MRI investigation of normal fetal lung maturation using signal intensities on different imaging sequences

To purpose of this paper is to study the relation between normal lung maturation signal and changes in intensity ratios (SIR) and to determine which magnetic resonance imaging sequence provides the strongest correlation of normal lung SIs with gestational age. 126 normal singleton pregnancies (20–37 weeks) were examined with a 1.5 Tesla unit. Mean SIs for lungs, liver, and gastric fluid were assessed on six different sequences, and SIRs of lung/liver (LLSIR) and lung/gastric fluid (LGSIR) were correlated with gestational age for each sequence. To evaluate the feasibility of SIRs in the prediction of the state of the lung maturity, accuracy of the predicted SIRs (D*) was measured by calculating relative residuals (D*−D)/D for each sequence. LLSIRs showed significant changes in every sequence (p<0.05), while LGSIRs only on two sequences. Significant differences were shown for the mean of absolute residuals for both LLSIRs (p<0.001) and for LGSIRs (p=0.003). Relative residuals of LLSIRs were significantly smaller on T1-weighted sequence, whereas they were significantly higher for LGSIRs on FLAIR sequence. Fetal liver seems to be adequate reference for the investigation of lung maturation. T1-weighted sequence was the most accurate for the measurement of the lung SIs; thus, we propose to determine LLSIR on T1-weighted sequence when evaluating lung development. Electronic supplementary material Supplementary material is available for this article at and is available for authorized users.     

胎儿继发性肺发育不良的MRI评价

目的 评价联合应用胎儿肺总体积(TLV)和肺肝信号强度比(LLSIR)预测胎儿肺发育不良的应用价值. 资料与方法 回顾性分析40例随访胎儿的MRI表现.40例胎儿分为对照组(A组,n=20)和肺发育不良组(B组,n=20),采用感兴趣区测量所有胎儿的TLV和LLSIR并进行统计分析. 结果 对照组胎儿TLV为12.9～120.8 cm3,与孕周呈线性正相关 (r=0.931,P=0.000＜0.01),随孕周增加而增加;对照组LLSIR为1.20～2.24,与孕周相关性不明显(r=0.479,P=0.033＞0.01).肺发育不良胎儿TLV较正常胎儿明显减小(P=0.000＜0.05),其中羊水过少的肺发育不良胎儿LLSIR明显减低(P=0.002＜0.05). 结论 联合应用胎儿TLV和LLSIR能很好地预测胎儿肺发育不良.     

Assessment of lung development using hyperpolarized helium-3 diffusion MR imaging

PURPOSE: To determine whether hyperpolarized helium-3 (HHe) diffusion MR can detect the expected enlargement of alveoli that occurs with lung growth during childhood. MATERIALS AND METHODS: A total of 29 normal subjects aged four to 30 years underwent HHe diffusion MR imaging with the b-value pair 0, 1.6 second/cm(2). A second acquisition during a separate breathhold was performed using the b-value pair 0, 4 second/cm(2) to evaluate the dependence on b-value. The mean apparent diffusion coefficient (ADC) and lung volume for each acquisition and each subject was determined. RESULTS: Subjects as young as four years of age were able to cooperate with the imaging procedure. The mean ADC increased with increasing subject age (r = 0.8; P < 0.001), with a 55% increase in mean ADC from the youngest to oldest subject. Lung volumes measured on MR were highly repeatable for the two HHe MR acquisitions (r = 0.980, P < 0.001). The mean ADC values measured with the two different b-value pairs were highly correlated (r = 0.975; P < 0.001), but the higher b-value pair resulted in slightly lower mean ADCs (P < 0.001). CONCLUSION: HHe diffusion MR appears to detect the expected increase in alveolar size during childhood, and thus HHe MR may be a noninvasive method to assess development of the lung microstructure.     

Magnetic resonance imaging of the fetal lung: a pictorial essay

Ultrasound, which is now a widely available and generally accepted, low-cost technique with real-time properties, is the screening investigation of choice in fetal medicine. However, enthusiasm for fetal prenatal magnetic resonance imaging (MRI) is rising, because of the absence of known biological risks, the increasing ease of performing of fetal MRI and the superb contrast resolution provided. Over the last 10 years, the technology has advanced dramatically. Fast imaging sequences have allowed better MRI visualization of the unborn patient than ever before. As a consequence, experience with fetal MRI is gradually expanding. We are beginning to appreciate the clinical conditions where fetal MRI can complement the ultrasound findings. Apart from the central nervous system, MRI of the fetal lung has received the most attention. Fetal MRI can be used to assess thoracic structural anomalies, lung development as well as maturation. The introduction of fetal therapy for severe lung hypoplasia, associated with congenital diaphragmatic hernia (CDH), has recently boosted the application. This review aims to highlight MRI techniques used to image the lungs of the unborn child and to point out their strengths and limitations in specific conditions.     

Diffusion-weighted MR imaging of the normal fetal lung

To quantify apparent diffusion coefficient (ADC) changes in fetuses with normal lungs and to determine whether ADC can be used in the assessment of fetal lung development. In 53 pregnancies (20–37th weeks of gestation), we measured ADC on diffusion-weighted imaging (DWI) in the apical, middle, and basal thirds of the right lung. ADCs were correlated with gestational age. Differences between the ADCs were assessed. Fetal lung volumes were measured on T2-weighted sequences and correlated with ADCs and with age. ADCs were 2.13 ± 0.44 μm²/ms (mean ± SD) in the apex, 1.99 ± 0.42 μm²/ms (mean ± SD) in the middle third, and 1.91 ± 0.41 μm²/ms (mean ± SD) in the lung base. Neither the individual ADC values nor average ADC values showed a significant correlation with gestational age or with lung volumes. Average ADCs decreased significantly from the lung apex toward the base. Individual ADCs showed little absolute change and heterogeneity. Lung volumes increased significantly during gestation. We have not been able to identify a pattern of changes in the ADC values that correlate with lung maturation. Furthermore, the individual, gravity-related ADC changes are subject to substantial variability and show nonuniform behavior. ADC can therefore not be used as an indicator of lung maturity.     

Impact of lung volume on MR signal intensity changes of the lung parenchyma

PURPOSE: To test the hypothesis that, in magnetic resonance (MR) imaging of healthy individuals, equal relative changes in lung volume cause equal relative changes in MR signal intensity of the lung parenchyma. MATERIALS AND METHODS: In two experimental runs, 10 volunteers underwent spirometrically monitored MR imaging of the lungs, with MR images acquired at 10 incremental lung volumes ranging from total lung capacity to 10% above residual volume. Average signal intensity, signal variability, and signal intensity integrals were calculated for each volunteer and for each lung volume. The effect of lung volume on signal intensity was quantified using linear regression analysis complemented by the runs test. Slopes and intercepts of regression lines were compared with an analysis of covariance. Slopes of the lines of best fit for lung volumes and signal intensities from the two runs were compared to the slope of the line of identity. Comparisons between the two runs were visualized using Bland and Altman plots. RESULTS: The slopes of the 10 individual regression lines yielded no significant differences (F = 1.703, P = 0.101; F = 1.321, P = 0.239). The common slopes were -0.556 +/- 0.027 (P = 0.0001) for the first and -0.597 +/- 0.0031 (P = 0.0001) for the second experimental run. Both slopes displayed no significant nonlinearity (P = 0.419 and P = 0.067). There was a strong association between changes in lung volumes (rs = 0.991, P = 0.0001) and changes in signal intensity (rs = 0.889, P = 0.0001) in the two experimental runs. Lines of best fit for lung volume and signal intensities were not significantly different from the slope of the line of identity (P = 0.321 and P = 0.212, respectively). CONCLUSION: Equal changes in lung volume cause equal changes in MR signal intensity of the lung parenchyma. This linear and reproducible phenomenon could be helpful in comparing pulmonary MR signal intensity between individuals.     

Extrafetal findings at fetal MR: evaluation of the normal placenta and correlation with ultrasound

Despite the increasing use of magnetic resonance (MR) for fetal imaging, few studies have addressed the MR appearance of the normal placenta. The goal of this paper is to describe the MR features and thickness of the normal placenta during the second and early third trimester of gestation, based on a retrospective study comparing MR with the reference standard of obstetric imaging, ultrasound.     

Investigation of normal organ development with fetal MRI

The understanding of the presentation of normal organ development on fetal MRI forms the basis for recognition of pathological states. During the second and third trimesters, maturational processes include changes in size, shape and signal intensities of organs. Visualization of these developmental processes requires tailored MR protocols. Further prerequisites for recognition of normal maturational states are unequivocal intrauterine orientation with respect to left and right body halves, fetal proportions, and knowledge about the MR presentation of extrafetal/intrauterine organs. Emphasis is laid on the demonstration of normal MR appearance of organs that are frequently involved in malformation syndromes. In addition, examples of time-dependent contrast enhancement of intrauterine structures are given.     

Gravity-dependent signal gradients on MR images of the lung in supine and prone positions: a comparison with isogravitational signal variability

PURPOSE: To investigate the tendency of proton MR signal intensity (SI) gradients to be steeper in the supine than in the prone body position, and to quantify the relation between gravity-related and isogravitational changes of SI on proton MR images of the lung. MATERIALS AND METHODS: In eight healthy volunteers, MR images were obtained in the supine and prone positions using a multiple inversion recovery turbo spin-echo (TSE) sequence. The variation in SI along the gravity-dependent direction and within isogravitational planes was measured on a pixel-by-pixel basis. Ratios of slopes were calculated for comparisons among volunteers. Comparisons of ratios were made using Fisher's exact test. Isogravitational variability was compared with the mean SI, the signal-to-noise ratio (SNR), and the image noise. RESULTS: The average ratios of slopes showed that the overall SI gradient was steeper in the supine than the prone position, with a substantial difference in the supine/prone ratios between inspiration (1.21) and expiration (1.72). In both the supine and prone positions, gravity-dependent gradients were steeper in expiration than in inspiration (P = 0.001). The SI variability along the gravitational direction was larger than the isogravitational variability. The isogravitational variability in turn was larger than the image noise but smaller than the mean SI of the MR images. CONCLUSION: Gravity-dependent gradients in proton MR SI are steeper in the supine than in the prone position. The magnitudes of these gradients were larger than the isogravitational signal variability, showing that MRI is sensitive to gravitationally induced effects.     

胎儿肝脏MRI信号强度的意义

目的：探讨宫内胎儿肝脏的生长发育状况。材料和方法：采用超导0．35T磁共振成像仪，对44例中、晚孕程孕妇进行MR成像，观察、分析其中42例正常胎儿肝脏的形态结构、信号变化。结果：胎肝的形态、大小以及肝内的门静脉、肝静脉等被良好显示。在T1WI上，孕32周前，胎儿肝脏的信号呈不均匀的高信号；在孕32周后，胎儿肝脏的信号呈均匀的一致的中等信号。在PDWI及T2WI上，胎儿肝脏的信号在不同的孕周均呈等信号。结论：MRI对胎肝的生长发育状态的研究有重要价值，胎肝在T1WI上均匀一致和中等信号（约孕32周后）代表着胎肝的发育成熟；而PDWI及T2WI对揭示胎肝的发育成熟不敏感。     

Assessment of lung development in isolated congenital diaphragmatic hernia using signal intensity ratios on fetal MR imaging

Objectives

To investigate developmental changes in the apparently unaffected contralateral lung by using signal intensity ratios (SIR) and lung volumes (LV), and to search for correlation with clinical outcome.

Methods

Twenty-five fetuses (22–37 weeks’ gestation) were examined. Lung/liver signal intensity ratios (LLSIR) were assessed on T1-weighted and T2-weighted sequences for both lungs, then together with LV compared with age-matched controls of 91 fetuses by using the U test. Differences in LLSIRs and lung volumes were correlated with neonatal outcomes.

Results

LLSIRs in fetuses with congenital diaphragmatic hernia (CDH) were significantly higher in both lungs on T1-weighted images and significantly lower on T2-weighted images, compared with normals (p?<?0.05), increasing on T2-weighted imaging and decreasing on T1-weighted imaging during gestation. Total LV were significantly smaller in the CDH group than in controls (p?<?0.05). No significant differences in LLSIR of the two lungs were found. Outcomes correlated significantly with total LV, but not with LLSIR.

Conclusion

Changes in LLSIR seem to reflect developmental impairment in CDH; however, they provide no additional information in predicting outcome. LV remains the best indicator on fetal MR imaging of neonatal survival in isolated, left-sided CDH.     

Measurement of MR signal and T2* in lung to characterize a tight skin mouse model of emphysema using single-point imaging

PURPOSE: To evaluate whether MRI signal and T2* measurements of lung tissue acquired at ultrashort detection times (tds) can detect emphysematous changes in lungs. MATERIALS AND METHODS: MR signal intensity of in vivo mouse lungs was measured at 4.7 T at tds of 0.2 and 0.4 msec using single-point imaging (SPI). T2* was calculated from the measurements obtained at the two tds. Two groups of 8- and 30-week-old Tight Skin (TS) and aged-matched CB57BL/6 mice were examined. The TS mice spontaneously developed emphysema-like alveolar enlargement. In vivo micro-computed tomography (microCT) scanning and histology were used as reference methods. RESULTS: MR signal and T2* were significantly lower in the lungs of TS mice than in controls. There were no significant differences between the different age groups. MR signal in lung parenchyma correlated linearly (P < 0.0001, r = 0.89) with microCT mass density, and T2* correlated linearly (P < 0.0001, r = -0.91) with the alveoli size (mean linear intercept [MLI]). CONCLUSION: The MR signal intensity and T2* measured at short tds can be used as imaging biomarkers to characterize parenchyma density and alveolar size, respectively.     

MR fetography using heavily T2-weighted sequences: comparison of thin- and thick-slab acquisitions

Purpose

To evaluate the use of MR-fetography sequences in identifying the major fetal structures and to compare thick- and thin-slab acquisitions for their diagnostic value.

Materials and methods

Twenty-one consecutive, pregnant women with suspected fetal pathology underwent fetal magnetic resonance imaging (MRI) using a 1.5 T MRI unit. Heavily T2-weighted, single-shot fast spin-echo (SSFSE) sequences with a long echo train (MR-fetography) were acquired in a thick- and thin-slab modus. Thick- and thin-slab acquisitions were reviewed by two experienced radiologists with regard to the overall image quality and landmark anatomical structures (spinal canal, spinal cord, posterior fossa, cerebellum, brainstem, basal cisterns, stomach, urinary bladder and umbilical cord according to a three-scale grading system (good, moderate and poor). Visibility scores were calculated and compared between both sequences.

Results

Overall image quality was graded good in 76.2%, moderate in 19.0% and poor in 4.8% for thick-slab images and good in 81%, moderate in 14.3% and poor in 4.8% for thin-slab images. The visibility scores of the thick/thin-slab images for evaluation of the main fetal structures were as follows: for the spinal canal 2.8 ± 0.4/2.9 ± 0.54 (p > 0.05), spinal cord 2.4 ± 0.75/2.7 ± 0.66 (p > 0.05), posterior fossa components (cerebellum, brainstem and basal cisterns) 2.4 ± 0.68/2.8 ± 0.54; 2.4 ± 0.67/2.7 ± 0.66; 2.5 ± 0.51/2.7 ± 0.56 (p < 0.05), stomach 2.8 ± 0.44/2.9 ± 0.48 (p > 0.05), urinary bladder 2.8 ± 0.51/2.8 ± 0.54 (p > 0.05) and umbilical cord 2.9 ± 0.30/2.6 ± 0.60 (p < 0.05).

Conclusion

Heavily T2-weighted MR-fetography renders a quick overview of fetal contours, fetal position, amount of amniotic fluid and integrity and presence of several major fluid containing structures. Thick- and thin-slab acquisitions render complementary information. Thick-slab images display the entire fetus in one projection while thin-slab images provide more detailed anatomical information. The short imaging time usually allows measuring both thick- and thin-slab images. MR-fetography is as a helpful addition to conventional fetal MRI. MR-fetography should not be viewed as a single, stand alone sequence but as a supporting fast MR sequence in a well-designed multisequence fetal MRI protocol. Future studies evaluating larger patient groups are mandatory.     

Paradoxical signal pattern of mediastinal cysts on T2-weighted MR imaging: phantom and clinical study

Purpose

To evaluate the intracystic MRI (magnetic resonance imaging) signal intensity of mediastinal cystic masses on T2-weighted images.

Materials and methods

A phantom study was performed to evaluate the signal intensity of a mediastinal cystic mass phantom (rubber balloon containing water) adjacent to a cardiac phantom pulsing at the rate of 60/min. T2-weighted images (sequence, fast spin echo [FSE] and single shot fast spin echo [SSFSE]) were acquired for the mediastinal cystic mass phantom. Further, a clinical study was performed in 33 patients (16 men, 17 women; age range, 19-85 years; mean, 65years) with thymic cysts or pericardial cysts. In all patients, T2-weighted images (FSE and SSFSE) were acquired. The signal intensity of cystic lesion was evaluated and was compared with that of muscle. A region of interest (ROI) was positioned on the standard MR console, and signal intensity of the cystic mass (cSI), that of the muscle (mSI), and the rate of absolute value of cSI–mSI to standard deviation (SD) of background noise (|cSI–mSI|/SD = CNR [contrast-to-noise ratio]) were measured.

Results

The phantom study demonstrated that the rate phantom-ROI/saline-ROI was higher in SSFSE (0.36) than in FSE (0.19). In clinical cases, the degree of the signal intensity was higher in SSFSE than in FSE. The CNR was significantly higher in SSFSE (mean ± standard deviation, 111.0 ± 47.6) than in FSE (72.8 ± 36.6) (p < 0.001, Wilcoxon signed-rank test).

Conclusions

Anterior mediastinal cysts often show lower signal intensity than the original signal intensity of water on T2-weighted images. SSFSE sequence reduces this paradoxical signal pattern on T2-weighted images, which may otherwise cause misinterpretation when assessing cystic lesions.     

Helium-3 diffusion MR imaging of the human lung over multiple time scales

RATIONALE AND OBJECTIVES: Diffusion magnetic resonance imaging (MRI) with hyperpolarized (3)He gas is a powerful technique for probing the characteristics of the lung microstructure. A key parameter for this technique is the diffusion time, which is the period during which the atoms are allowed to diffuse within the lung for measurement of the signal attenuation. The relationship between diffusion time and the length scales that can be explored is discussed, and representative, preliminary results are presented from ongoing studies of the human lung for diffusion times ranging from milliseconds to several seconds. MATERIALS AND METHODS: (3)He diffusion MRI of the human lung was performed on a 1.5T Siemens Sonata scanner. Using gradient echo-based and stimulated echo-based techniques for short and medium-to-long diffusion times, respectively, measurements were performed for times ranging from 2 milliseconds to 6.5 seconds in two healthy subjects, a subject with subclinical chronic obstructive pulmonary disease and a subject with bronchopulmonary dysplasia. RESULTS: In healthy subjects, the apparent diffusion coefficient decreased by about 10-fold, from approximately 0.2 to 0.02 cm(2)/second, as the diffusion time increased from approximately 1 millisecond to 1 second. Results in subjects with disease suggest that measurements made at diffusion times substantially longer than 1 millisecond may provide improved sensitivity for detecting certain pathologic changes in the lung microstructure. CONCLUSIONS: With appropriately designed pulse sequences it is possible to explore the diffusion of hyperpolarized (3)He in the human lung over more than a 1,000-fold variation of the diffusion time. Such measurements provide a new opportunity for exploring and characterizing the microstructure of the healthy and diseased lung.     

Correlation of lung parenchymal MR signal intensity with pulmonary function tests and quantitative computed tomography (CT) evaluation: a pilot study

PURPOSE: To evaluate the effect of ventilatory impairment on MR signal intensity of the lung parenchyma. MATERIALS AND METHODS: Subjects were five normal volunteers (age = 30 +/- 7.9 years, mean +/- SD) and 19 male patients with chronic obstructive lung disease (COPD) (mean age = 70.4 +/- 6.5 years). Coronal MR images were obtained over entire lung fields at full inspiration and full expiration with cardiac triggering on a 1.5T system. Changes in the mean lung intensity between the two respiratory states were normalized by each intercept of the linear regression lines of the signal changes, and the slope of the relationship was calculated. Computed tomography (CT) images were also obtained in COPD patients at full inspiration using a multidetector row CT scanner. Attenuation values less than -950 Hounsfield units (HU) (RA-950) represented the percentage of relative lung area on the CT. RESULTS: The mean slope of COPD patients (0.365 +/- 0.074) was less steep than that of the normal subjects (0.570 +/- 0.124, P < 0.001). In COPD patients, the slope correlated significantly with forced expiratory volume in one second (FEV1, r = 0.508, P = 0.026), but not with RA-950. CONCLUSION: In COPD patients, lung signal change measured by MRI correlates with airflow obstruction, but not with volume of the emphysema measured by lung CT.     

Configurational MR characteristics of metastatic brain tumors

Magnetic resonance (MR) characteristics of metastatic brain tumors (MBTs) were studied using 15 cases (13 males and 2 females whose ages ranged from 32–78 yr, with the mean age of 57.8 yr;12 adenocarcinomas, 2 squamous-cell carcinomas, 2 large-cell carcinomas). Nine cases showed hypointensities and five showed isointensities on T1-weighted images. Six cases showed markedly hypo- or hypointensities, two showed isointensities, and six showed markedly hyper- or hyperintensities on T2-weighted images. One case was markedly hyperintense on both T1- and T2-weighted images. The decrease of the signal intensity on the T2-weighted image was the main MR characteristic. A hypointense peritumoral rim was seen in four of the six hyperintense tumors on T2-weighted images. There was no correlation between the signal intensity and the histological classification.