Factors Affecting Image Contrast

If it is desired to emphasize Tt differences, one easy choice is to repeat the pulse sequence with a short repetition time TR, so that magnetizations are unable to relax completely back to equilibrium. Thus signals Corn water molecules with long Tj are reduced in intensity. Alternatively, an inversion recovery sequence (180°, T, 90°) can be applied instead of just a 90° pulse to excite the resonance. With careful selection of the value of r, as indicated in Fig. 14.4, T, differences can be emphasized. 

In many instances, it has been found that pathological conditions can most easily be observed by emphasizing differences in T2. Because images are normally obtained by use of a spin echo sequence (90°, r, 180°), adjustment of r permits 

FIGURE 14.4 Recovery of magnetization to its equilibrium value after inversion in a 180°, r, 90° pulse sequence. The relative intensities of two components with different values of Ti can be varied by choice of value of r. 

---

Several factors affect the contrast of an image, namely count density, scattered radiations, type of film, size of the lesion, and patient motion. Each contributes to the contrast to a varying degree. These factors are briefly discussed here. 

Brandsch, R., Bar, G., and Whangbo, M.-H. 1997. On the factors affecting the contrast of height and phase images in tapping mode atomic force microscopy, Langmuir 13,6349-6353. 

Define contrast of an image. Elucidate the different factors that affect the contrast. 

Various factors affect the quahty of image that can be achieved. The way in which the specimen is prepared and mounted influences the type of objectives that can be used and which type of contrast enhancement filters are appropriate. 

In HREM images of inorganic crystals, phase information of structure factors is preserved. However, because of the effects of the contrast transfer function (CTF), the quality of the amplitudes is not very high and the resolution is relatively low. Electron diffraction is not affected by the CTF and extends to much higher resolution (often better than lA), but on the other hand no phase information is available. Thus, the best way of determining structures by electron crystallography is to combine HREM images with electron diffraction data. This was applied by Unwin and Henderson (1975) to determine and then compensate for the CTF in the study of the purple membrane. 

Although PET/CT modality is widely employed for diagnostic purpose, CT data acquired either for fusion of PET and CT images or for photon attenuation correction are affected by several factors that cause artifacts in PET/CT images. The factors include patient positioning, metal objects, CT contrast agents, truncation artifacts, and respiratory movement, which are discussed below. 

Image quality in CT is determined both by spatial resolution and contrast. It is the tube current (mA) that primarily affects spatial resolution, whilst the peak tube potential (kVp) affects both spatial and contrast resolution. Other factors influencing spatial resolution - the ability to observe small details - include in the imaging plane the collimation (in SDCT) - or section collimation (in MDCT) and the focal spot size of the tube. The slice (section) thickness and the choice of pitch influence spatial resolution along the Z-(long) axis of the patient - thinner slices and a lower pitch leading to an improvement in spatial resolution (Huda et al. 2002). 

---