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Thoracic Magnetic Resonance Imaging: Technique and Approach to Diagnosis


Introduction

Uses of Thoracic Magnetic Resonance Imaging

Chest radiography (CXR) and chest tomography (CT) have been the mainstays of thoracic imaging for decades, and 18-fluorodeoxyglucose positron emission tomography (FDG-PET)–CT has more recently proven its value in staging lung cancer and metastatic disease within the thorax. Upon technical improvements that have addressed challenges related to cardiorespiratory motion, magnetic resonance imaging (MRI) has become appreciated for its capability in tissue diagnosis, without ionizing radiation exposure. The term thoracic MRI is defined here to encompass all of thoracic MRI except cardiovascular and breast MRI.

Problem Solving

Magnetic resonance imaging has proven impactful as a problem solver in the thorax when lesions are indeterminate by CT. Although problem solving with MRI has been largely focused on the mediastinum and pleura, its applications in the lung have been increasing, with examples including serial follow-up of patients with cystic fibrosis, tissue characterization of indeterminate but probably benign pulmonary nodules on CT greater than or equal to 1 cm, and discernment of tumor from postobstructive atelectasis.

Full Chest Imaging and Screening

In addition to full chest MRI for mesothelioma, there has been increased use of MR to screen for and follow various disease entities in the thorax without ionizing radiation exposure, such as endometriosis, chronic lymphocytic leukemia and lymphoma in young and pregnant patients ( Fig. 3.1 ), cystic fibrosis, paragangliomas (e.g., SDHD mutation), teratomas (NMDA [ N -methyl- d -aspartate] receptor antibodies), carcinoid tumors (multiple endocrine neoplasia type 1), and syndromes with high risk for malignancy (e.g., Li-Fraumeni syndrome).

FIGURE 3.1, “No-dose imaging”: a 29-year-old woman with Hodgkin disease in need of staging while pregnant. A and B, Axial in-phase T1-weighted image and coronal single-shot fast spin-echo T2-weighted image show anterior mediastinal (long arrows) , left axillary (medium-sized arrows) , and left supraclavicular (short arrow) lymphadenopathy.

Strengths of Magnetic Resonance Imaging Over Computed Tomography

No Ionizing Radiation

The ability to make a diagnosis and impact clinical management without ionizing radiation exposure is a substantial benefit of MR over CT, provided MR is used for appropriate indications (see Fig. 3.1 ).

Better Soft Tissue Contrast

The high soft tissue contrast of MRI and chemical composition analysis by this modality offer a multitude of diagnostic benefits, including:

  • Better definition of soft tissue planes

    • More accurate determination of the compartment where a lesion resides ( Fig. 3.2 )

      FIGURE 3.2, Accurate compartmental localization aids differential diagnosis. Intrapleural bronchogenic cyst. Coronal (A) and axial (B) computed tomography (CT) scans demonstrate a complex, cystic hypoattenuating mass in the right posteromedial hemithorax. It is unclear by CT whether this mass resides in the paravertebral mediastinum or the pleural space, with the differential diagnosis including paravertebral masses such as cystic schwannomas and cystic pleural lesions such as extralobar sequestration and intrapleural bronchogenic cyst. Coronal single-shot fast spin-echo T2-weighted (C) and axial cardiac-gated double inversion recovery T1-weighted (D) magnetic resonance images show this lesion to be T1- and T2-hyperintense (secondary to hemorrhagic or proteinaceous content), thick walled, and septated. The axial T1-weighted MR image (D) shows the preserved right paravertebral fat plane (arrows) and proves this mass to be outside the paravertebral mediastinum and within the pleural space, excluding neurogenic tumors from the differential diagnosis. The absence of a systemic arterial supply to this lesion favors a rare intrapleural bronchogenic cyst over an extralobar sequestration. Surgical pathology confirmed an intrapleural bronchogenic cyst.

  • Better demonstration of neurovascular, esophageal ( Fig. 3.3 ), and chest wall involvement

    FIGURE 3.3, Detection of invasion: subcarinal metastatic bladder cancer invading the esophagus. A, Artistic rendering showing the correlative names for the various high- and low-signal layers of the esophagus. Matched axial computed tomography (CT) (B) and axial cardiac-gated (CG) double inversion recovery (IR) T2-weighted (C) magnetic resonance images (MRIs) and artistic rendering (D) of an amorphous heterogeneous attenuation mass on CT, effacing the fat plane between it and the right half of the esophageal wall. The presence of esophageal wall invasion by the mass is indeterminate by CT. Correlative T2-weighted MRI and artistic rendering reveal partial encasement of the esophagus by tumor without invasion through the laminar wall of the esophagus. Matched axial CT (E) and CG double IR T2-weighted (F) MR images and artistic rendering (G) of the mass at a higher level, again revealing partial encasement of the esophagus and effacement of the fat plane between these two entities on CT. T2-weighted MRI and rendering reveal effacement of a portion of the wall (muscularis propria and submucosa) of the esophagus (arrow) , compatible with invasion of these layers of the esophageal wall by tumor.

  • Definitive differentiation of cystic from solid lesions (hyperattenuating hemorrhagic and proteinaceous cystic lesions can be misperceived as solid on CT) ( Fig. 3.4 )

    FIGURE 3.4, Distinction of cystic from solid lesions: indeterminate thymic mass on computed tomography (CT) characterized as a unilocular proteinaceous or hemorrhagic cyst by magnetic resonance imaging (MRI). A, Axial CT image shows a homogeneous attenuation, 45 HU (Hounsfield units) mass with saccular morphology filling much of the thymic bed. The differential diagnosis includes thymic hyperplasia, thymic cyst, thymic neoplasm, and lymphoma. B, Axial in-phase T1-weighted, C, Cardiac-gated double-inversion recovery T2-weighted, D, Precontrast ultrafast three-dimensional (3D) gradient echo (GRE), fat-saturated T1-weighted, E, Postcontrast ultrafast 3D GRE, fat-saturated T1-weighted, and F Postcontrast, postprocessed subtracted MR images, respectively, show the mass to be of intermediate T1 signal and homogeneously and markedly T2-hyperintense and to exhibit no internal enhancement, proving the mass to represent a thymic cyst. Thin smooth wall enhancement is present and commonly appreciable in thymic cysts by MRI, albeit seldom by CT.

  • More thorough and sensitive depiction of lesion complexity (heterogeneous composition, small nodules, septations, wall asymmetries, and irregularities)

  • Detection of microscopic fat (in addition to macroscopic fat), blood products, fibrous tissue, cartilage, and smooth muscle

  • Differentiation of muscles from nerves, tendons, and ligaments

Impact on Clinical Decision Making

These strengths of MR regarding tissue characterization and compartmental localization often yield higher diagnostic specificity, more accurate assessment of resectability, guidance to the interventionist regarding optimum (solid, cellular) sites for biopsy for higher diagnostic yield ( Fig. 3.5 ), guidance to the thoracic surgeon regarding the surgical approach, prevention of unnecessary diagnostic intervention, and prevention of unnecessary follow-up imaging and related clinical care.

FIGURE 3.5, Magnetic resonance imaging (MRI) differentiates solid tissue from fluid in a mass, guiding biopsy for higher diagnostic yield: indeterminate hemorrhagic mass involving visceral mediastinum on computed tomography (CT), shown to be mixed solid and hemorrhagic–necrotic on MRI. Axial noncontrast (A) and coronal contrast-enhanced (B) CT images show amorphous, heterogeneous attenuation material in the visceral mediastinum, anteriorly displacing the heart and esophagus (short arrow) , with an adjacent or contiguous right pleural effusion and partial right lower lobe relaxation atelectasis. It is unclear by CT whether this finding represents pure hemorrhage or a hemorrhagic mass and, if a hemorrhagic mass, where its solid components are. C, Coronal single-shot fast spin-echo T2-weighted, D, Precontrast fat-saturated T1-weighted, E, Postcontrast ultrafast three-dimensional gradient echo, fat-saturated T1-weighted, F, Postcontrast, postprocessed subtraction MR images reveal the indeterminate CT finding to represent a large, well-circumscribed mediastinal mass of heterogeneous T1- and T2-weighted signal, with areas of T1-hyperintensity representing hemorrhage (e.g., short arrow ). The postcontrast image and, in particular, the subtraction image clearly delineate the solid, cellular, enhancing components of this mass that would be most promising for diagnostic biopsy, including a subcarinal solid component (long arrow) accessible to minimally invasive bronchoscopic needle biopsy.

Value

Used judiciously, MR therefore offers great value, defined by Harvard Business School professor Michael E. Porter as quality divided by cost. Professor Porter puts it simply: “To reduce cost, the best approach is often to spend more on some services to reduce the need for others.” He stresses thinking about the full cost over the care cycle of the patient, rather than the cost of an individual test. When MRI makes a more precise tissue diagnosis than CT, better defines tissue planes and invasiveness for the surgeon, prevents unnecessary surgery and its associated morbidity and mortality, and prevents unnecessary follow-up imaging, then it has saved a substantial amount over the care cycle of the patient and is invaluable.

Tissue Characterization and Magnetic Resonance Imaging Interpretation

The basic principles of MR tissue characterization in other parts of the body are fully applicable to lesions in the thorax. Generally, the signal intensity of a lesion is described in reference to skeletal muscle on the same image and referred to as hypointense, isointense, or hyperintense (to muscle). If another reference standard is used besides muscle, it is helpful, when reporting the study, to specify the tissue to which the signal of the lesion is being compared. Table 3.1 lists the typical MR signal characteristics of various lesion tissue types encountered in the thorax (and elsewhere). Knowledge of the classic signal characteristics of these tissue types can markedly narrow the differential diagnosis of a lesion.

TABLE 3.1
Magnetic Resonance Tissue Characterization Features *
Tissue Type T1 Signal Intensity T2 Signal Intensity Enhancement Pattern Restricted Diffusion
Serous cyst Hypointense Hyperintense No internal enhancement; may see thin, smooth wall enhancement No
Proteinaceous or hemorrhagic cyst Isointense or hyperintense Hyperintense No internal enhancement; may see thin, smooth wall enhancement No, unless old congealed blood or hematoma, which can be restricted
Microscopic fat Not discernible on in-phase images; hypointense on opposed-phase images NA NA NA
Macroscopic fat Hyperintense Hyperintense No enhancement No
Cartilage Hypointense Hyperintense Little to no enhancement of cartilage substance; enhancement of scaffolding or rim or septations, however No
Fibrous tissue Isointense Iso- to hypointense (T2-hypointense compared with most lesions) Gradual, sometimes limited DWI hypo intense (collagen, no water), ADC hypointense
Smooth muscle Isointense Isointense (T2-hypointense, compared with most lesions) Gradual Variable
ADC, Apparent diffusion coefficient; DWI, diffusion-weighted imaging.

* Signal intensity is described relative to skeletal muscle . Skeletal muscle is intermediate in signal on T1-weighted images and of low signal on T2-weighted images.

Cyst Versus Solid

Cysts and fluid collections are almost always very T2-hyperintense (to muscle) or -isointense to cerebrospinal fluid (CSF) ( Fig. 3.6 ), with the exception of endometriomas and other long-standing hemorrhagic cysts, which may be T2-hypointense, exhibiting “T2-shading” secondary to their highly concentrated iron content ( Fig. 3.7 ). Because cysts contain fluid, they do not internally enhance, with the exception of lymphangiomatous locules into which intravenous (IV) contrast may eventually seep with time. The walls of benign cysts lined by epithelium may exhibit thin, smooth wall enhancement. Unilocular cysts with no wall enhancement or thin smooth wall enhancement are virtually always benign. Inflammatory cyst walls typically enhance. Irregular, nodular, or asymmetric wall enhancement of a cystic lesion or enhancing septations warrant consideration of a more complex benign multilocular lesion (e.g., a multilocular lymphangioma (see Fig. 7.11 ) or multilocular thymic cyst), an inflammatory lesion, and a cystic neoplasm. The T1 signal of cysts is variable, depending on the nature of the fluid. If the fluid is serous, the T1 signal will be hypointense. Hemorrhagic or proteinaceous fluid and fatty fluid are generally T1-isointense or -hyperintense to muscle. Cysts occasionally exhibit fluid–fluid levels that have been referred to as “hematocrit levels,” in the context of hemorrhagic cysts (see Fig. 5.10 ).

FIGURE 3.6, Classic right paratracheal bronchogenic cyst. A, Axial computed tomography (CT) shows a well-circumscribed, homogeneous, water attenuation mass with an imperceptible wall filling the right paratracheal space. These CT features and location are characteristic of a bronchogenic cyst. Although a mesothelial cyst and unilocular lymphangioma could also have this appearance, they are less common in this location. B, Axial in-phase T1-weighted, C, Cardiac-gated double-inversion recovery T2-weighted, and Pre- (D) and postcontrast (E) ultrafast three-dimensional gradient echo, fat-saturated T1-weighted images reveal the mass to be homogeneously T1-hypointense (excluding artifacts) and T2-hyperintense (reflecting the serous nature of the fluid), with a thin, smooth wall and no internal enhancement or lesion complexity.

FIGURE 3.7, Low T2 signal secondary to concentrated iron: indeterminate, rim-calcified, soft tissue attenuation right paratracheal mass on computed tomography (CT) shown to be a hemosiderin-laden bronchogenic cyst by magnetic resonance imaging. A, Axial CT shows a rim-calcified, ovoid, right paratracheal mass with a slightly irregular contour (arrow) . B, Axial in-phase T1-weighted image, C, Cardiac-gated double-inversion recovery T2-weighted, and D, Postcontrast ultrafast three-dimensional gradient echo, fat-saturated T1-weighted images reveal the mass (arrow) to be markedly T1- and T2-hypointense and nonenhancing. These features are compatible with an old, contracted, hemorrhagic bronchogenic cyst containing concentrated iron or hemosiderin. Its slightly irregular contour reflects partial contraction over time. A remote neck CT (not shown) of this same patient showed a larger water attenuation, well-circumscribed unilocular cyst in this area at that time.

Blood Products

Magnetic resonance imaging has higher sensitivity and specificity than CT for the detection of and distinction between blood products. For this reason, it is the test of choice when screening for endometriomas ( Fig. 3.8 ). The signal characteristics of hemorrhage and hematomas vary over time and depend on the age and volume of blood. The most useful signal characteristics of blood products to remember are:

  • Methemoglobin is T1-hyperintense, whether intracellular or extracellular, T2-hypointense if intracellular, and T2-hyperintense if extracellular.

  • Chronic recurrent hemorrhage within a cyst may yield iron accumulation, with resultant T2-hypointensity or “T2 shading,”

  • Hemosiderin is invariably T1- and T2-hypointense.

FIGURE 3.8, High sensitivity and specificity of magnetic resonance imaging (MRI) for blood product detection: tiny endometrioma along right hemidiaphragmatic pleura. Axial (A) and coronal (B) precontrast, ultrafast three-dimensional gradient echo, fat-saturated T1-weighted images reveal a 4-mm, ovoid, well-circumscribed T1-hyperintense nodule (arrows) along the diaphragm in this young woman with right upper quadrant and pleuritic chest pain during menses over the past 1 to 2 years. Initially, her full chest screening MRI was negative for endometriomas; however, it was performed when she was not symptomatic. This MR, performed 6 weeks later, during menses, when she was symptomatic, manifested a tiny endometrioma.

Calcification

Magnetic resonance imaging cannot readily identify calcification, unlike CT. Nevertheless, when foci or a rim of T2-hypointensity is observed in a lesion in which calcification can occur, the presence of calcification can be suggested.

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