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Image-guided proton MRS (1H MRS) is a technique that provides metabolic information about the prostate gland, which may be used for in situ characterisation, diagnosis and therapy evaluation of prostate cancer (Fig. 6). Although the examination is comparable with MR imaging, the spatial resolution is lower (down to 0.24 cm3 has been reported for the prostate) [43] and the information obtained is related to metabolites rather than anatomy. It has been shown that prostate cancer is characterised by a decreased level of citrate and an increased level of (phos-pho)choline [44]. Especially in the PZ, tumour tissue can be identified by an increased choline/citrate (or choline+creatine/citrate) signal-ratio [44, 45]. Correlations have been reported between metabolite ratios and the his-tologic grade in human prostate cancers [46]. The addition of !H MRS to (dynamic) MR imaging can improve tumour visualisation and spatial extent [43, 47]. Potential areas of

Fig. 6. MRSI. Metabolic map showing high choline/citrate ratio in left peripheral zone. Prostatectomy confirmed cancer prostate cancer management that may benefit from the 1H MRS information include targeted TRUS-guided biopsies for patients with PSA levels indicative of cancer but negative previous biopsies, therapy monitoring (watchful waiting) and guiding focal prostate cancer therapies [48].

Nodal Staging Using Nano-Particles [48,49]

New MR contrast agents with ultra small super paramagnetic iron oxide (USPIO) particles are currently under investigation. In normal lymph nodes with functioning macrophages, the iron oxide particles are phagocytosed and thereby decrease the signal intensity on MR imaging. Metastatic nodes, lacking macrophages, do not take up the contrast agent and hence show no change in signal on post contrast images. These agents may increase sensitivity for nodal metastasis, by detection of metastasis in normal sized nodes. When using high resolution MR-technique, small metastases (3-7 mm) can be prospectively recognised in small (5-10 mm) size lymph nodes. These small lymph nodes would be considered to be benign in plain MRI or CT examinations. In addition, hyperplastic enlarged nodes can be correctly recognised as non-metastatic, based on their low signal intensity. Using USPIO, MRI patients may be reliably selected for prostatectomy or radiotherapy without the need for invasive and costly procedures such as open and laparoscopic pelvic lymphadenec-tomy (PLND). Furthermore, USPIO can identify large malignant nodes and extracapsular extension. If the node is not too small the presence of a malignancy can be confirmed by image-guided biopsy in 70% of the cases, and thus also avoid PLND in these patients. Finally, Fowler and Whitmore showed that 12% of all positive-node patients or 5% of all patients subjected to PLND will have the positive nodes missed, because they are located in the internal iliac or common iliac region only and thus are not included in the modern modified PLND for prostate cancer. With USPIO MRI all pelvic nodes are visualised. Harisinghani et al. [49] showed that in 9 of their 80 patients with prostate cancer in which post USPIO MRI suggested metastases outside the classical field of lymph node resection (Fig. 7), and who underwent more extensive exploration, true metastases were confirmed at these sites.

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