1. © Jimoid.com 2005 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 Structure e.g. chest X-ray Organ function e.g. renal scan Organ function e.g. renal scan A combination of the above e.g. functional MR A combination of the above e.g. functional MR The applications of medical images include: Screening Screening Diagnosis Diagnosis Treatment planning Treatment planning Intervention Intervention

2. © Jimoid.com 2005 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. © Jimoid.com 2005 Imaging Modalities Imaging Modality ProbeInteractionPropertyImage Radiography 80 kV X-ray Photoelectric, Compton Attenuation map X-ray transmission 2D projection Computed Tomography 120 kV X-ray Photoelectric, Compton Attenuation map 3D reconstruction from multiple tomographs Ultrasound Megahertz sound pulses Reflection, refraction Tissue density, boundaries 2D sector, reflection Magnetic Resonance Electromagnetic pulses Resonance of nuclear spins Proton density 3D reconstruction, EM signal map Nuclear Medicine γ -ray emitting isotope Uptake of radiopharmaceu tical Concentration of pharmaceutical Distribution of emissions

4. © Jimoid.com 2005 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. © Jimoid.com 2005 Image Noise All medical images are subject to noise. The sources of noise include: Scattering Scattering Detector inaccuracies Detector inaccuracies Electronic interference Electronic interference Signal processing 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.