T2* relaxation refers to decay of transverse magnetization caused by a combination of spin-spin relaxation and magnetic field inhomogeneity. T2* relaxation is seen only with gradient-echo (GRE) imaging because transverse relaxation caused by magnetic field inhomogeneities is eliminated by the 180 degrees pulse at spin-echo imaging. T2* relaxation is one of the main determinants of image contrast with GRE sequences and forms the basis for many magnetic resonance (MR) applications, such as susceptibility-weighted (SW) imaging, perfusion MR imaging, and functional MR imaging. GRE sequences can be made predominantly T2* weighted by using a low flip angle, long echo time, and long repetition time. GRE sequences with T2*-based contrast are used to depict hemorrhage, calcification, and iron deposition in various tissues and lesions. SW imaging uses phase information in addition to T2*-based contrast to exploit the magnetic susceptibility differences of the blood and of iron and calcification in various tissues. Perfusion MR imaging exploits the signal intensity decrease that occurs with the passage of a high concentration of gadopentetate dimeglumine through the microvasculature. Change in oxygen saturation during specific tasks changes the local T2*, which leads to the blood oxygen level-dependent effect seen at functional MR imaging. The basics of T2* relaxation, T2*-weighted sequences, and their clinical applications are presented, followed by the principles, techniques, and clinical uses of four T2*-based applications, including SW imaging, perfusion MR imaging, functional MR imaging, and iron overload imaging.

Steady-state sequences are a class of rapid magnetic resonance (MR) imaging techniques based on fast gradient-echo acquisitions in which both longitudinal magnetization (LM) and transverse magnetization (TM) are kept constant. Both LM and TM reach a nonzero steady state through the use of a repetition time that is shorter than the T2 relaxation time of tissue. When TM is maintained as multiple radiofrequency excitation pulses are applied, two types of signal are formed once steady state is reached: preexcitation signal (S-) from echo reformation; and postexcitation signal (S+), which consists of free induction decay. Depending on the signal sampled and used to form an image, steady-state sequences can be classified as (a) postexcitation refocused (only S+ is sampled), (b) preexcitation refocused (only S- is sampled), and (c) fully refocused (both S+ and S- are sampled) sequences. All tissues with a reasonably long T2 relaxation time will show additional signals due to various refocused echo paths. Steady-state sequences have revolutionized cardiac imaging and have become the standard for anatomic functional cardiac imaging and for the assessment of myocardial viability because of their good signal-to-noise ratio and contrast-to-noise ratio and increased speed of acquisition. They are also useful in abdominal and fetal imaging and hold promise for interventional MR imaging. Because steady-state sequences are now commonly used in MR imaging, radiologists will benefit from understanding the underlying physics, classification, and clinical applications of these sequences.

In this retrospective pilot study, PMSA was significantly lower in children with ESLD compared with healthy age- and gender-matched controls. Because this finding was independent of growth in ESLD subjects, PMSA may represent a novel objective nutritional biomarker in children with advanced liver disease.

Background Sarcopenia, the unintentional loss of skeletal muscle mass, is associated with poor outcomes in adult patient populations. In adults, sarcopenia is often ascertained by cross-sectional imaging of the psoas muscle area (PMA). Although children with chronic medical illnesses may be at increased risk for muscle loss because of nutritional deficiencies, physical deconditioning, endocrine anomalies, and systemic inflammation, consistent quantitative definitions for sarcopenia in children are lacking. We aimed to generate paediatric reference values for PMA at two intervertebral lumbar levels, L3-4 and L4-5. Methods In this cross-sectional study, we analysed abdominal computed tomography scans of consecutive children presenting to the emergency department. Participants were children 1-16 years who required abdominal cross-sectional imaging after paediatric trauma between January 1, 2005 and December 31, 2015 in a large Canadian quaternary care centre. Children with a documented chronic medical illness or an acute spinal trauma at presentation were excluded. Total PMA (tPMA) at levels L3-4 and L4-5 were measured in square millimetres (mm 2 ) as the sum of left and right PMA. Age-specific and sex-specific tPMA percentile curves were modelled using quantile regression. Results Computed tomography images from 779 children were included. Values of tPMA at L4-5 were significantly larger than at L3-4 at all ages, but their correlation was high for both girls (r = 0.95) and boys (r = 0.98). Amongst girls, tPMA 50th percentile values ranged from 365 to 2336 mm 2 at L3-4 and from 447 to 2704 mm 2 for L4-5. Amongst boys, 50th percentile values for tPMA ranged between 394 and 3050 mm 2 at L3-4 and from 498 to 3513 mm 2 at L4-5. Intraclass correlation coefficients were excellent at L3-4 (0.97, 95% CI 0.94 to 0.981) and L4-5 (0.99, 95% CI 0.986 to 0.995). Weight and tPMA were correlated, stratified by sex for boys (L3-4 r = 0.90; L4-5 r = 0.90) and for girls (L3-4 r = 0.87; L4-5 r = 0.87). An online application was subsequently developed to easily calculate age-specific and sex-specific z-scores and percentiles. Conclusions We provide novel paediatric age-specific and sex-specific growth curves for tPMA at intervertebral L3-4 and L4-5 levels for children between the ages of 1-16 years. Together with an online tool (https://ahrc-apps.shinyapps.io/sarcopenia/ ), these tPMA curves should serve as a reference enabling earlier identification and targeted intervention of sarcopenia in children with chronic medical conditions.

Every major vessel in the human body has a characteristic flow pattern that is visible in spectral waveforms obtained in that vessel with Doppler ultrasonography (US). Spectral waveforms reflect the physiologic status of the organ supplied by the vessel, as well as the anatomic location of the vessel in relation to the heart. In addition, the waveforms may be affected by age- and development-related hemodynamic differences. For example, adults tend to have higher flow velocities, whereas neonates, particularly those born prematurely, have higher resistance to flow, especially in the cerebral and renal vascular beds. As Doppler US is performed with increasing frequency for vascular evaluation in children, the recognition of normal flow patterns has become imperative. Familiarity with the waveforms characteristic of specific veins and arteries in children is important. In addition, an understanding of the hemodynamic factors involved provides a useful basis for interpreting waveform abnormalities.

Apart from serving as an important landmark for description and staging of pathologic conditions, the diaphragm is also affected by various types of pathologic conditions in children. Congenital abnormalities affecting the diaphragm include aplasia or hypoplasia, accessory diaphragm, eventration, and hernias. Congenital diaphragmatic hernias (CDHs) include Bochdalek, Morgagni, and hiatal hernias. Although survival rates are improving with the advent of new therapies, there is still significant morbidity and mortality associated with CDH. The morbidity and mortality depend on the associated congenital anomalies, the size of the hernia, and the degree of lung hypoplasia. Newer surgical repair techniques for the diaphragm have resulted in new postoperative complications and imaging appearances. Ultrasonography has become the modality of choice for evaluation of diaphragmatic paralysis. The diaphragm is uncommonly affected by trauma in children; when diaphragmatic injury is present, it usually indicates high-impact trauma associated with other severe injuries. Primary diaphragmatic tumors are very rare in children, with rhabdomyosarcoma being the most common. The diaphragm may also be secondarily involved by invasion of an adjacent tumor. Radiologists should have an understanding of the embryology, anatomy, and anatomic variations of the diaphragm and should be aware of the imaging appearances of pathologic conditions affecting the diaphragm in children.

Disorders of sex development (DSDs) are congenital conditions in which the development of chromosomal, gonadal, or anatomic sex is atypical. DSDs can be classified broadly into four categories on the basis of gonadal histologic features: female pseudohermaphroditism (46,XX with two ovaries); male pseudohermaphroditism (46,XY with two testes); true hermaphroditism (ovotesticular DSD) (both ovarian and testicular tissues); and gonadal dysgenesis, either mixed (a testis and a streak gonad) or pure (bilateral streak gonads). Imaging plays an important role in demonstrating the anatomy and associated anomalies. Ultrasonography is the primary modality for demonstrating internal organs; genitography is used to assess the urethra, vagina, and any fistulas or complex tracts; and magnetic resonance imaging is used as an adjunct modality to assess for internal gonads and genitalia. Early and appropriate gender assignment is necessary for healthy physical and psychologic development of children with ambiguous genitalia. Gender assignment can be facilitated with a team approach that involves a pediatric endocrinologist, geneticist, urologist, psychiatrist, social worker, neonatologist, nurse, and radiologist, allowing timely diagnosis and proper management.

Sarcopenia predicts morbidity and mortality in adults with end-stage liver disease (ESLD) and is determined by total psoas muscle area (tPMA) measurement from computed tomography (CT) imaging. Recently developed pediatric age-and sex-specific tPMA growth curves provide the opportunity to ascertain the prevalence and impact of sarcopenia in children awaiting liver transplantation (LT). We performed a retrospective single-center study to evaluate sarcopenia in children with ESLD between 1 and 16 years of age with a clinically indicated abdominal CT less than 3 months before first isolated LT. Sarcopenia was defined as a tPMA z score less than −2 measured at the intervertebral L4-5 level. Patient demographic, biochemical, and outcome data were recorded. tPMA was compared to other measures of nutritional and growth status using univariate and multivariate logistic analyses. Outcome measures included 1-year morbidity events and mortality after LT. CT images from 25 (64% female) children with a median age of 5.50 (interquartile range [IQR], 3.75, 11.33) years were reviewed. Of the 25 children with ESLD, 10 (40%) had a tPMA z score less than −2. Sarcopenia was associated with lower z scores for weight (odds ratio [OR], 0.38; P = 0.02) and height (OR, 0.32; P = 0.03) and nutritional support before LT (OR, 12.93; P = 0.01). Patients with sarcopenia had a longer duration of pediatric intensive care unit (PICU) stay (3.50 [IQR, 3.00, 6.00] vs. 2.00 [IQR, 2.00, 3.50] days; P = 0.03). Sarcopenia was prevalent in 40% of children with ESLD who underwent clinically indicated CT while awaiting LT, and lower tPMA z score was associated with anthropometrics and need for nutritional support before LT. Post-LT PICU duration was increased in children with sarcopenia, reflecting adverse outcomes associated with muscle loss. Further studies are needed to elucidate the underlying mechanisms of sarcopenia in children with ESLD. Sarcopenia, defined as a decrease in skeletal muscle mass and function, is a frequent finding in adults with cirrhosis. (1-3) In a recent consensus statement by the North American Working Group on Sarcopenia in Liver Transplantation, computed tomography (CT) assessment of total psoas muscle area (tPMA) was recommended as the gold-standard technique to assess sarcopenia in patients with cirrhosis. (4) Unaffected by ascites, tPMA is linearly related to whole body mass, providing an estimation of overall lean muscle mass in patients with end-stage liver disease (ESLD).