Get Parent value in Child Record in MS CRM

function GetParentValue() {
//Fetch the case id.
//calling this function on Email form
    var regardingObject = Xrm.Page.getAttribute("regardingobjectid");
    if (regardingObject.getValue() != null) {
        //OData URI to get address information from parent account record
        var oDataURI = Xrm.Page.context.getClientUrl()
            + "/XRMServices/2011/OrganizationData.svc/"
            + "IncidentSet(guid'" + regardingObject.getValue()[0].id + "')"
            + "?$select=Title,ContactId";

        //Asynchronous XMLHttpRequest to retrieve account record
        var req = new XMLHttpRequest();
        req.open("GET", encodeURI(oDataURI), true);
        req.setRequestHeader("Accept", "application/json");
        req.setRequestHeader("Content-Type", "application/json; charset=utf-8");
        req.onreadystatechange = function () {
            debugger;
            if (this.readyState == 4 /* complete */) {
                req.onreadystatechange = null; //avoids memory leaks
                if (this.status == 200) {
                    //parse the response string as a JSON object into the successCallback method.
                    //successCallback(JSON.parse(this.responseText).d);
                    //Directly show the alert and assign accordingly to other values
                    alert(JSON.parse(JSON.parse(this.responseText).d.Title));
                }
                else {
                    errorCallback(CaseID);
                }
            }
        };
        req.send();
    }
}

//Call the GetParentValue method on OnLoad.