Magnetic fields are generated by moving charges (equivalent to electrical current). Ampere’s law or Fleming’s right-hand rule determines the magnitude and direction (i.e. clockwise or anti-clockwise) of ...

T1 values are a few hundred milliseconds (ms) for most tissues examined. The following are approximate T1 values (ms) of several tissues for B0 = 1.5 T fat = 260 liver = 500 muscle = 870 brain white matter = 780 brain grey matter = 920 CSF = 2500 Tissues that will have high signal on T1-...

Factors affecting T1 and T2 relaxation times of different tissues are generally based on molecular motion, size and interactions. The protons giving rise to an NMR signal are mainly those in cell water and lipids (i.e. protons that are free to move), while those in protein and solids usually do...

The average rate at which molecules tumble (and therefore T1 and T2 time) is related to the molecular size. Small molecules (e.g. water/CSF) have a broad distribution of motional frequencies with poor matching with the Larmor frequency and therefore have long T1 values. Medium sized molecules (e...

The chemical shift is the local change in resonant frequency due to different chemical environments. The external magnetic field causes the electron cloud surrounding the nucleus to induce an electron current, which in turn produces a local magnetic field at the nucleus opposed in direction to t...

Proton density (PD) weighted images are related to the number of nuclei in the area being imaged (number of hydrogen protons), as opposed to the magnetic characteristics of the hydrogen nuclei. They are produced from the first echo. Proton density weighted images result when the contribution of ...

Fast or turbo spin echo (FSE/TSE) is an adaptation of conventional spin-echo (SE) acquisition technique designed to reduce imaging time. It has largely supplanted the original spin-echo technique due to vastly improved imaging speed. Basic spin echo sequence In a basic SE sequence, a single ec...

Signal localization for image construction in MR is based on adding a magnetic field gradient onto the main (constant) magnetic field. In 1973, Paul Lauterbur published the idea in Nature of deliberately superimposing linear field gradients on the main magnetic field. Along each gradient, the s...

Slice thickness is dependent on gradient strength and bandwidth, to attain a slice thickness, a range of frequencies must be transmitted to produce resonance across the whole slice. This range of frequencies is the transmitter RF (radiofrequency) bandwidth (tBW). Slice thickness is determined by...

One of the key concepts in assessing the quality of a magnet is that of magnetic field homogeneity, as it will relate to image quality and the presence of artifacts.Field homogeneity refers to the uniformity of the main magnetic field when no patient is present, measured in parts per million (pp...

Magnetic shimming refers to the process of reducing magnetic field inhomogeneities. It is an important aspect of optimizing image quality. Field inhomogeneities can be the result by both intrinsic magnet properties and the surrounding environment of the MR magnet – such as the presence of nearby...