Clinical implications of nuclear magnetic resonance lung research

Molecular Imaging of Pulmonary Inflammation and Infection

Infectious and inflammatory pulmonary diseases are a leading cause of morbidity and mortality worldwide. Although infrequently used in this setting, molecular imaging may significantly contribute to their diagnosis using techniques like single photon emission tomography (SPET), positron emission tomography (PET) with computed tomography (CT) or magnetic resonance imaging (MRI) with the support of specific or unspecific radiopharmaceutical agents. ¹⁸F-Fluorodeoxyglucose (¹⁸F-FDG), mostly applied in oncological imaging, can also detect cells actively involved in infectious and inflammatory conditions, even if with a low specificity. SPET with nonspecific (e.g., ⁶⁷Gallium-citrate (⁶⁷Ga citrate)) and specific tracers (e.g., white blood cells radiolabeled with ¹¹¹Indium-oxine (¹¹¹In) or ^99mTechnetium (^99mTc)) showed interesting results for many inflammatory lung diseases. However, ⁶⁷Ga citrate is unfavorable by a radioprotection point of view while radiolabeled white blood cells scan implies complex laboratory settings and labeling procedures. Radiolabeled antibiotics (e.g., ciprofloxacin) have been recently tested, although they seem to be quite unspecific and cause antibiotic resistance. New radiolabeled agents like antimicrobic peptides, binding to bacterial cell membranes, seem very promising. Thus, the aim of this narrative review is to provide a comprehensive overview about techniques, including PET/MRI, and tracers that can guide the clinicians in the appropriate diagnostic pathway of infectious and inflammatory pulmonary diseases.

Lung MRI as a possible alternative to CT scan for patients with primary immune deficiencies and increased radiosensitivity

BACKGROUND

Patients with common variable immunodeficiency (CVID) suffer from respiratory infections leading over time to permanent lung damage. Increased radiosensitivity has been described, and clinicians should consider a risk-benefit assessment when ordering a CT scan, in that the exact level of "safe" radiation exposure is unknown.

METHODS

Twenty-one patients with CVID were evaluated with chest CT scan, MRI, and pulmonary function tests on the same day. MRI protocol included a T2-weighted rotating blade-like k-space covering sequence (time repetition, 2,000; echo train = 27; field of view, 400 mm; flip angle, 150; slice thickness, 5 mm) on axial and coronal planes. The bronchial and parenchymal abnormalities were compared with those identified by CT scan applying a modified Bhalla scoring system to assess bronchiectasis, bronchial wall thickening, number of bronchial generations involved, mucous plugging, consolidations, emphysema, bullae, and nodules.

RESULTS

CT scan and MRI findings were comparable for moderate to severe degrees of bronchial and parenchymal alterations. A low concordance was found between MRI and CT scan for lower scores of bronchial abnormalities. CT scan allowed a better identification of peripheral airways abnormalities.

CONCLUSIONS

Lung alterations in patients with higher radiation sensitivity, such as patients with CVID, might be evaluated by MRI, a radiation-free technique alternative to CT scan.

Antimicrobial chemotherapy and lung microdialysis: a review

Pneumonia is a form of lung infection that may be caused by various micro-organisms. The predominant site of infection in pneumonia is debatable. Advances in the fields of diagnostic and therapeutic medicine have had a less than optimal effect on the outcome of pneumonia and one of the many causes is likely to be inadequate antimicrobial concentrations at the site of infection in lung tissue. Traditional antimicrobial therapy guidelines are based on indirect modelling from blood antimicrobial levels. However, studies both in humans and animals have shown the fallacy of this concept in various tissues. Many different methods have been employed to study lung tissue antimicrobial levels with limited success, and each has limitations that diminish their utility. An emerging technique being used to study the pharmacokinetics of antimicrobial agents in lung tissue is microdialysis. Development of microdialysis catheters, along with improvement in analytical techniques, has improved the accuracy of the data. Unfortunately, very few studies have reported the use of microdialysis in lung tissue, and even fewer antimicrobial classes have been studied. These studies generally suggest that this technique is a safe and effective way of assessing the pharmacokinetics of antimicrobial agents in lung tissue. Further descriptive studies need to be conducted to study the pharmacokinetics and pharmacodynamics of different antimicrobial classes in lung tissue. Data emanating from these studies could inform decisions for appropriate dosing schedules of antimicrobial agents in pneumonia.

T2* and proton density measurement of normal human lung parenchyma using submillisecond echo time gradient echo magnetic resonance imaging

OBJECTIVE

To obtain T2* and proton density measurements of normal human lung parenchyma in vivo using submillisecond echo time (TE) gradient echo (GRE) magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Six normal volunteers were scanned using a 1.5-T system equipped with a prototype enhanced gradient (GE Signa, Waukausha, WI). Images were obtained during breath-holding with acquisition times of 7-16 s. Multiple TEs ranging from 0.7 to 2.5 ms were tested. Linear regression was performed on the logarithmic plots of signal intensity versus TE, yielding measurements of T2* and proton density relative to chest wall muscle. Measurements in supine and prone position were compared, and effects of the level of lung inflation on lung signal were also evaluated.

RESULTS

The signal from the lung parenchyma diminished exponentially with prolongation of TE. The measured T2* in six normal volunteers ranged from 0.89 to 2.18 ms (1.43 +/- 0.41 ms, mean +/- S.D.). The measured relative proton density values ranged between 0.21 and 0.45 (0.29 +/- 0.08, mean +/- S.D.). Calculated T2* values of 1.46 +/- 0.50, 1.01 +/- 0.29 and 1.52 +/- 0.18 ms, and calculated relative proton densities of 0.20 +/- 0.03, 0.32 +/- 0.13 and 0.35 +/- 0.10 were obtained from the anterior, middle and posterior portions of the supine right lung, respectively. The anterior-posterior proton density gradient was reversed in the prone position. There was a pronounced increase in signal from lung parenchyma at maximum expiration compared with maximum inspiration. The ultrashort TE GRE technique yielded images demonstrating signal from lung parenchyma with minimal motion-induced noise.

CONCLUSION

Quantitative in vivo measurements of lung T2* and relative proton density in conjunction with high-signal parenchymal images can be obtained using a set of very rapid breath-hold images with a recently developed ultrashort TE GRE sequence.

Magnetic resonance imaging of lung signal intensity and dimensions in patients with advanced lung disease before and after single lung transplantation

A retrospective analysis of lung signal intensity normalized for fat and lung cross-sectional area normalized for body surface area was obtained from cardiac gated spin-echo (echo time 40 ms) images before and after single-lung transplantation in 12 patients with pulmonary airway disease (seven emphysema, two bronchiectasis and three obliterative bronchiolitis) and 14 with interstitial lung disease (three pulmonary fibrosis, four fibrosing alveolitis and seven lymphangioleiomyomatosis). Nine healthy volunteers were studied for comparison. The native lung in pulmonary airway disease without inflammatory processes has normalised signal intensity (79 +/- 12%) similar to that of the control (100 +/- 6%) while interstitial lung disease had a higher normalised signal intensity (172 +/- 33%; P = < 0.05). The transplanted lung had a similar normalised signal intensity (103 +/- 12%) to that of the control except when there was a rejection reaction (one patient) or infection (two patients). Compared with the control, the native lung was smaller in pulmonary fibrosis (50 +/- 8%) and fibrosing alveolitis (78 +/- 8%), while larger in lymphangioleiomyomatosis (136 +/- 6%) and pulmonary airway disease (165 +/- 8%). The cross-sectional area of the transplanted lung was comparable to that of the control (100 +/- 13%). Distinctive features of lung normalised signal intensity and cross-sectional area were demonstrated in patients with pulmonary airway disease and interstitial lung disease before lung transplantation. These measurements could be useful noninvasive indices for assessment of the transplanted lung and for frequent follow-up of patients without exposure to ionizing radiation. Future developments are required to enhance lung signal sufficiently to detect less severe diseases.

Thoracic magnetic resonance imaging in the evaluation of HIV-1/AIDS pneumonitis

Magnetic resonance imaging of the thorax was performed on ten occasions in eight HIV-positive patients with a clinical picture suggestive of Pneumocystis carinii pneumonia. The diagnosis of PCP was subsequently confirmed on six occasions. Patients without PCP had low MRI profusion scores, while four of six patients with PCP had MRI profusion scores greater than 6/21. Neither the chest roentgenogram appearance nor computer-generated T1 and T2 relaxation times could reliably distinguish between these two groups. Magnetic resonance imaging may be useful in the early and noninvasive diagnosis of PCP in HIV-positive patients.