Liver, Pancreas and Biliary TractUsefulness of chemical-shift MRI in discriminating increased liver echogenicity in glycogenosis
Introduction
Glycogen storage diseases (GSDs) are inherited defects of glycogen metabolism, which cause accumulation of glycogen, of normal or abnormal quality, in the tissues.
Most of these diseases are inherited as autosomal recessive traits. The overall frequency of all forms of GSDs is approximately 1 in 20,000 live births. More than 12 types are recognized and were categorized in the order in which the enzymatic defects were identified. The most common childhood disorders are type I, II, III and IX. Type IA GSD is caused by a deficiency of glucose-6 phosphatase activity in the liver, kidneys and intestinal mucosa with excessive accumulation of glycogen in these organs. Clinical manifestations of type IA GSDs are growth retardation, hepatomegaly, hypoglycemia, lacticacidemia, hyperuricemia and hyperlipidemia. Type IB GSD is a variant of type IA caused by a defect in the system for transporting glucose-6-phosphate across the microsomal membrane. Type III glycogen storage disease is caused by deficiency of glycogen debranching enzyme activity. Deficiency of glycogen debranching enzyme produces hepatomegaly, hypoglycemia, short stature, variable skeletal myopathy and cardiomyopathy. Histology of the liver is characterized by distention of hepatocytes by glycogen [1].
On sonography (US), most patients with type I and type III GSDs show increased liver echogenicity [2], [3], [4]. On computed tomography (CT), glycogen deposition results in increased density [5].
Fatty liver or steatosis is defined as accumulation of fat within hepatocytes [6]. Hepatic fat can result from a variety of diseases including increased production or mobilization of fatty acids or decreased hepatic clearance of fatty acids due to hepatocellular injury [7]. On US, fatty liver produces a diffuse increased echogenicity with posterior beam attenuation [8], [9]. On CT, fatty change determines a low-density hepatic parenchyma [7]. Chemical-shift magnetic resonance imaging (MRI) (opposed-phase imaging) provides a method to differentiate tissues containing water “only” from those containing both fat and water [6]. Chemical-shift results when nuclei in different chemical environments experience slightly different magnetic fields strengths. Chemical-shift MRI is best suited to suppressing the signal from tissues that contain similar amounts of lipid and water, for example fatty liver; it is not a fat-suppressed sequence, as subcutaneous fat remains hyperintense on opposed-phase images [6], [10], [11].
On US, liver shows increased echogenicity both in GSD and in steatosis. Liver hyperechogenicity in GSDs may depend on the accumulation of either glycogen or fat [2], [4].
The primary aim of the present study was to evaluate the usefulness of liver chemical-shift MRI for detecting liver steatosis in patients with metabolic impairment due to glycogen storage diseases.
Section snippets
Patients and methods
A sample of 36 subjects admitted to the San Paolo Hospital, Milan, Northern Italy, between January 2004 and April 2006 was studied. The local Ethical Committee approved the protocol. A written informed consent was obtained from patients (pts), or their parents, after the details of the protocol had been fully explained.
Twelve pts (8 males, 4 females, aged 10–24 years) consecutively referred to the Department of Paediatrics for type I (n = 8) or type III (n = 4) GSDs were studied. Because no
Results
Table 1 shows some clinical characteristics of the studied subjects.
No focal liver lesions were observed in any patient.
Fat fraction calculated with MRI was significantly associated with liver echogenicity in overweight-obese subjects (Spearman's correlation coefficient, r = 0.881, p < 0.0001) but not in subjects with GSDs (r = 0.510, p = 0.102).
Liver echogenicity among patients with GSDs was classified as follows: grade 0 in five cases, grade 1 in two cases, grade 2 in two cases and grade 3 in the
Discussion
Diagnostic imaging is useful in the detection and quantification of fatty infiltration of the liver. In a study comparing US with histological examinations, US showed a sensitivity of 90%, and a specificity of 82%, with a positive predictive value of 87%, and a negative predictive value of 87% for steatosis [15]. Moreover, when compared with biopsies, US was 100% and 93% sensitive in detecting moderate or severe (>30%) fatty infiltration with positive predictive values of 62% and 76%,
Conflict of interest statement
None declared.
Acknowledgement
The Authors wish to thank Mr Guido Pasini, medical radiation technologist, for his skilled assistance.
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