Erro ao tranformar de base64 para imagem

Galera estou convertendo de base64 para imagem pelo asp.net com C# no site aqui, porém esta dando erro, alguém pode me auxiliar?

Segue erro:

Compilation Error 

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS0246: The type or namespace name 'MemoryStream' could not be found (are you missing a using directive or an assembly reference?)

Source Error:

Line 17: // Convert Base64 String to byte[]

Line 18: byte[] imageBytes = Convert.FromBase64String(base64String);

Line 19: MemoryStream ms = new MemoryStream(imageBytes, 0, imageBytes.Length);

Line 20:

Line 21: // Convert byte[] to Image

Erro na linha 19

Segue o Código:

<%@ Page Language="C#" ContentType="text/html" ResponseEncoding="utf-8" %>
<script runat="server">
void Page_Load(object sender, System.EventArgs e){

string base64String;
base64String = Request["teste"];

byte[] imageBytes = Convert.FromBase64String(base64String);
MemoryStream ms = new MemoryStream(imageBytes, 0, imageBytes.Length);

ms.Write(imageBytes, 0, imageBytes.Length);
Image image = Image.FromStream(ms, true);

image.Save("imagens/" + image);
Response.Write(image);

}
</script>