US & MRI

4. Ultrasound (US)

- probes use acoustical energy above the audible frequency of humans to produce images

-the probe or transducer both produces ultrasonic signal and records it; the signal is processed by an on-board computer and the images recorded digitally and easily stored on PACS system

-advantages: relatively inexpensive, widely available, can be portable, does not use IR so particularly useful in imaging women of child-bearing age during pregnancy and in children

-disadvantages: cannot penetrate bones, large gas filled structures disrupt the ultrasound signal, difficult to visualize deep structures in the obese, operator-dependent

-study of first choice in imaging the female pelvis, paediatric patients, differentiating cystic vs solid, non-invasive vascular imaging, imaging foetus and placenta during pregnancy, in real-time image-guided fluid aspirations and biopsies

-other common uses- breast masses, thyroid nodules, tendons; assessing brain hips and spine in newborns

5.  Magnetic Resonance Imaging (MRI)

- utilizes the potential energy stored in the body’s hydrogen atoms- mostly those in water

- hydrogen atoms contain a single proton that can be made to act like a small magnet

- extremely strong magnetic fields and radiofrequency pulses can manipulate the hydrogen atoms to produce enough localizing and tissue-specific energy to allow highly sophisticated computer programs to generate 2D or 3D images

- gadolinium used primarily for better detection of lesions such as tumours abscesses or metastases

-bone is black, fluid is white

-advantages: no ionizing radiation, superior contrast between soft tissues to CT and can differentiate better between fat, water, muscle, and other soft tissues, can characterize and discriminate among tissues using their physical and biochemical properties (e.g., water, iron, fat, and extravascular blood and its breakdown products); Blood flow, cerebrospinal fluid flow, and contraction and relaxation of organs, both physiologic and pathologic, can be evaluated

-other advantages: Because calcium emits no signal on most MRI images, tissues surrounded by bone, such as the contents of the posterior fossa and the spine, can be imaged; MRI can produce images of equal resolution in any projection without moving the patient; MRI protocols can be programmed to acquire data on physiologic phenomena such as the velocity of moving blood or the diffusion of water (useful in detecting stroke).

-disadvantages: not as widely available, expensive, high ongoing operating cost, safety issues with the extremely strong magnetic fields, side effects from radio-frequency waves produce by the scanners, possible adverse effects from some MRI contrast agents

-widely used in neurologic imaging; particularly sensitive in imaging soft tissues like muscles tendons and ligaments