Items tagged “physics”

Slew rate refers to the speed at which a gradient can be turned on and off, and is defined as the maximum gradient strength of the gradient divided by the rise time.  MR imaging is a product of magnetic field gradients which are created by magnetic gradient coils. The quality and performance of...

The radiofrequency (RF) transmitter is the generator of the radiofrequency current which is delivered to the transmitting coil. This creates a signal which is used to excite protons in the imaging field. Radiofrequency coils can be both transmitters and receivers of the radiofrequency signal or ...

Radiofrequency coils are the radiofrequency receivers (as well as sometimes the transmitters) of the electromagnetic signal used in MRI. Radiofrequency coils can be either receive-only, or both receivers and transmitters of the radiofrequency signal. The receiver coils detect the electromagnetic...

Volume coils are the transmit and receive radiofrequency coils which are used to both transmit and receive the radiofrequency signal in MRI. Most MRI scanners have what is called a body coil – which is a volume coil built into the bore of the magnet which transmits the radiofrequency for most ex...

Surface coils are a type of receive-only radiofrequency coil used in MRI to receive the radiofrequency signal (transmitted by the body coil). Surface coils are small and are shaped so that they can be placed near the part of anatomy being imaged. By their nature, surface coils have good signal-t...

Phased array coils are an example of a receive-only radiofrequency coil system which receives the radiofrequency signal in MRI. It involves the collection of multiple surface coils into a larger array whose individual signals are combined to create one image. As signal coils detect signal based ...

Magnetic shielding refers to the attempt to isolate or block the magnetic field of the MRI magnet.  This can be done to prevent unwanted interference from the MRI magnet on nearby electronic devices.  This is different from radiofrequency shielding, which is the attempt to prevent the unwanted i...

The fringe field is the peripheral magnetic field outside of the magnet core. It is important because it can cause interference with nearby electronic devices, such as pacemakers. Although the strength of the magnetic fields decreases with distance from the core of the magnet, the effect of the ...

Voxel size is an important component of image quality. Voxel is the 3-D analog of a pixel. Voxel size is related to both the pixel size and slice thickness.  Pixel size is dependent on both the field of view and the image matrix. The pixel size is equal to the field of view divided by the matrix...

Computed tomography (CT) fluoroscopy combines the conventional advantages of both CT and fluoroscopy and has an important role in image-guided interventions where real-time imaging is required. Historically, fluoroscopy was the main image guidance tool for interventional radiology procedures. T...

Image reconstruction in CT fluoroscopy requires the computer to provide very fast reconstructions of images in order to provide immediate and accurate spatial and anatomical feedback during a procedure. This is achieved by: use of a small reconstruction matrix – e.g. 256 x 256 (compared with co...

Consideration needs to be made to the radiation dose to both the patient and radiologist in CT fluoroscopy, given the relatively high and continuous radiation exposure involved. Radiation exposure to the patient The patient surface dosage may range between 2 and 10 mGy/sec, with exposure times...

The amplitude and intensity of ultrasound waves decrease as they travel through tissue, a phenomenon known as attenuation. Given a fixed propagation distance, attenuation affects high frequency ultrasound waves to a greater degree than lower frequency waves. This dictates the use of lower freque...

Commonly referred to as B (brightness) mode, the use of grey scale imaging in ultrasound renders a two-dimensional image in which the organs and tissues of interest are depicted as points of variable brightness. The formation of a B-mode image relies on the pulse-echo principle; assuming the spe...

The electron binding energy is the minimum energy that is required to remove an electron from an atom, as the negatively charged electrons are held in place by the electrostatic pull of the positively charged nucleus. The electron binding energy is measured in electronvolts (eV), where 1 eV = 1....

X-ray spectrum quantity refers to the number of photons within the primary X-ray beam and is measured by mAs (milliampere-seconds) 2. Meanwhile, the intensity of the beam is defined as rate of flow of energy per unit area perpendicular to the beam 3, also known as energy fluence rate, with units...

Specific absorption rate (SAR) is the rate that electromagnetic energy in the radiofrequency pulses is absorbed by tissues during MR image acquisition measured in watts per kilogram (W/kg).  Both the International Electrotechnical Commission and the Food and Drug Administration (in the USA) lim...

Ionization chambers measure exposure by detecting liberated electron charge when x-ray photons ionize the gas within the chamber. The chambers need a high positive voltage applied at the collecting anode to attract the liberated electrons. The electron charge is collected and used to determine t...

Film dosimeters are used to measure radiation exposure to workers to monitor radiation safety and ensuring that they receive doses below the appropriate limit.  Film badges are the cheapest and most common monitoring device. They consist of a small case with a piece of film situated between fil...

The kernel, also known as a convolution algorithm, refers to the process used to modify the frequency contents of projection data prior to back projection during image reconstruction in a CT scanner 1. This process corrects the image by reducing blurring 1. The kernel affects the appearance of i...