I am still not convinced. (Maybe I could ask, and answer, the same thing about abdominal radiography!)
In recent years in my practice we have had examples of echographic diagnosis of intramural gas in stable infants who had an abdominal ultrasound for other reasons (follow up of antenatal diagnosis of pylectasis for example). In follow up exams of babies with a clinical diagnosis of NEC we have seen ultrasound giving a diagnosis of necrotic bowel loops which were well perfused at laparotomy. One problem is the lack of a gold-standard non-invasive diagnosis, so determining sensitivity and specificity is not possible.
A new article suggests in contrast that ultrasound is useful for prognosis (Garbi-Goutel A, Brevaut-Malaty V, Panuel M, Michel F, Merrot T, Gire C. Prognostic value of abdominal sonography in necrotizing enterocolitis of premature infants born before 33weeks gestational age. J Pediatr Surg. 2014;49(4):508-13).
This article only includes infants who had a diagnosis of NEC and had both ultrasound and x-rays, so it would exclude our first group of problematic babies, and it does suggest some findings that might be useful for prognostication.
Free intra-peritoneal air was more frequently seen on ultrasound than on x-ray, so for diagnosis of perforation it may be more sensitive. It has already been shown that portal venous gas is much more commonly seen on ultrasound, this study confirms that, and I think gives you a definite diagnosis of NEC if you are not sure; at least if it happens to be present.
Peritoneal fluid was often seen on ultrasound in the new study, and had a positive correlation with a poorer outcome. One of the problems with this study though, is that a poorer outcome includes having surgery, and, as a retrospective study, it is not clear how many babies had surgery because the ultrasound showed peritoneal fluid.
One thing that the authors do not discuss is evaluation of bowel perfusion, which has been described previously, but I am very unsure how reliable it is, even with fancy colour doppler and the latest machines and software. I think we need more investigation of these techniques, with prospective studies evaluating reliability of diagnoses and prognoses.
On the other hand, abdominal radiography is subject to a lot of inter-observer variability, (see for example here) except in the most severe cases with extensive pneumatosis, where the agreement is higher (see here), and probably a third of perforations do not have visible free air on abdominal radiography, so we need more reliable, more objective ways of diagnosing NEC and determining need for surgery.