Imaging Modality

·         In an x-ray tube, x-rays are produced when an energetic electron passes close to an atomic nucleus in the anode of the x-ray tube. The attractive force of the positively charged nucleus causes the electron to change direction and lose energy. The energy difference between the initial energy of the electron and the energy of the electron after it changes direction is released as an x-ray photon. This process is called bremsstrahlung, which means "braking radiation" in German.

·         If kV increased, X-ray penetration increases, exposure increases (darker film), and contrast goes down. The maximum energy of the x-rays produced is increased. Film contrast is primarily dependent on kV.

Increasing mAs means increased film exposure (more x-rays produced), which darkens the film. The maximum energy of the x-rays produced is not changed.

·         inverse square law: The intensity of the x-ray beam decreases with the square of the distance from the x-ray tube. In other words, if one doubles the distance between oneself and the x-ray tube, x-ray exposure decreases by a factor of 4. This concept is important in determining x-ray exposure and absorbed dose.

·         The focal spot is the source of x-rays in the tube.

A small focal spot produces sharper images and is used in mammography.

A large focal spot can tolerate more heat and is used in fluoroscopy.

·         Mammography uses lower kV (for higher image contrast) and higher mA (for shorter exposure times) compared with the technique for chest and abdominal examinations

·         On fluoroscopy, things that are white on x-rays (like bones or oral contrast) are presented as dark. Things that are dark on x-ray (like air) are presented as white on fluoroscopy.

·         Posterior acoustic enhancement refers to the increased brightness seen beyond objects that transmit a lot of sound waves. Since fluid-filled cysts transmit a great deal of sound, the area beyond cysts often displays posterior acoustic enhancement.

Posterior acoustic shadowing has the opposite effect-decreased brightness seen beyond objects that reflect a great deal of sound. Gallstones, which are echogenic, reflect a lot of sound, and shadowing is often seen in the tissue beyond the gallstones.

·         Once the CT data are acquired and an HU value is assigned to each voxel in a slice, the data can be viewed in different ways. The window is a range of HU values that are chosen to be viewed as shades of gray; assigned HU values greater than this range are depicted as white, and values less than this range are depicted as black. The level is the HU value that is the center value of the window. Commonly used window/level combinations include those for viewing lung, bone, liver, and the brain.

·         In diagnostic imaging, matrix size refers to the number of pixels contained on each axis of a CT or MRI slice, such as a square matrix of 512 × 512 pixels (typical in CT) or a rectangular matrix of 256 × 192 pixels (common in MRI).

·         T1-weighted images are created primarily by using data from differential proton relaxation rates in the plane longitudinal to the main magnetic field.

T2-weighted images primarily make use of data from differential proton relaxation rates in the plane transverse to the main magnetic field.

·         Most commonly used MRI sequences:

T1-weighted images: short TR and short TE signal. Good for showing anatomic detail.

T2-weighted images: long TR and long TE. Good for detecting most types of pathology.

Proton-density weighted images: long TR and short TE.

Fluid-attenuated inversion recovery (FLAIR): this is essentially a T2-weighted image, which is manipulated to suppress water (cerebrospinal fluid) and make it appear darker. Such images are very sensitive for detecting tissue abnormalities and most types of pathology.

Diffusion-weighted images: this technique is particularly sensitive for water diffusion and thus for detecting edema.

Gradient echo: these sequences are very sensitive to flow and hence are used for MR angiography.

·         Diagnostic x-ray imaging is referred to as "transmission imaging." This term is used because images are formed as x-ray photons from an external source traverse tissue and emerge to form the image. Nuclear medicine is referred to as "emission imaging" because photons are emitted from inside the patient and subsequently detected by the gamma camera imaging system.

·         Tc-99m DTPA is a glomerular agent, meaning that it is cleared primarily by glomerular filtration and is neither reabsorbed nor secreted by the renal tubules. Thus it can be used to measure the glomerular filtration rate. Tc-99m mertiatide (MAG3) is a tubular agent, meaning that its mechanism of renal clearance is solely tubular secretion. Tc-99m dimercaptosuccinic acid (DMSA) is a cortical agent, meaning that it binds to tubular cells in the renal cortex, making possible static imaging of the renal parenchymal cortex.

·         PET stands for positron emission tomography. A tracer that emits positrons is injected into the patient. After the positrons are emitted from the nucleus of an atom, they travel through surrounding tissue and collide with electrons. This collision between positrons and electrons is called annihilation and produces gamma rays. The gamma rays are detected by a PET scanner and analyzed by a computer to form an image.

·         COMPUTERS IN RADIOLOGY

1.       The storage size for a given image is determined by the spatial resolution and bit depth of the image.

2.       DR systems eliminate the plate and cassette completely and acquire digital images directly, using flat-panel detectors.

3.       RIS and PACS are essential elements of digital radiology.