MR Imaging of the Thorax

Abstract

Magnetic resonance (MR) imaging is a new modality that makes sectional images of the body similar to computed tomography (CT). The basic principles governing this modality are well documented in the radiology literature.1Partain CL Price RR Patton JA Stephens WH Price AC Runge VM et al.Nuclear magnetic resonance imaging.Radiographics. 1984; 4: 5-25Crossref Google Scholar, 2Young SW Nuclear magnetic resonance imaging—basic principles. Raven Press, New York1984Google Scholar, 3Kean D Smith M Magnetic resonance imaging: principles and applications. Williams & Williams, Baltimore1986Google Scholar Briefly, many atomic nuclei in the body behave as if they are tiny bar magnets because they are charged and they spin. When they are placed inside a magnetic field, the nuclei tend to align themselves with the direction of the external field and assume an equilibrium state. If a radio-frequency pulse is applied to them at a specific frequency determined by the nature of the nuclei and the strength of the external field, their energy level is raised and they become unstable. The excited nuclei return to their original stable energy level by emitting signals at the same frequency. These signals are received and constructed by computers to form a sectional image. The magnitude of the resonant signal is determined by the density of the nuclei, the rate at which the perturbed nuclei return to their equilibrium state (the T1 and T2 relaxation times) and motion of the nuclei, if any. Currently, most medical MR imaging uses hydrogen protons as the nuclei of interest because they are abundant in the body.The advantages of MR include: (1) ionizing radiation not being used; (2) direct coronal, sagittal or oblique, as well as transverse imaging; (3) intrinsic contrast in blood vessels due to “flow-void” effect; (4) high soft tissue contrast due to multiple MR parameters; (5) absence of bone artifacts; and (6) no known hazards or side effects. On the other hand, there are also disadvantages: (1) long image time; (2) more susceptible to artifacts caused by involuntary movements such as respiratory and cardiac motion; (3) calcifications not detected; (4) contraindicated in patients with cardiac pacemakers and aneurysm clips; and (5) high cost.The introduction and refinement of ECG-gated imaging have led to the acquisition of high-resolution cardiac MR images. Potentially, MR can provide information on cardiovascular anatomy, cardiovascular function, myocardial tissue characterization, and myocardial metabolism. Only the first capability is now being used for clinical diagnosis. Even though the place of MR in the noninvasive diagnosis of heart disease compared with radionuclide imaging, ultrasound and CT is still unsettled, it is showing considerable promise in evaluation of pericardial disease, paracardiac and intracardiac masses, thoracic aortic disease, congenital and ischemic heart disease, and cardiomyopathies.4Miller SW Brady TJ Dinsmore RE Edelman RR Johnston DL Okada RD et al.Cardiac magnetic resonance imaging: The Massachusetts General Hospital experience.Radiol Clin North Am. 1985; 23: 745-764PubMed Google Scholar, 5Higgins CB Overview of MR of the heart—1986.AJR. 1986; 146: 907-918Crossref PubMed Scopus (58) Google ScholarMany investigators have reported their experience in using MR for diagnosing mediastinal diseases.6Gamsu G Webb WR Sheldon R Kaufman L Crooks LE Bimberg FA et al.Nuclear magnetic resonance imaging of the thorax.Radiology. 1983; 147: 473-480Crossref PubMed Scopus (67) Google Scholar, 7Cohen AM Creviston S LiPuma JP Bryan PJ Hagga JR Alfidi RJ NMR evaluation of hilar and mediastinal lymphadenopathy.Radiology. 1983; 148: 739-742Crossref PubMed Scopus (28) Google Scholar, 8Epstein DM Kressel H Gefter W Axel L Thickman D Aronchick J Miller W MR imaging of the mediastinum: a retrospective comparison with computed tomography.J Computed Assist Tomogr. 1984; 8: 670-676Crossref PubMed Scopus (14) Google Scholar, 9Poon PY Bronskill MJ Henkelman RM Dunlap HJ Blend R Herman S et al.Magnetic resonance imaging of the mediastinum.J Can Assoc Radiol. 1986; 37: 173-181Google Scholar, 10Musset D Grenier P Carette MF Frija G Hauuy MP Desbleds MT et al.Primary lung cancer staging: prospective comparative study of MR imaging with CT.Radiology. 1986; 160: 607-611Crossref PubMed Scopus (118) Google Scholar The high signal intensity of mediastinal fat and the low signal intensity of vascular structures, due to the flow-void phenomenon, provide excellent contrast for identifying soft tissue masses without the use of intravenous contrast medium. Sagittal and coronal images are useful in many instances.11Webb WR Gamsu G Crooks LE Multisection sagittal and coronal magnetic resonance imaging of the mediastinum and hila.Work in progress. Radiology. 1984; 150: 475-478Google Scholar However, a significant disadvantage of MR is that it cannot detect calcifications in mediastinal masses, particularly in lymph nodes, and yet, very often calcification is a sign of benignity. Also, tissue characterization on MR imaging is limited. There is significant overlap of MR parameters of benign and malignant tissues. In the detection of lymphadenopathy, MR depends upon size to differentiate normal from abnormal nodes. Current results show that the accuracy of MR and CT are very comparable in the diagnosis of such lymphadenopathy.10Musset D Grenier P Carette MF Frija G Hauuy MP Desbleds MT et al.Primary lung cancer staging: prospective comparative study of MR imaging with CT.Radiology. 1986; 160: 607-611Crossref PubMed Scopus (118) Google Scholar, 12Poon PY Bronskill MJ Henkelman RM Rideout DF Shulman HS Weisbrod GL et al.Mediastinal lymph node metastases from bronchogenic carcinoma: detection with MR imaging and CT.Radiology. 1987; 162: 651-656Crossref PubMed Scopus (63) Google Scholar This is not unexpected, considering that both modalities use nodal size as the sole criterion to predict involvement.MR imaging of the lung parenchyma is severely handicapped by the long imaging time and the impracticality of respiratory gating. McFadden et al, in this issue of Chest (see page 31), report their experience in evaluating active interstitial lung disease (ILD) with MR. They observed that qualitative MR data are as useful as other radiographic, physiologic and scintigraphic techniques in predicting the clinical outcome of the 34 patients with ILD they studied. These results are encouraging. However, the fact that qualitative MR data correlate better than quantitative data with clinical findings reiterates that respiratory movement is a significant, adverse factor against the clinical application of MR imaging in pulmonary parenchymal diseases. This also explains why relatively little has been reported on this subject in the literature. Recent technical development of fast-scan is very promising. Such scanning techniques will soon be available in most if not all of the commercially available MR systems. The usefulness of MR in pulmonary parenchymal disease can then be properly assessed and compared with other techniques, particularly CT.While MR has firmly established its value and, in many instances, its superiority over CT in the diagnosis of diseases of the central nervous system,13Bradley WG Waluch V Yadley RA Wycoff RR Comparison of CT and MR in 400 patients with suspected disease of the brain and cervical cord.Radiology. 1984; 152: 695-702Crossref PubMed Scopus (185) Google Scholar, 14New PF Bachow TB Wismer GL Rosen BR Brady TJ MR imaging of the acoustic nerves and small acoustic neuromas at 0.6T: Prospective study.AJNR. 1985; 6: 165-170Google Scholar, 15Lee BCP Kneeland JB Walker RW Posner JB Cahill PT Deck MDF MR imaging of brainstem tumors.AJNR. 1985; 6: 159-163PubMed Google Scholar it is comparable to CT in detecting mediastinal masses. It is very promising in the evaluation of heart diseases and, after having overcome involuntary respiratory motion, it can be valuable in assessing pulmonary parenchymal pathology. Magnetic resonance (MR) imaging is a new modality that makes sectional images of the body similar to computed tomography (CT). The basic principles governing this modality are well documented in the radiology literature.1Partain CL Price RR Patton JA Stephens WH Price AC Runge VM et al.Nuclear magnetic resonance imaging.Radiographics. 1984; 4: 5-25Crossref Google Scholar, 2Young SW Nuclear magnetic resonance imaging—basic principles. Raven Press, New York1984Google Scholar, 3Kean D Smith M Magnetic resonance imaging: principles and applications. Williams & Williams, Baltimore1986Google Scholar Briefly, many atomic nuclei in the body behave as if they are tiny bar magnets because they are charged and they spin. When they are placed inside a magnetic field, the nuclei tend to align themselves with the direction of the external field and assume an equilibrium state. If a radio-frequency pulse is applied to them at a specific frequency determined by the nature of the nuclei and the strength of the external field, their energy level is raised and they become unstable. The excited nuclei return to their original stable energy level by emitting signals at the same frequency. These signals are received and constructed by computers to form a sectional image. The magnitude of the resonant signal is determined by the density of the nuclei, the rate at which the perturbed nuclei return to their equilibrium state (the T1 and T2 relaxation times) and motion of the nuclei, if any. Currently, most medical MR imaging uses hydrogen protons as the nuclei of interest because they are abundant in the body. The advantages of MR include: (1) ionizing radiation not being used; (2) direct coronal, sagittal or oblique, as well as transverse imaging; (3) intrinsic contrast in blood vessels due to “flow-void” effect; (4) high soft tissue contrast due to multiple MR parameters; (5) absence of bone artifacts; and (6) no known hazards or side effects. On the other hand, there are also disadvantages: (1) long image time; (2) more susceptible to artifacts caused by involuntary movements such as respiratory and cardiac motion; (3) calcifications not detected; (4) contraindicated in patients with cardiac pacemakers and aneurysm clips; and (5) high cost. The introduction and refinement of ECG-gated imaging have led to the acquisition of high-resolution cardiac MR images. Potentially, MR can provide information on cardiovascular anatomy, cardiovascular function, myocardial tissue characterization, and myocardial metabolism. Only the first capability is now being used for clinical diagnosis. Even though the place of MR in the noninvasive diagnosis of heart disease compared with radionuclide imaging, ultrasound and CT is still unsettled, it is showing considerable promise in evaluation of pericardial disease, paracardiac and intracardiac masses, thoracic aortic disease, congenital and ischemic heart disease, and cardiomyopathies.4Miller SW Brady TJ Dinsmore RE Edelman RR Johnston DL Okada RD et al.Cardiac magnetic resonance imaging: The Massachusetts General Hospital experience.Radiol Clin North Am. 1985; 23: 745-764PubMed Google Scholar, 5Higgins CB Overview of MR of the heart—1986.AJR. 1986; 146: 907-918Crossref PubMed Scopus (58) Google Scholar Many investigators have reported their experience in using MR for diagnosing mediastinal diseases.6Gamsu G Webb WR Sheldon R Kaufman L Crooks LE Bimberg FA et al.Nuclear magnetic resonance imaging of the thorax.Radiology. 1983; 147: 473-480Crossref PubMed Scopus (67) Google Scholar, 7Cohen AM Creviston S LiPuma JP Bryan PJ Hagga JR Alfidi RJ NMR evaluation of hilar and mediastinal lymphadenopathy.Radiology. 1983; 148: 739-742Crossref PubMed Scopus (28) Google Scholar, 8Epstein DM Kressel H Gefter W Axel L Thickman D Aronchick J Miller W MR imaging of the mediastinum: a retrospective comparison with computed tomography.J Computed Assist Tomogr. 1984; 8: 670-676Crossref PubMed Scopus (14) Google Scholar, 9Poon PY Bronskill MJ Henkelman RM Dunlap HJ Blend R Herman S et al.Magnetic resonance imaging of the mediastinum.J Can Assoc Radiol. 1986; 37: 173-181Google Scholar, 10Musset D Grenier P Carette MF Frija G Hauuy MP Desbleds MT et al.Primary lung cancer staging: prospective comparative study of MR imaging with CT.Radiology. 1986; 160: 607-611Crossref PubMed Scopus (118) Google Scholar The high signal intensity of mediastinal fat and the low signal intensity of vascular structures, due to the flow-void phenomenon, provide excellent contrast for identifying soft tissue masses without the use of intravenous contrast medium. Sagittal and coronal images are useful in many instances.11Webb WR Gamsu G Crooks LE Multisection sagittal and coronal magnetic resonance imaging of the mediastinum and hila.Work in progress. Radiology. 1984; 150: 475-478Google Scholar However, a significant disadvantage of MR is that it cannot detect calcifications in mediastinal masses, particularly in lymph nodes, and yet, very often calcification is a sign of benignity. Also, tissue characterization on MR imaging is limited. There is significant overlap of MR parameters of benign and malignant tissues. In the detection of lymphadenopathy, MR depends upon size to differentiate normal from abnormal nodes. Current results show that the accuracy of MR and CT are very comparable in the diagnosis of such lymphadenopathy.10Musset D Grenier P Carette MF Frija G Hauuy MP Desbleds MT et al.Primary lung cancer staging: prospective comparative study of MR imaging with CT.Radiology. 1986; 160: 607-611Crossref PubMed Scopus (118) Google Scholar, 12Poon PY Bronskill MJ Henkelman RM Rideout DF Shulman HS Weisbrod GL et al.Mediastinal lymph node metastases from bronchogenic carcinoma: detection with MR imaging and CT.Radiology. 1987; 162: 651-656Crossref PubMed Scopus (63) Google Scholar This is not unexpected, considering that both modalities use nodal size as the sole criterion to predict involvement. MR imaging of the lung parenchyma is severely handicapped by the long imaging time and the impracticality of respiratory gating. McFadden et al, in this issue of Chest (see page 31), report their experience in evaluating active interstitial lung disease (ILD) with MR. They observed that qualitative MR data are as useful as other radiographic, physiologic and scintigraphic techniques in predicting the clinical outcome of the 34 patients with ILD they studied. These results are encouraging. However, the fact that qualitative MR data correlate better than quantitative data with clinical findings reiterates that respiratory movement is a significant, adverse factor against the clinical application of MR imaging in pulmonary parenchymal diseases. This also explains why relatively little has been reported on this subject in the literature. Recent technical development of fast-scan is very promising. Such scanning techniques will soon be available in most if not all of the commercially available MR systems. The usefulness of MR in pulmonary parenchymal disease can then be properly assessed and compared with other techniques, particularly CT. While MR has firmly established its value and, in many instances, its superiority over CT in the diagnosis of diseases of the central nervous system,13Bradley WG Waluch V Yadley RA Wycoff RR Comparison of CT and MR in 400 patients with suspected disease of the brain and cervical cord.Radiology. 1984; 152: 695-702Crossref PubMed Scopus (185) Google Scholar, 14New PF Bachow TB Wismer GL Rosen BR Brady TJ MR imaging of the acoustic nerves and small acoustic neuromas at 0.6T: Prospective study.AJNR. 1985; 6: 165-170Google Scholar, 15Lee BCP Kneeland JB Walker RW Posner JB Cahill PT Deck MDF MR imaging of brainstem tumors.AJNR. 1985; 6: 159-163PubMed Google Scholar it is comparable to CT in detecting mediastinal masses. It is very promising in the evaluation of heart diseases and, after having overcome involuntary respiratory motion, it can be valuable in assessing pulmonary parenchymal pathology.