Medical brain Notes 8

  Scope of Diagnostic Imaging For almost half a century following the discovery of x-rays by Roentgen in 1895, radiologic imaging was mainly based on plain and contrast-enhanced radiography. Those images were created by exposing film to an x-ray beam attenuated after penetrating the body. In the recent half century, diagnostic radiology has undergone dramatic changes and developments. Conventional angiog-raphy, nuclear medicine, ultrasonography, and computed tomography (CT) were developed between 1950 and 1970. Magnetic resonance (MR) imaging, interventional radiology, and positron emission tomography (PET) were developed later. Conventional radiology, including contrast-enhanced radiography and CT, uses ionizing radiation created from x-ray equipment. Nuclear medicine uses ionizing radiation that is emitted from injected or ingested radioactive pharmaceuticals in various parts of the body. Ultrasonography and MR imag-ing modalities use sound waves and magnetism, respectively, rath...

  CONVENTIONAL RADIOGRAPHY Conventional radiography  refers to plain radiographs that are   generated when x-ray film is exposed to ionizing radiation and developed by photochemical process. During develop-ment, the metallic silver on the x-ray film is precipitated, ren-dering the latent image black. The amount of blackening on the film is proportional to the amount of x-ray radiation ex-posure. Plain radiography relies on natural and physical con-trast based on the density of material through which the x-ray radiation must pass. Thus, gas, fat, soft tissue, and bone produce black, gray-black, gray, and white radiographic im-ages, respectively, on film (Figure 1-1). Although other image modalities such as CT, ultra-sonography, and MR imaging are being used with increasing frequency to replace plain radiographs, conventional radiog-raphy remains a major modality in the evaluation of chest, breast, bone, and abdominal diseases. Computed radiography (CR) or digital radiograp...

  ULTRASONOGRAPHY   Diagnostic ultrasound is a noninvasive imaging technique that uses high-frequency sound waves greater than 20 kilo-hertz (kHz). A device known as a  transducer  is used to emit and to receive sound waves from various tissues in the body. Figure 1-7.  A transverse ultrasound image of the gallbladder demonstrates a gallstone (arrow) with the characteristic distal acoustic shadowing (S) because sound waves cannot penetrate the gallstone.   The transducer is placed against the patient’s skin with a thin layer of coupling gel. This gel displaces the air that would oth-erwise reflect virtually the entire incident ultrasound beam. As sound travels into the patient, wave fronts spread out, di-minishing the overall beam intensity. Beam attenuation also occurs secondary to partial tissue absorption with associated heat conversion. At tissue interfaces, the beam is partially re-flected and transmitted. The reflected sound waves, or echoes, travel b...

  MAGNETIC RESONANCE IMAGING   In 1952, Felix Bloch and Edward Purcell were awarded the Nobel Prize for their independent discovery of the magnetic resonance phenomenon in 1946. Between 1950 and 1970, nuclear magnetic resonance (NMR) was developed and used for chemical and physical molecular analysis. In 1971, Raymond Damadian demonstrated that NMR had utility in cancer di-agnosis, based on prolonged relaxation times in pathologic tissue. The first 2D proton NMR image of a water sample was generated in 1972 by Paul Lauterbur using a back-projection technique, similar to that used in CT. In 1975, Richard Ernst used phase and frequency encoding, as well as Fourier trans-form analysis, to form the basis of current magnetic resonance imaging (MRI) techniques. All of these experiments used de-fined, nonuniform magnetic fields or linear variations in field strength along all coordinate axes. The application of these nonuniform fields (magnetic field gradients) permitted dis-criminat...

  NUCLEAR MEDICINE   Nuclear medicine studies, in general, are very sensitive, but relatively nonspecific in the detection of pathology. It is very important, therefore, to correlate nuclear medicine examina-tions with pertinent history, physical findings, laboratory data, and other diagnostic imaging studies in order to opti-mize the diagnostic utility of these studies. Nuclear medicine imaging examinations are performed by administering vari-ous radiopharmaceuticals to the patient and subsequently recording in vivo distribution. Radiopharmaceuticals consist of two main components: (1) the main component that is distributed to various organs via a number of different mech-anisms, and (2) the radionuclide that is tagged to the main component, which emits gamma rays, permitting detection of the compound in the body.   Most nuclear medicine studies are performed with gamma cameras, which provide planar (2D) images. Single photon emission computed tomography (SPECT) is a spe...