In parasitic cysts, some anaerobic metabolism is usually present, even if the aerobic pathway is active to some extent (Garg et al. 2002). As a result, a variety of metabolites involved in anaerobic metabolism are usually found. Of the parasitic cysts, hydatid cyst is a typical example that shares many similarities and some differences with other parasitic cysts. The frequently described metabolites found in hydatid fluid in reasonable amounts are succinate, pyruvate, acetate, lactate and alanine, while a few others such as malate, fumarate, glycine, other amino acids, Cho (and Cho-containing compounds), propionate, etc. are found in smaller amounts or traces (McManus and Smyth 1978; Chang et al. 1998; Garg et al. 2002; Jayakumar et al. 2003). The frequency and success of detection of metabolites depend largely on the technique used.

Pyruvate is a product of glycolysis and reported to be found in abundance in hydatid fluid; investigators have confirmed its presence using mass spectroscopic measurements (Jayakumar et al. 2003). It is also thought to be an indicator of viability as it has not been detectable in cysts that show degeneration on histopathologic examination (Jayakumar et al. 2003). Pyruvate can be metabolized to acetate and lactate which are important and easily detectable metabolites in hydatid fluid, lactate being more prominently seen. Under anaerobic conditions, the partial tricarboxylic acid (TCA) cycle is active, in which phosphoenolpyruvate formed during glycolysis partially converts into oxaloacetate and then further into malate, using malate dehydrogenase (MDH). It further transforms into fumarate and succinate (McManus and Bryant 1995; Garg et al. 2002). In hydatid, the activity of MDH and other related enzymes has been reported as more favourable for succinate production by some of the investigators (Garg et al. 2002), who regard succinate as a very important metabolite present in significant amount in hydatid cysts and have confirmed its presence by specialized techniques (Garg et al. 2002). However, other investigators have given more importance to pyruvate, describing it as a marker of cestodial infestation and have downplayed the importance of succinate by citing low biochemical activity of succinate dehydrogenase (Jayakumar et al. 2003). Therefore, in the literature, there is no consensus on the relative importance of succinate and pyruvate, and as discussed below, it has important ramifications for ¹H MRS.

The only significant amino acid found is alanine, which is derived from pyruvate (Kohli et al. 1995; Jayakumar et al. 2003), while others such as leucine, valine and isoleucine are found in small amounts. Fumarate and malate are only detectable in traces but are considered as important metabolites that indicate fertility and can also be useful for differentiation from NCC cysts (Garg et al. 2002).

While both PRESS and STEAM techniques have been used, PRESS is more commonly performed. Both SVS and CSI techniques have been successfully used. At 1.5 T, different researchers use (time to repeat) TR of 1,500–3,000 ms with satisfactory results. Usually, a (time to echo) TE of 135–144 ms is used, as the metabolites usually found in hydatids are reasonably detected at these TE values, although some researchers have also used shorter TEs in addition (Chang et al. 1998). A variable NEX can be performed, depending on several factors. It is preferable to obtain the spectrum from the cystic part, avoiding the solid tissue/wall/host tissue as it can result in further metabolites (Fig. 13.3).

Of the several metabolites described, only a few are routinely picked up reliably on in vivo ¹H MRS due to sensitivity issues. In particular the peak at 2.4 ppm is important as both pyruvate and succinate are detected at 2.4 ppm. These are difficult to differentiate on in vivo ¹H MRS, and as discussed above, there is no consensus in the literature as to which of these metabolites are predominantly represented at 2.4 ppm, although their presence has been confirmed in separate studies using different techniques (Garg et al. 2002; Jayakumar et al. 2003, 2004). While this issue is pending, awaiting further investigation, still, a prominent peak at 2.4 ppm (pyruvate/succinate) is one of the most important features in in vivo ¹H MRS.

Other important metabolites detected by in vivo technique are lactate (1.3 ppm, seen as a doublet), acetate (1.9 ppm) and alanine (1.5 ppm) (Chang et al. 1998; Garg et al. 2002; Jayakumar et al. 2003). While these could be variable, lactate is usually seen as a significant peak. Acetate is detectable although it is a smaller peak relative to the succinate/pyruvate. As a result, the ratio of succinate/acetate is generally quite high in hydatid cysts and is an important differentiating feature with pyogenic abscesses, where acetate/succinate ratio is high (Poptani et al. 1995; Martinez-Perez et al. 1997; Dev et al. 1998; Chand et al. 2005). It has been further suggested that various metabolites described in hydatid cysts may not be seen in all spectra, due to changes in internal milieu during evolution of the cyst (Jayakumar et al. 2003). Also, various forms of echinococcus may have different consumptions of oxygen and glycogen, thereby producing different concentration of metabolites (Jayakumar et al. 2003). Thus, despite having some uniformity in basic spectra, variations are possible.

Ex vivo ¹H MRS detects a larger number of metabolites from hydatid fluid. These include amino acids leucine, valine and isoleucine (0.9–1.2 ppm), glycine (3.56 ppm), Cho (3.2 ppm), malate (4.3 ppm), fumarate (6.5 ppm), glucose and some of the aromatic amino acids (Garg et al. 2002).

The size and appearance of NCC are usually fairly different from hydatid cysts and rarely present a diagnostic dilemma; however, large and racemose lesions can create difficulty in diagnosis on routine MRI (Jayakumar et al. 2003). The issue becomes important due to the different management of these conditions (medical in NCC versus surgical in hydatid).