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Imaging Basics

Imaging Basics. A medical image is a 2D or 3D distribution of signals which represent properties of an object. The purpose of medical images include: Structure e.g. chest X-ray Organ function e.g. renal scan A combination of the above e.g. functional MR

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Imaging Basics

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  1. Imaging Basics • A medical image is a 2D or 3D distribution of signals which represent properties of an object. • The purpose of medical images include: • Structure e.g. chest X-ray • Organ function e.g. renal scan • A combination of the above e.g. functional MR • The applications of medical images include: • Screening • Diagnosis • Treatment planning • Intervention

  2. Image Production • There are few natural signals emitted by the human body which are useful to create medical images. It is therefore necessary to use external (X-rays, ultrasound etc.) or internal (radiopharmaceuticals) probes to image. Images are created by measuring the transmitted or emitted signals, after the signal has interacted with the tissue. The signal needs to be strong enough to penetrate the tissue, but not so strong that its interactions with tissue cannot be detected. Also the image quality versus dose to patient must be optimised. • Resonant probes have wavelengths similar to that of tissue and the probe is attenuated and transmitted (X-rays, γ-rays, RF, infrared). • Non-resonant probes do not have similar to tissue. The signals are reflected and refracted (ultrasound).

  3. Imaging Modalities

  4. Image Display • It is important that the probe is sampled frequently enough so that no information is lost. If it is under-sampled aliasing can occur and image information lost. • Image contrast is very important. With analogue images it is necessary to choose optimal parameters before image acquisition as post exposure processing is not possible. With digital images windowing is possible to create maximum contrast. • With film images it is necessary to use a dark room to allow maximum image contrast for the human eye.

  5. Image Noise • All medical images are subject to noise. The sources of noise include: • Scattering • Detector inaccuracies • Electronic interference • Signal processing • The effect of noise on the image is to decrease contrast and detail. The important quantity in any image is the signal to noise ratio SNR. The higher this value the better the quality of the image.

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