The United States Supreme Court held that a state must equip a defendant with the primary tools needed to present a defense. Ake v. Oklahoma, 470 U.S. 68, 77, 84 L. Ed. 2d 53, 105 S. Ct. 1087 (1985). The Texas Court of Criminal Appeals extended this to apply to an expert in any field. Rey v. State, 897 S.W.2d 333, 338 (Tex. Crim. App. 1995). However, this right does not imply that an indigent defendant may be given "'all the assistance that his wealthier counterparts might buy.'" Griffith v. State, 983 S.W.2d 282, 286 (Tex. Crim. App. 1998), cert. denied, U.S., 120 S. Ct. 77, 145 L. Ed. 2d 65 (1999) (citing Ake, 470 U.S. at 77). The aim behind it is to provide him with access to an expert able to help him evaluate, prepare, and present his defense. Griffith, 983 S.W.2d at 286. The defendant carries the burden of making a sufficient threshold showing that he needs the expert's service. Id.