Perfusion imaging techniques

By far, the most widely used and most empirically studied tissue viability imaging techniques are those that study tissue perfusion, and discussion of perfusion imaging techniques will dominate this section. We will also mention a few emerging techniques that currently are not as widely used in the acute stroke setting, but show promise for the future. 

Perfusion imaging techniques study pathophysiologic events that occur in capillaries and other microscopic blood vessels that cannot be seen by angiographic techniques like CTA or MRA. The perfusion imaging techniques in most widespread clinical use are performed using CT or MRl, and generally obtain or... 

Aksoy, F.G. and M.H. Lev, Dynamic contrast-enhanced brain perfusion imaging technique and clinical applications. Semin Ultrasound CT MR, 2000. 21(6) p. 462-77. 

Table 8.2 highlights some of the difficulties entailed in using PWl to identify the ischemic core and penumbra. One such difficulty is the use of PWl in delineating the infarct core. It has been suggested that the quantitative analysis of CBF maps could be used to identify the core, because only blood flow below a certain level would be expected to result in infarction. However, this cannot be done reliably for several reasons. First, non-viable core tissue may exhibit postischemic hyperperfusion, in which case, obviously, no threshold of hypoperfusion would identify the tissue as part of the core. Second, perfusion imaging techniques that rely on... 

Precise quantification of CBF and ischemic duration is not possible in the clinical setting [110]. Instead, a semi-quantitative approach has been used to estimate the ischemic penumbra. Using perfusion imaging techniques, a... 

The principle of todays myocardial perfusion imaging techniques, no matter what modality actually being used, is based on the comparison of stress and rest studies. This strategy actually rehably differentiates persistent from reversible myocardial perfusion deficits. Apphcation of this principle to CT techniques would in addition double the radiation dose. OveraU, these current limitations prohibit CT dynamic perfusion imaging being used in clinical appfications. 

Nuclear imaging techniques, like single photon emission tomography (SPECT) and positron emission tomography (PET), directly assess myocardial perfusion, cell membrane integrity, cellular metabolism, and the molecular mechanisms of ischemic viable or necrotic myocardium, thereby indicating revascularization procedures or not. 

Earlier we described how pharmacokinetic and dynamic properties of inhaled drugs are relevant for pulmonary selectivity. The assessment of pharmacokinetic and dynamic properties is consequently relevant for drug development and clinical practice. This section reviews some of the relevant techniques for assessing such properties. The available tools range from cell culture or isolated lung perfusion models to mucociliary clearance analysis, imaging techniques, and in vivo pharmacokinetic and dynamic analysis of the inhaled drug. 

The most recent imaging techniques are CMR (Figure 1.4) and CMDCT (Figure 1.1). The latter is used for non-invasive study of coronary tree. CMR, which may also be used for perfusion and func-... 

Non-invasive imaging techniques, especially SPECT, are very useful in detecting perfusion defects during exercise test. 

The injury vector has approximately the same direction as that of the vector of ischaemia and infarction but opposite sense (see p. 35, 60 and 131 and Figures 3.6, 4.8 and 5.3). Therefore, most probably, in case of injury of the lateral wall, an ST-segment depression will be especially recorded in V1-V2, and in case of injury of the inferobasal wall, the ST-segment depression will be recorded especially in V2-V3. However, further perfusion studies, with imaging techniques in the acute phase have to be done to validate this hypothesis. 

V1-V3 and deficits of perfusion detected by SPECT (nuclear medicine or other imaging techniques) will tell us if this hypothesis is correct. 

Influence of several nitrosyl-iron complexes on relaxation rates was assessed to establish a MR imaging technique for nitric oxide (NO). " To induce formation of nitrosyl-ion complexes in a biological tissue, isolated rat liver was perfused with a saline solution of the NO donor sodium nitroprus-side (SNP), and the MR signal intensity was examined thereafter. Nitrosyl-iron complexes exhibit a contrast effect at MR imaging that can be exploited for NO imaging in living animals and patients with conventional MR imaging. 

Among the neutron-deficient isotopes of Tc, " Tc seems to have suitable nuclear properties for application in positron emission tomography (PET) (Browne and Firestone 1986). Considering the relatively new lipophilic Tc-labeled flow tracers used in cardiac perfusion imaging (2-methoxyisobutyl isocyanide [MlBl], teboroxime, tetrofosmin) it would be reasonable to replace Tc with " Tc in order to carry out quantitative investigations of biodistribution and clearance with PET. With this replacement, the pharmacokinetics of useful Tc compounds could be studied in a number of cases, which would enhance the introduction of new Tc-labeled pharmaceuticals into clinical practice. The labeling and quality control methods are well known and partly subject of this book. Our task is to review the different methods for the production of Tc and also the separation technique from irradiated targets. 

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