Low-dose technique

Martin et alJ106b have employed low-dose techniques to obtain selected area electron diffraction and high-resolution electron microscopy of these phenylacetylene dendrimer s. 

Most studies use a low dose technique, without additional use of IV contrast. The reasons are the care of radiation dose in a technique requiring prone-supine imaging, and the fear of adverse reaction using IV contrast. 

The low dose technique capitalizes on the high contrast that exists between the air-filled lumen and the soft tissue density wall. Polyps protruding into air-filled lumen can be detected using these low-dose technique. 

In our institution, we use a low dose technique (140 KV, with 10 mAs for supine, and 30 mAs for prone scanning), without IV contrast. For the evaluation of the colon, 0.6-mm slices are reconstructed at every 0.3 mm for the evaluation of extracolonic findings, 3 mm thick slices are reconstructed at every 1.5 mm. The latter reduces noise, enabling visualisation of solid organ lesions (Fig. 10.4). 

It can thus be concluded that low dose technique without IV contrast results in possible, but low (<1.5%) false negative diagnosis (Kara 2005). 

Most studies using low dose CT without IV contrast report upon hepatic or renal cysts that were further evaluated. False positives are thus caused by a low dose technique without IV contrast. For example, in the study of Hellstrom et al. (Hellstrom et al. 

Dynamic scanning can be performed as a continuous acquisition with uninterrupted radiation exposure or as an intermittent series with variable intervals ranging from 0.5 to 30 s between acquisitions. Additionally, as a low-dose technique is preferable, several rotations can be combined to generate one time-averaged acquisition to reduce noise and improve image quahty. The rotation time, tube current, and voltage can be set individually for each acquisition, and it is possible to switch from continuous to intermittent acquisition and vice versa. 

An important issue in perfusion imaging is to obtain diagnostic information with a reasonable radiation exposure, which increases with the number of scans acquired. Only strict use of low-dose techniques (e.g., 80 kV with a dramatically reduced mA) wiU restrict radiation exposure to the scope of a conventional diagnostic CT examination. 

Zaporozhan J, Ley S, Eberhardt R et al. (2005) Paired inspira-tory/expiratory volumetric thin-slice CT scan for emphysema analysis comparison of different qimntitative evaluations and pulmonary function test. Chest 128 3212-3220 Zaporozhan J, Ley S, Weinheimer O etal. (2006) Multi-detector CT of the chest influence of dose onto quantitative evaluation of severe emphysema a simulation study. J Comput Assist Tomogr 30 460-468 Zhang J, Hasegawa I, Feller-Kopman D, Boiselle PM (2003) 2003 AUR Memorial Award. Dynamic expiratory volumetric CT imaging of the central airways comparison of standard-dose and low-dose techniques. Acad Radiol 10 719-724... 

When the low dose technique works well, the resolution in the image of a beam sensitive material approaches its theoretical limit (Section 3.4). Obtaining optimum imaging conditions beyond this point means improving this theoretical limit. The limit depends on the following four factors, which will be discussed further ... 

If possible, a low-dose technique should be used, which means focusing on one place on the sample and taking micrographs of a different (previously nonirradiated) place. So, the electron beam is directed onto the feature only when it is being recorded. 

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