![]() ![]() What is MRI imaging? How MRI Images are produced? Fat is a darker shade of grey than water (-70 HU), while blood is a lighter shade of grey as compared to water (70 HU). Bone, on the other hand, absorbs radiation completely (1000 HU) and appears completely white. For instance, air does not absorb any radiation (-1000 HU), and so appears completely black. Tissues with a greater number of Hounsfield Units appear whiter than the rest, while tissues with a lower HU value appear blacker. The amount of absorbed radiation is measured as Hounsfield Units (HU). Various tissues in the body absorb different amounts of radiation, and emit the rest. Knowing the ‘color’ that a particular structure takes on is helpful during identification. Once you are oriented to the plane and direction, begin identifying different structures present in a single cross-section. This is right, 12 o’clock is anterior, 3 o’clock is left, and 6 o’clock is the posterior part of the cross-section. ![]() To get your bearings, hold the film in front of you and begin at the part of the image that lies in the 9 o’clock position. You would be looking at one of these slices as though you are lying on the floor, staring upwards. Imagine the patient’s body as being divided into several slices, using a cutting disc that is parallel to the ground surface. Most CT images are presented in the transverse or axial section. However, it is always helpful to keep the following tips in mind when a CT scan is being interpreted: It requires a few years of training and study to learn how to properly interpret a CT scan and make a clinical diagnosis from it. Reading CT scan images requires a good knowledge of anatomy and a sound sense of the orientation of different body structures. These slices may be stacked together to obtain a three dimensional image of the body part. Several such rotations occur, until a series of slices representing the entire body part is obtained. The next rotation of the source brings out a different slice of the body. Each slice can vary from 1 mm to 10 mm in thickness, depending on the type of machine used. The reconstructed image appears as a two-dimensional, cross sectional image ‘slice’ of the body part. Data obtained from one complete rotation of the X-ray source is reconstructed using mathematical techniques. The data from the detector is then transmitted to the computer. The X-ray detector within the CT scanner is more sensitive than the traditional X-ray film, and can pick up several degrees of distribution density. Once the X-rays pass through the body, they are picked up by special digital X-ray detectors, rather than films. As a specific body part crosses the tunnel, X-rays enter the body in all directions. There is a motorized X-ray source that rotates all around the circumference of the gantry, emitting several narrow X-ray beams. The patient lies down on an imaging table, which then slowly passes through this gantry. The CT machine consists of a circular, donut shaped device, called a gantry.
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