GetListItems returns null for LookUp columns

I have a list which contains a LookUp field from another list. What I want to do is get the value of this column using the usual ListService.GetListItems. But the data I get is null. Same thing happens when I try to get the value of a calculated value field, I get a "float;#17.97000" (17.97 being the value).

Can I manage to get those fields correctly?

public List<Cartridge> getData()
        {
            XmlDocument xmlDoc = new XmlDocument();
            XmlNode query = xmlDoc.CreateNode(XmlNodeType.Element, "Query", "");
            XmlNode viewFields = xmlDoc.CreateNode(XmlNodeType.Element, "ViewFields", "");

            query.InnerXml = "<OrderBy>" +"<FieldRef Name='Imprimante'/>" +"</OrderBy>";

            viewFields.InnerXml = "<FieldRef Name='Title'/>" +"<FieldRef Name='Imprimante'/>" +"<FieldRef Name='Stock'/>" +"<FieldRef Name='Fournies'/>" +"<FieldRef Name='PrixUnitaire'/>" +"<FieldRef Name='PrixTotalFournies'/>";

            XmlNode listItems = ListService.GetListItems("Cartouches d'encre", null, query, viewFields, "10000000", null, null);

            List<Cartridge> cartridges = new List<Cartridge>();

            for (int i = 1; i != listItems.ChildNodes[1].ChildNodes.Count; i += 2)
            {
                cartridges.Add(new Cartridge(listItems.ChildNodes[1].ChildNodes[i].Attributes[0].InnerXml,
                                                listItems.ChildNodes[1].ChildNodes[i].Attributes[2].InnerXml,
                                                listItems.ChildNodes[1].ChildNodes[i].Attributes[3].InnerXml,
                                                listItems.ChildNodes[1].ChildNodes[i].Attributes[4].InnerXml,
                                                listItems.ChildNodes[1].ChildNodes[i].Attributes[5].InnerXml));
            }
            return cartridges;
        }