Translational Research: Moving Discovery to Practice

In the first week of October, I announced the launch of a national consortium that will transform how clinical and translational research is conducted; ultimately enabling researchers to provide new treatments more efficiently and quickly to patients. This new consortium, funded through Clinical and Translational Science Awards (CTSAs), begins with 12 academic health centers (AHCs) located throughout the nation. An additional 52 AHCs are receiving planning grants to help them prepare to apply for a CTSA.

Noninvasive assessment of myocardial stunning from short-term coronary occlusion using tagged magnetic resonance imaging

Brief myocardial ischemia of less than 20 min duration, followed by reperfusion, is known to cause transient contractile dysfunction, often termed myocardial stunning. Tagged magnetic resonance imaging offers a noninvasive method that can be used to quantify this regional mechanical dysfunction in stunned myocardium. To this end, a closed-chest canine model of myocardial stunning was created by short-term (approximately 20-min) coronary occlusion, via inflation of an angioplasty balloon placed fluoroscopically in the left anterior descending (LAD) coronary, followed by reperfusion. Changes in myocardial strain before occlusion, during occlusion, and at 15 and 30 min after reperfusion were determined using repeated-measures analysis of variance. After instrumentation but before coronary occlusion, global reductions in myocardial strain were observed relative to animals that did not undergo coronary catheterization procedures. Declines of 46% and 49% in regional myocardial blood flow in the LAD and left circumflex bed, respectively, from preinstrumentation levels occurred due to coronary angiography and placement of a deflated angioplasty balloon in the LAD for 1 hr. During LAD occlusion, maximum myocardial shortening was significantly reduced in the anterior and anteroseptal regions of the left ventricular apex (i.e., ischemic region) but returned to baseline values by 30 min after reperfusion. No augmentation of myocardial function was observed in the nonischemic regions during occlusion or reperfusion. Thus, this noninvasive technique to evaluate myocardial ischemia demonstrated a graded response in myocardial function to ischemia and persistent regional dysfunction or "myocardial stunning" after short-term coronary occlusion.

Transmural contractile reserve after reperfused myocardial infarction in dogs

OBJECTIVES

The goal of this study was to characterize detailed transmural left ventricular (LV) function at rest and during dobutamine stimulation in subendocardial and transmural experimental infarcts.

BACKGROUND

The relation between segmental LV function and the transmural extent of myocardial necrosis is complex. However, its detailed understanding is crucial for the diagnosis of myocardial viability as assessed by inotropic stimulation.

METHODS

Short-axis tagged magnetic resonance images were acquired at five to seven levels encompassing the LV from base to apex in seven dogs 2 days after a 90-min closed-chest left anterior descending coronary occlusion, followed by reflow. Myocardial strains were measured transmurally in the entire LV by harmonic phase imaging at rest and 5 ig x kg(-1) x min(-1) dobutamine. Risk regions were assessed by radioactive microspheres, and the transmural extent of the infarct was assessed by 2,3,5 triphenyltetrazolium chloride staining.

RESULTS

Circumferential shortening (Ecc), radial thickening (Err) and maximal shortening at rest were greater in segments with subendocardial versus transmural infarcts, both in subepicardium (-1.1+/-1.0 vs. 2.5+/-0.6% for Ecc, -0.5+/-1.9 vs. -1.8+/-1.0% for Err, p < 0.05) and subendocardium (-2.0+/-1.4 vs. 2.8+/-0.8%, 2.4+/-1.7 vs. 0.0+/-0.9%, respectively, p < 0.05). Under inotropic stimulation, risk regions retained maximal contractile reserve. Recruitable deformation was found in outer layers of subendocardial infarcts (p < 0.01 for Ecc and Err) but also in inner layers (p < 0.01). Conversely, no contractile reserve was observed in segments with transmural infarcts.

CONCLUSIONS

Under dobutamine challenge, recruitment of myofiber shortening and thickening was observed in inner layers of segments with subendocardial infarcts. These results may have important clinical implications for the detection of myocardial viability.

Quantitative tagged magnetic resonance imaging of the normal human left ventricle

Magnetic resonance imaging with tissue tagging is a noninvasive technique for measuring three-dimensional motion and deformation in the human heart. Tags are regions of tissue whose longitudinal magnetization has been altered before imaging so that they appear dark in subsequent magnetic resonance images. They then move with the underlying tissue and serve as easily identifiable landmarks within the heart for the detailed detection of motion. Many different motion and strain parameters can be determined from tagged magnetic resonance imaging. Strain components that are based on a high density of tag data, such as circumferential and longitudinal shortening, or parameters that are combinations of multiple strain components, have highest measurement precision and tightest normal ranges. The pattern of three-dimensional motion and strain in the heart is important clinically, because it reflects the basic mechanical function of the myocardium at both local and global levels. Localized abnormalities can be detected and quantified if the pattern of deformation in a given heart is compared to the normal range for that region, because normal motion and strain in the left ventricle is spatially heterogeneous. Contraction strains typically are greatest in the anterior and lateral walls and increase toward the apex. The direction of greatest contraction lies along a counter clockwise helix from base to apex (viewed from the base) and approximates the epicardial muscle fiber direction. This fiber geometry also results in long-axis torsion during systole. Ejection is accomplished primarily by radially inward motion of the endocardium and by descent of the base toward the apex during systole.

Fast determination of regional myocardial strain fields from tagged cardiac images using harmonic phase MRI

BACKGROUND

Tagged MRI of the heart is difficult to implement clinically because of the lack of fast analytical techniques. We investigated the accuracy of harmonic phase (HARP) imaging for rapid quantification of myocardial strains and for detailed analysis of left ventricular (LV) function during dobutamine stimulation.

METHODS AND RESULTS

Tagged MRI was performed in 10 volunteers at rest and during 5 to 20 microg(-1). kg(-1). min(-1) dobutamine and in 9 postinfarct patients at rest. We compared 2D myocardial strains (circumferential shortening, Ecc; maximal shortening, E(2); and E(2), direction) as assessed by a conventional technique and by HARP. Full quantitative analysis of the data was 10 times faster with HARP. For pooled data, the regression coefficient was r=0.93 for each strain (P<0.001). In volunteers, Ecc and E(2) were greater in the free wall than in the septum (P<0.01), but recruitable myocardial strain at peak dobutamine was greater in the LV septum (P<0.01). E(2) orientation shifted away from the circumferential direction at peak dobutamine (P<0.01). HARP accurately detected subtle changes in myocardial strain fields under increasing doses of dobutamine. In patients, HARP-determined Ecc and E(2) values were dramatically reduced in the asynergic segments as compared with remote (P<0.001), and E(2) direction shifted away from the circumferential direction (P<0.001).

CONCLUSIONS

HARP MRI provides fast, accurate assessment of myocardial strains from tagged MR images in normal subjects and in patients with coronary artery disease with wall motion abnormalities. HARP correctly indexes dobutamine-induced changes in strains and has the potential for on-line quantitative monitoring of LV function during stress testing.

Three-dimensional systolic strain patterns in the normal human left ventricle: characterization with tagged MR imaging

PURPOSE

To present a database of systolic three-dimensional (3D) strain evolution throughout the normal left ventricle (LV) in humans.

MATERIALS AND METHODS

In 31 healthy volunteers, magnetic resonance (MR) tissue tagging and breath-hold MR imaging were used to generate and then detect the motion of transient fiducial markers (ie, tags) in the heart every 32 msec. Strain and motion were calculated from a 3D displacement field that was fit to the tag data. Special indexes of contraction and thickening that were based on multiple strain components also were evaluated.

RESULTS

The temporal evolution of local strains was linear during the first half of systole. The peak shortening and thickening strain components were typically greatest in the anterolateral wall, increased toward the apex, and increased toward the endocardium. Shears and displacements were more spatially variable. The two specialized indexes of contraction and thickening had higher measurement precision and tighter normal ranges than did the traditional strain components.

CONCLUSION

In this study, the authors noninvasively characterized the normal systolic ranges of 3D displacement and strain evolution throughout the human LV. Comparison against this multidimensional database may permit sensitive detection of systolic LV dysfunction.

Switchable multicoil array for MR micro-imaging of breast lesions

High-resolution breast imaging may improve differentiation between benign and malignant lesions and may be important for refining treatment strategy. This article presents a new, flexible design of a breast-imaging coil capable of providing breast images of a high level of spatial resolution. Referred to as a switchable coil array, the design uses small-diameter surface coils that provide high sensitivity of detection, which, combined with a relatively small field of view, affords a high degree of spatial resolution (up to 200 microm). Remote selection of the coil pair closest to the position of the lesion in the breast permits coverage of the whole breast without changing the position of the coils or the patient. High-resolution MR images of phantom and volunteer patients with benign and malignant breast lesions are presented.

Differentiation of viable and nonviable myocardium by the use of three-dimensional tagged MRI in 2-day-old reperfused canine infarcts

BACKGROUND

To limit ischemic myocardial injury, it is important to differentiate viable from infarcted myocardium. Three dimensional (3D) tagged MRI has the ability to quantify myocardial 3D deformation and strain (noninvasively and precisely), and can achieve a true comparison of contraction not only from region to region, but also at different levels of function. In this study, we investigated whether regional strain mapping obtained by 3D-tagged MRI can differentiate between viable but stunned myocardium and nonviable myocardium.

METHODS AND RESULTS

We examined 7 dogs 2 days after a 90-minute closed-chest left anterior descending coronary artery occlusion followed by 48 hours of reperfusion. 3D-tagged MR images spanning the entire left ventricle were acquired both at rest and during dobutamine infusion (5 microg. kg-1. min-1 IV). Regional blood flow was measured with radioactive microspheres and used to define risk regions. Infarcted regions were defined as 2,3,5 triphenyltetrazolium chloride negative regions. Strains in infarcted regions were greatly impaired compared with remote regions (P<0.001) and remained unchanged during dobutamine stress. Risk regions showed a dysfunction at rest, with improved function during dobutamine infusion. Receiver operating characteristics analysis showed that radial strain was more accurate for identifying viable regions.

CONCLUSIONS

When coupled with a stress test, 3D strain mapping by the use of tagged MRI is a sensitive and noninvasive method for characterizing ischemic injury. Regional strain can be used to differentiate between viable but stunned and nonviable myocardium within the postischemic injured myocardium.

Noninvasive measurement of three-dimensional myocardial deformation with tagged magnetic resonance imaging during graded local ischemia

The purpose of this study was to investigate the relationship between three-dimensional (3D) deformation patterns in the canine left ventricle and localized graded reductions in perfusion. Magnetic resonance (MR) tissue tagging in a clinical scanner was used to determine systolic 3D deformation throughout the left ventricle with 32-msec time resolution. Six dogs were studied at normal and reduced left anterior descending coronary artery flow levels, for a total of 14 studies. Deformation was calculated by fitting a 3D displacement field to tag displacement data from three orthogonal sets of tags and taking spatial derivatives. A novel index of 3D radial mechanical function, calculated from the 3D strain tensor components and the tissue incompressibility constraint, had a higher correlation (R = 0.94) with perfusion (colored microspheres) than any of the 3D Lagrangian finite strain tensor components or wall thickening. As a function of the fraction of baseline perfusion, it was well fit by a linear relationship for subnormal perfusion with a slope of 0.46 +/- 0.05 and an intercept of -0.156 +/- 0.026. Longitudinal strain was lost first with decreasing perfusion (48%), followed by circumferential (40%) and finally radial function (35%). The strain method detected perfusion drops as small as 20%, and early paradoxical strain transients lasting 100 msec were seen only with ischemia. 3D strain changes can be noninvasively measured throughout the left ventricle with MR tissue tagging. MR imaging-derived strain indices, unique to 3D analysis, correlate most sensitively with regional perfusion in the canine left ventricle.

Quantification and time course of microvascular obstruction by contrast-enhanced echocardiography and magnetic resonance imaging following acute myocardial infarction and reperfusion

OBJECTIVES

We aimed to validate contrast-enhanced echocardiography (CE) in the quantification of microvascular obstruction (MO) against magnetic resonance imaging (MRI) and the histopathologic standards of radioactive microspheres and thioflavin-S staining. We also determined the time course of MO at days 2 and 9 after infarction and reperfusion.

BACKGROUND

Postinfarction MO occurs because prolonged ischemia produces microvessel occlusion at the infarct core, preventing adequate reperfusion. Microvascular obstruction expands up to 48 h after reperfusion; the time course beyond 2 days is unknown. Though used to study MO, CE has not been compared with MRI and thioflavin-S, which yield precise visual maps of MO.

METHODS

Ten closed-chest dogs underwent 90-min coronary artery occlusion and reperfusion. Both CE and MRI were performed at 2 and 9 days after reperfusion. The MO regions by both methods were quantified as percent left ventricular (% LV) mass. Radioactive microspheres were injected for blood flow determination. Postmortem, the myocardium was stained with thioflavin-S and 2,3,5-triphenyltetrazolium chloride.

RESULTS

Expressed as % total LV, MO by MRI matched in size MO by microspheres using a flow threshold of <40% remote (4.96+/-3.52% vs. 5.32+/-3.98%, p=NS). For matched LV cross sections, MO by CE matched in size MO by microspheres using a flow threshold of <60% remote (13.27+/-4.31% vs. 13.5+/-4.94%, p=NS). Both noninvasive techniques correlated well with microspheres (MRI vs. CE, r=0.87 vs. 0.74; p=NS). Microvascular obstruction by CE corresponded spatially to MRI-hypoenhanced regions and thioflavin-negative regions. For matched LV slices at 9 days after reperfusion, MO measured 12.94+/-4.51% by CE, 7.11+/-3.68% by MRI and 9.18+/-4.32% by thioflavin-S. Compared to thioflavin-S, both noninvasive techniques correlated well (CE vs. MRI, r=0.79 vs. 0.91; p=NS). Microvascular obstruction size was unchanged at 2 and 9 days (CE: 13.23+/-4.11% vs. 12.69+/-4.97%; MRI: 5.53+/-4.94% vs. 4.68+/-3.44%; p=NS for both).

CONCLUSIONS

Both CE and MRI can quantify MO. Both correlate well with the histopathologic standards. While MRI can detect regions of MO with blood flow <40% of remote, the threshold for MO by CE is <60% remote. The extent of MO is unchanged at 2 and 9 days after reperfusion.

Prognostic significance of microvascular obstruction by magnetic resonance imaging in patients with acute myocardial infarction

BACKGROUND

The extent of microvascular obstruction during acute coronary occlusion may determine the eventual magnitude of myocardial damage and thus, patient prognosis after infarction. By contrast-enhanced MRI, regions of profound microvascular obstruction at the infarct core are hypoenhanced and correspond to greater myocardial damage acutely. We investigated whether profound microvascular obstruction after infarction predicts 2-year cardiovascular morbidity and mortality.

METHODS AND RESULTS

Forty-four patients underwent MRI 10 +/- 6 days after infarction. Microvascular obstruction was defined as hypoenhancement seen 1 to 2 minutes after contrast injection. Infarct size was assessed as percent left ventricular mass hyperenhanced 5 to 10 minutes after contrast. Patients were followed clinically for 16 +/- 5 months. Seventeen patients returned 6 months after infarction for repeat MRI. Patients with microvascular obstruction (n = 11) had more cardiovascular events than those without (45% versus 9%; P=.016). In fact, microvascular status predicted occurrence of cardiovascular complications (chi2 = 6.46, P<.01). The risk of adverse events increased with infarct extent (30%, 43%, and 71% for small [n = 10], midsized [n = 14], and large [n = 14] infarcts, P<.05). Even after infarct size was controlled for, the presence of microvascular obstruction remained a prognostic marker of postinfarction complications (chi2 = 5.17, P<.05). Among those returning for follow-up imaging, the presence of microvascular obstruction was associated with fibrous scar formation (chi2 = 10.0, P<.01) and left ventricular remodeling (P<.05).

CONCLUSIONS

After infarction, MRI-determined microvascular obstruction predicts more frequent cardiovascular complications. In addition, infarct size determined by MRI also relates directly to long-term prognosis in patients with acute myocardial infarction. Moreover, microvascular status remains a strong prognostic marker even after control for infarct size.

Precision of myocardial contour estimation from tagged MR images with a "black-blood" technique

RATIONALE AND OBJECTIVES

The authors determined whether blood presaturation of tagged magnetic resonance (MR) images affects identification of left ventricular endocardial borders.

MATERIALS AND METHODS

Three healthy volunteers underwent MR imaging performed with a breath-hold segmented spoiled gradient-recalled-echo sequence with tissue tagging. Two saturation pulses (in the basal and apical regions) were used to generate black-blood images. Manual segmentation of endocardial contours on black-blood and white-blood images was performed independently by five observers.

RESULTS

Endocardial borders were better identified on black-blood images compared with white-blood images, especially in the early systolic phases. Interobserver variability in contour estimation was significantly higher for white-blood images (P < .001) and was twice that for corresponding black-blood images during early systole. Contour variability appeared to be affected mainly by tag-to-myocardium contrast (P = .009) and myocardium-to-chamber contrast (P = .05).

CONCLUSION

Blood presaturation of tagged MR images improves reliability of contour segmentation.

Intravascular magnetic resonance imaging of aortic atherosclerotic plaque composition

Magnetic resonance imaging (MRI) may be an excellent tool to define atherosclerotic plaque composition, but surface MRI (SMRI) suffers from a low signal-to-noise ratio and low resolution of arterial images. Intravascular MRI (IVMRI) represents a potential solution for acquiring high-quality in vivo images of atherosclerotic plaques. Isolated segments of 11 thoracic human aortas obtained at autopsy were imaged by IVMRI using an intravascular receiver catheter coil designed and built at our institution. Images obtained by IVMRI were compared with corresponding images obtained by SMRI and with histopathological aortic cross sections. The intensity of intimal thickness and plaque components was graded by IVMRI and histopathology using a score of 1 for mild, 2 for moderate, and 3 for severe intensity. IVMRI had an agreement of 75% with histopathology in fibrous cap grading (37.5% expected, kappa = 0.60, P < 0.001) and of 74% in necrotic core grading (39% expected, kappa = 0.57, P < 0.001). Intraplaque calcification was correctly graded by IVMRI in six of the eight plaques in which histopathology recognized calcium. The analysis of intimal thickness showed 80% agreement between IVMRI and histopathology (52% expected, kappa = 0.59, P < 0.001). IVMRI image features were similar to those of SMRI. In addition, IVMRI accurately determined atherosclerotic plaque size in comparison with histopathology and SMRI (slope = 1.25 cm2, r = 0.99, P < 0.001 for luminal area by IVMRI vs histopathology; slope = 0.97 cm2, r = 0.996, P < 0.001 for luminal area by IVMRI vs SMRI). IVMRI has the potential to provide important prognostic information in patients with atherosclerosis because of its ability to accurately assess both plaque composition and size.

Extremity veins: evaluation with fast-spin-echo MR venography

The authors evaluated a flow-independent magnetic resonance (MR) imaging technique in extremity veins with slow flow that relies on the prolonged T2 of blood to create angiographic contrast. A commercially available heavily T2-weighted fast-spin-echo sequence was optimized for MR venography in volunteer and phantom studies. Good depiction of venous anatomy was routinely obtained with the optimized method. Fast-spin-echo MR venography allowed evaluation of slow-flow systems such as the calf and forearm veins.

Thoracoabdominal aorta in Marfan syndrome: MR imaging findings of progression of vasculopathy after surgical repair

PURPOSE

To determine the natural history of vasculopathy of the thoracoabdominal aorta in patients with Marfan syndrome after composite graft repair of the aortic root.

MATERIALS AND METHODS

A total 224 magnetic resonance (MR) images obtained in 48 patients with Marfan syndrome over a period of 2.3-9.4 years (mean, 5.0 years) were retrospectively reviewed. On each image, the diameter of the thoracoabdominal aorta was measured and the presence of dissection or major peripheral artery aneurysms was determined.

RESULTS

In 31 (65%) of the 48 patients, no statistically significant change (3 mm or less increase in diameter) in the diameter of the aorta occurred during the study (group 1); in the remaining 17 (35%) patients, a significant change occurred (greater than 3 mm increase) (group 2). The mean initial diameter of the native aorta was slightly larger in group 2 (mean, 27 mm +/- 8 [standard deviation]) than in group 1 (mean, 23 mm +/- 6). In group 1, the mean rate of dilation was 0.07 mm/y +/- 0.2; in group 2, the rate was 2.3 mm/y +/- 3.3. Two patients with aortic dissection were in group 1, whereas 14 such patients were in group 2 (P < .001). Aneurysms that involved major peripheral arteries were present in four (13%) of the 31 group 1 patients and in 12 (71%) of the 17 group 2 patients (P < .001). Surgical intervention was necessary in two group 1 patients and in 14 group 2 patients (P < .001).

CONCLUSION

A subset of patients with Marfan syndrome manifested multiple forms of vasculopathy, including progressive aortic dilation, dissection, and peripheral artery aneurysm after composite aortic graft repair of the ascending aorta. Patients with these characteristics merit more frequent MR follow-up since further surgery was often necessary in these individuals.

Fast 23Na magnetic resonance imaging of acute reperfused myocardial infarction. Potential to assess myocardial viability

BACKGROUND

The ability of the myocyte to maintain an ionic concentration gradient is perhaps the best indication of myocardial viability. We studied the relationship of 23Na MRI intensity to viability and explored the potential of fast-imaging techniques to reduce 23Na imaging times in rabbits and dogs.

METHODS AND RESULTS

Eighteen rabbits underwent in situ coronary artery occlusion and reperfusion. The hearts were then either imaged following isolation and perfusion with cardioplegic solution (n = 6), imaged in vivo (n = 6), or analyzed for 23Na content and relaxation times (n = 12). Normal rabbits (n = 6) and dogs (n = 4) were imaged to examine the effect of animal size on 23Na image quality. 23Na imaging times were 7, 11, and 4 minutes for isolated rabbits, in vivo rabbits, and in vivo dogs, respectively. Infarcted, reperfused regions, identified by triphenyltetrazolium chloride staining, showed a significant elevation in 23Na image intensity compared with viable regions (isolated, 42 +/- 5%, P < .02; in vivo, 95 +/- 6%, P < .001), consistent with increased tissue sodium content. Similarly, 23Na MR spectroscopy showed that [Na+] was higher in nonviable than viable myocardium (isolated, 99 +/- 4 versus 61 +/- 2 mmol/L; in vivo, 91 +/- 2 versus 38 +/- 1 mmol/L; P < .001 for both). Image signal-to-noise ratios were higher in dogs than rabbits despite shorter imaging times, primarily due to larger voxels.

CONCLUSIONS

Following acute infarction with reperfusion, a regional increase in 23Na MR image intensity is associated with nonviable myocardium. Fast gradient-echo imaging techniques reduce 23Na imaging times to a few minutes, suggesting that 23Na MR imaging has the potential to become a useful experimental and clinical tool.

Benign and malignant musculoskeletal masses: MR imaging differentiation with rim-to-center differential enhancement ratios

PURPOSE

To assess the diagnostic potential of the rim-to-center differential enhancement ratio in the magnetic resonance (MR) imaging differentiation of benign from malignant musculoskeletal masses.

MATERIALS AND METHODS

Dynamic gadolinium-enhanced fast multiplanar spoiled gradient-recalled acquisition in the steady state imaging was performed to evaluate 17 one and soft-tissue masses (10 malignant, seven benign) in 14 patients. The rim-to-center differential enhancement ratio was determined from images at the maximal enhancement rate. The difference in differential enhancement ratios between benign and malignant masses was evaluated and compared with the maximum rate of enhancement for each mass.

RESULTS

Nine of 10 malignant masses showed rapid rim enhancement with delayed central fill-in. This enhancement pattern was absent in benign masses. The average rim-to-center differential enhancement ratio was 0.64 +/- 0.26 for malignant masses and -0.16 +/- 0.33 for benign masses (P < .001). This difference was statistically significant. The average maximum rate of enhancement was 3.41%/sec +/- 2.20 for malignant masses and 2.74%/sec +/- 2.46 for benign masses (P > .05).

CONCLUSION

Intratumoral enhancement patterns of malignant and benign masses differ because of differences in vascular architecture. These preliminary results suggest that the rim-to-center differential enhancement ratio has potential as an additional parameter for the MR imaging differentiation of indeterminate musculoskeletal masses.

High resolution intravascular MRI and MRS by using a catheter receiver coil

Potentially important diagnostic information about atherosclerosis can be obtained by using magnetic resonance imaging and spectroscopy techniques. Because critical vessels such as the aorta, coronary arteries, and renal arteries are not near the surface of the body, surface coils are not adequate to increase the data quality to desired levels. A few catheter MR receiver coil designs have been proposed for imaging the walls of large blood vessels such as the aorta. These coils have limited longitudinal coverage and they are too thick to be placed into small vessels. A flexible, long and narrow receiver coil that can be placed on the tip of a catheter and will enable multi-slice high resolution imaging of small vessels has been developed. The authors describe the theory of the coil design technique, derive formulae for the signal-to-noise ratio characteristics of the coil, and show examples of high resolution cross-sectional images from isolated human aortas acquired by using this catheter coil. In addition, high resolution in vivo rabbit aorta images were obtained as well as a set of spatially resolved chemical shift spectra from a dog circumflex coronary artery.

MRI of avascular necrosis of bone

Avascular necrosis (AVN) is characterized by death of both trabecular bone as well as bone marrow elements. Weight-bearing bone becomes mechanically weakened and may eventually collapse, secondarily leading to osteoarthritis and debilitating pain. Early diagnosis and treatment of this entity are crucial because it affects relatively young individuals, and treatment options for advanced disease are limited. Magnetic resonance imaging (MRI) has emerged as the modality of choice for the evaluation of avascular necrosis of bone. We will discuss applications of MRI for early diagnosis, for monitoring therapy, and for its potential role in assessing individuals at risk of AVN. Although bone scintigraphy using single photon emission computed tomography (CT) may be nearly as accurate as MRI, MRI offers a more specific diagnosis in the patient who presents with hip pain of uncertain etiology. In addition, lesion size and location can be more easily assessed on magnetic resonance images, and this has been shown to relate to prognosis and need for treatment. We will review the pathophysiologic mechanisms of AVN and the current use of MRI in the diagnosis of this condition.

CT and MR imaging in the staging of colorectal carcinoma: report of the Radiology Diagnostic Oncology Group II

PURPOSE

To prospectively evaluate the relative accuracy of computed tomography (CT) and magnetic resonance (MR) imaging in the staging of colorectal carcinoma.

MATERIALS AND METHODS

CT and MR studies were independently interpreted in a group of 478 patients with colorectal carcinoma in a study conducted from 1989 to 1993. The accuracy of each modality was assessed in a subset of 365 patients with primary tumors with respect to staging of local extent of tumor, status of local-regional lymph nodes, and the presence of liver metastases.

RESULTS

In the staging of local extent of tumor, CT is more accurate than MR imaging, particularly in the definition of penetration of the muscularis propria by rectal cancer (74% vs 58%). Accuracies of CT and MR imaging were equivalent in depiction of transmural extent in colon cancers. CT and MR imaging exhibited accuracies of 62% and 64% in assessment of lymph node involvement with sensitivities of 48% and 22%, respectively. The accuracy of MR imaging and of CT (85% for each) are better for evaluation of liver metastases; lower sensitivities (62% and 70%, respectively) than specificities (97% and 94%, respectively) were demonstrated for both modalities.

CONCLUSION

CT was more accurate than MR imaging in detection and characterization of transmural penetration of rectal tumors. Recent technologic advances in MR imaging may affect these results.

Quantitative cardiac perfusion: a noninvasive spin-labeling method that exploits coronary vessel geometry

PURPOSE

To quantitate myocardial arterial perfusion with a noninvasive magnetic resonance (MR) imaging technique that exploits the geometry of coronary vessel anatomy.

MATERIALS AND METHODS

MR imaging was performed with a spin-labeling method in six arrested rabbit hearts at 4.7 T. Selective inversion of magnetization in the short-axis imaging section along with all myocardium apical to that section produces signal enhancement from arterial perfusion. A linescan protocol was used for validation of flow enhancement. Flow was quantitated from two images and validated with spin-echo (SE) imaging. Regional perfusion defects were created by means of coronary artery ligation and delineated with gadolinium-enhanced imaging.

RESULTS

Linescan estimates of T1 obtained at physiologic flows agreed with model predictions. Flow-induced signal enhancement measured on SE images also agreed with expected values. Finally, perfusion abnormalities created by means of coronary artery ligation were detected.

CONCLUSION

This spin-labeling method provides quantitative estimates of myocardial arterial perfusion in this model and may hold promise for clinical applications.

Pencil excitation with interleaved fourier velocity encoding: NMR measurement of aortic distensibility

A technique is presented for rapidly and noninvasively determining aortic distensibility, by NMR measurement of pulse-wave velocity in the aorta. A cylinder of magnetization is excited along the aorta, with Fourier-velocity encoding and readout gradients applied along the cylinder axis. Cardiac gating and data interleaving improve the effective time resolution to as high as 3 ms. Wave velocities are determined from the position of the foot of the flow wave in the velocity profiles. Evidence of helical flow distal to the aortic arch can be seen in normal subjects, while disturbed flow patterns are visible in patients with aneurysms and dissections.

Renal cell carcinoma and tumor thrombus neovascularity: MR demonstration with pathologic correlation

A case of renal granular cell carcinoma with inferior vena cava and right atrium involvement is presented. Spin-echo and single breath-hold gradient-recalled-echo magnetic resonance pulse sequences demonstrate a patchy flow signal within the cavoatrial thrombus. This pattern, in correlation with the histopathologic findings, represents tumoral neovascularity characteristic of renal carcinoma venous invasion, which was previously reported by angiography, computed tomography, and color Doppler duplex ultrasound.

Peripheral primitive neuroectodermal tumors. CT and MRI evaluation

The clinical, radiographic, and pathologic features of 17 patients with documented peripheral primitive neuroectodermal tumor (PNET) were evaluated in a retrospective study. The age at diagnosis ranged from 9 months to 46 years (median, 15.8 years). Primary sites of involvement were the abdomen (n = 8), extremities (n = 5), chest (n = 1), temporal bone (n = 1), maxilla (n = 1), and diploe (n = 1). At the time of diagnosis, six patients had distant metastases; all of these patients died, with an average survival of 8.8 months. Radiologic workup included standard radiographs, ultrasound, computed tomography (CT), magnetic resonance imaging (MRI), and bone scintigraphy. The radiographic appearance of these tumors was not specific for differentiation of PNETs from other types of bone and soft tissue tumors. The typical appearance resembled large non-calcified, soft tissue masses with cystic or necrotic areas. Heterogeneous enhancement with intravenous contrast agents was evident on CT, as was an intermediate signal intensity on Tl-weighted images and hyperintense signal on T2-weighted and STIR sequences. After gadolinium administration, variable enhancement was seen. MRI and CT were useful in predicting resectability, in detecting distant metastases, and in the evaluation of response to treatment. Surgery was performed in all cases, either for definitive diagnosis or for therapy. All patients received combined chemotherapy and radiotherapy and five patients received autologous bone marrow transplantation. Clinical follow-up was obtained over a mean period of 3 years (range 1 day to 6 years). Prognosis was poor with a median survival of 3.4 years. Our experience in 17 patients with peripheral neuroectodermal tumors indicates that although their radiologic features are non-specific, they should be included in differential diagnosis of soft tissue tumors of aggressive behavior, especially in a young age group. CT and MRI are useful in delineating the extent and resectability of tumor and in monotoring treatment.

Fluid-fluid levels in injured knees: do they always represent lipohemarthrosis?

PURPOSE

To determine if fluid-fluid levels on horizontal-beam radiographs of posttraumatic knee joints always represent lipohemarthrosis.

MATERIALS AND METHODS

The files of 41 patients with tibial plateau fracture were reviewed retrospectively. Twelve with fluid-fluid levels on radiographs who underwent computed tomography (CT) or magnetic resonance (MR) imaging were selected; CT attenuation and MR signal intensity patterns were used to assess fluid-fluid levels. Simulated hemarthrosis and lipohemarthrosis underwent plain radiography, CT, and MR imaging.

RESULTS

Eight patients showed no evidence of fat in the supernatant fluid with either CT (n = 7) or CT and MR imaging (n = 1). Fluid levels resulted from blood separating into cellular elements and supernatant serum. Four patients showed evidence of lipohemarthrosis with either CT (n = 3) or MR imaging (n = 1); in one of these patients, a double fluid-fluid level could be demonstrated retrospectively on plain radiographs.

CONCLUSION

Single fluid-fluid levels in posttraumatic knee joints do not necessarily represent a lipohemarthrosis. Double fluid-fluid levels are a more specific finding.

Effect of slice acquisition direction on image quality in thoracic MRI

OBJECTIVE

We tested the basic observation that imaging the heart and pericardium in systole improves image quality compared with that in diastole.

MATERIALS AND METHODS

Twenty consecutive patients and two volunteers underwent sequential ECG-gated short TE transaxial prospective multislice SE MRI with both caudocranially and craniocaudally directed slice prescriptions, keeping other imaging parameters constant. Images of the heart and pericardium were obtained in systole and diastole and examined by three independent reviewers for image quality.

RESULTS

In the lower mediastinum, cardiac structures and the pericardium were better seen in 49 of 57 individual evaluations when imaged in systole, 15 of which were judged markedly better. Vascular structures and the pericardium in the upper mediastinum were imaged equally well with both prescriptions.

CONCLUSION

The findings suggest that in systole, the more mobile heart can maintain a more consistent shape during the acquisition of successive phase-encoding steps and preserve luminal flow void, factors critical to optimizing image quality in the transaxial plane.

Femoral head perfusion and composition: MR imaging and spectroscopic evaluation of patients with systemic lupus erythematosus and at risk for avascular necrosis

PURPOSE

To measure fat content and relative marrow perfusion of the femoral head in patients at risk for avascular necrosis.

MATERIALS AND METHODS

Thirteen patients with systemic lupus erythematosus (SLE) without avascular necrosis and with long-term corticosteroid treatment and 12 healthy subjects were evaluated. Hip perfusion was assessed with gadolinium-enhanced T1-weighted magnetic resonance (MR) imaging. Marrow composition in the femoral head and neck was measured with hydrogen-1 MR spectroscopy.

RESULTS

Percentage fat content in the femoral head was similar between the two groups (SLE patients, 77% +/- 5 [standard deviation]; healthy subjects, 78% +/- 6; P = .74). Rate of enhancement of the femoral head was significantly higher in the SLE than the healthy group (11%/min +/- 6 vs 6.9%/min +/- 3, respectively; P = .05), was inversely related to fat content in healthy subjects (r = -.74, P = .02), and was more variable in the SLE group (r = -.53, P = .11).

CONCLUSION

Perfusion of the femoral head is inversely related to marrow fat content in healthy and is higher in SLE patients without hip avascular necrosis.

Measurement of three-dimensional lung tree structures by using computed tomography

A method was devised to computationally segment and measure three-dimensional pulmonary trees in situ. Bronchi and pulmonary vessels were computationally extracted from volumetric computed tomography data based on radiopacity differences between airway wall and airway lumen and between blood and parenchyma, respectively. The tree was reduced to a central axis to facilitate measurement of branch segment length and angle. Cross-sectional area was measured on a reconstructed computed tomography slice perpendicular to this central axis. The method was validated by scanning two Plexiglas phantoms and an intact lung. Reconstructed diameters in the phantoms were accurate for branches > 2 mm. In the lung airway branches between 1 and 2 mm in diameter were often unresolved when their angle of orientation with respect to the axis of the scanner was > 45 degrees. However, if a branch was resolved, its reconstructed diameter was little affected by orientation. This method represents a significant improvement in the analysis of complex pulmonary structures in three dimensions.

Pulmonary nodules: improved detection with vascular segmentation and extraction with spiral CT. Work in progress

PURPOSE

To determine whether extraction of pulmonary vessels from computed tomographic (CT) images with automated segmentation improves the detection of pulmonary nodules.

MATERIALS AND METHODS

Simulated nodules were superimposed on normal spiral CT images. Eight patients referred for CT assessment of pulmonary nodules were selected for clinical evaluation. Vessels were extracted from both the simulation and clinical study with a three dimensional seeded region-growing algorithm. Three experienced radiologists were asked to locate the nodules and assign a level of confidence to their findings. Sensitivity and proportion of false-positive results per case (FPC) were calculated. Observer performance was evaluated by alternate free-response receiver operating characteristic analysis.

RESULTS

Extraction of vascular structures from CT scans improved sensitivity from 63% to 84% in the simulation study and from 58% to 78% in the clinical study. The proportion of FPC decreased from 52% to 24% and from 55% to 12%, respectively. Radiologists performed consistently better with the segmented images than with the original images in both the simulation (P = .006) and the clinical (P = .0013) study.

CONCLUSION

Automated vessel subtraction and extraction improves detection of pulmonary nodules.

Single- and double- lumen silicone breast implant integrity: prospective evaluation of MR and US criteria

PURPOSE

To evaluate the accuracy of magnetic resonance (MR) and ultrasound (US) criteria for breast implant integrity.

MATERIALS AND METHODS

One hundred twenty-two single-lumen silicone breast implants and 22 bilumen implants were evaluated with surface coil MR imaging and US and surgically removed. MR criteria for implant failure were a collapsed implant shell ("linguine sign"), foci of silicone outside the shell ("noose sign"), and extracapsular gel, US criteria were collapsed shell, low-level echoes within the gel, and "snowstorm" echoes of extracapsular silicone.

RESULTS

Among single-lumen implants, MR imaging depicted 39 of 40 ruptures, 14 of 28 with minimal leakage; 49 of 54 intact implants were correctly interpreted. US depicted 26 of 40 ruptured implants, four of 28 with minimal leakage, and 30 of 54 intact implants. Among bilumen implants, MR imaging depicted four of five implants with rupture of both lumina and nine of 10 as intact; US depicted one rupture and helped identify two of 10 as intact. Mammography accurately depicted the status of 29 of 30 bilumen implants with MR imaging correlation.

CONCLUSION

MR imaging depicts implant integrity more accurately than US; neither method reliably depicts minimal leakage with shell collapse. Mammography is useful in screening bilumen implant integrity.

Airway edema potentiates airway reactivity

Thickening of the airway wall has been hypothesized to be one of the mechanisms contributing to airway hyperresponsiveness in asthma. If such thickening of the wall is internal to the airway smooth muscle or otherwise causes a decrease in baseline airway caliber, it should also cause exaggerated airway responsiveness. In the present study, we used high-resolution computed tomography to directly measure the changes in the caliber and wall thickness of conducting airways after aerosol histamine challenge before and after normal saline volume loading. On separate days, five anesthetized dogs received either a baseline aerosol challenge of 3 mg/ml of histamine for five breaths or the same aerosol challenge immediately after a 100 ml/kg bolus of normal saline infused over a 10-min period. Baseline aerosol histamine challenge decreased airway area to 71 +/- 2% (SE) of the control value (P < 0.05). Intravenous administration of 100 ml/kg of normal saline increased wall area by decreasing airway luminal area to 78 +/- 3% of the control value (P < 0.01), with no change in outer airway area. Aerosol histamine challenge superimposed on this engorgement with normal saline challenge further decreased airway luminal area to 54 +/- 3% of the control value (P < 0.01). Quantitative modeling indicated that the edema in the airway wall was mostly outside the smooth muscle and that the smooth muscle shortening with histamine was similar with and without edema. We conclude that a moderate degree of acute airway wall thickening can lead to a potentiated constrictor response to histamine.

Physiological basis of myocardial contrast enhancement in fast magnetic resonance images of 2-day-old reperfused canine infarcts

BACKGROUND

Contrast-enhanced fast magnetic resonance (MR) images of acute, reperfused human infarcts demonstrate regions of hypoenhancement and hyperenhancement. The relations between the spatial extent and time course of these enhancement patterns to myocardial risk, infarct, and no-reflow regions have not been well characterized.

METHODS AND RESULTS

The proximal left anterior descending coronary artery was occluded in 11 closed-chest dogs for 90 minutes followed by 2 days of reperfusion. Regional blood flow was determined by use of radioactive microspheres. The animals were studied at the 2-day time point with contrast-enhanced fast MRI (Signa 1.5 T, General Electric). Thioflavin-S was administered to demarcate no-reflow regions. The hearts were then excised, sectioned into five base-to-apex slices, stained with 2,3,5-triphenyltetrazolium chloride (TTC), and photographed under room light (for TTC) and ultraviolet light (for thioflavin). The spatial extents of thioflavin-negative, TTC-negative, and risk regions were compared planimetrically with MRI hypoenhanced and hyperenhanced regions. The spatial locations of subendocardial hypoenhancement in MR images correlated closely with those of thioflavin-negative regions. Microsphere blood flow in these regions was significantly reduced compared with remote regions (0.37 +/- 0.09 versus 0.88 +/- 0.10 mL/min per gram, respectively, P < .001) and with baseline (0.37 +/- 0.09 versus 0.87 +/- 0.15 mL/min per gram, P < .01). The spatial extent of hyperenhancement was smaller than the risk region (r = .64, slope = 0.48, P < .001) but highly correlated with TTC-negative regions and were, on average, 12% larger (r = .93, slope = 1.12, P = .035).

CONCLUSIONS

In contrast-enhanced MR images of 2-day-old reperfused canine infarcts, myocardial regions of hypoenhancement are related to the no-reflow phenomenon. Approximately 90% of the myocardium within hyperenhanced regions is nonviable.

Blood oxygenation dependence of T1 and T2 in the isolated, perfused rabbit heart at 4.7T

An MR line scan protocol has been used to measure relaxation parameters (T1 and T2) in isolated, blood perfused rabbit hearts at various blood oxygenations. Hearts were retrogradely perfused at 37 degrees C with a cardioplegic solution (modified St. Thomas' solution) containing sheep red blood cells and adenosine (1 mM) to maximally vasodilate the coronary vascular bed. Arresting the hearts eliminated motion complications and minimized arteriovenous oxygenation differences. The authors have found that under conditions of stable flow, there is a strong correlation between T2 in myocardial septa and hemoglobin (Hb) saturation, while tissue T1 is virtually independent of blood oxygenation. These effects are believed to be due to the paramagnetic agent deoxyhemoglobin.

Regional heterogeneity of human myocardial infarcts demonstrated by contrast-enhanced MRI. Potential mechanisms

BACKGROUND

Myocardial reperfusion is pivotal to the prognosis of patients with acute myocardial infarction. In these patients, coronary flow is generally assessed by angiography and tissue perfusion by tracer scintigraphy. This study was designed to examine whether magnetic resonance imaging (MRI) provides information on myocardial perfusion and damage beyond that supplied by angiography and thallium scintigraphy after acute myocardial infarction.

METHODS AND RESULTS

Twenty-two patients with recent myocardial infarction had ECG, echocardiography, coronary angiography, and fast contrast-enhanced MRI. Twelve patients also had exercise thallium scintigraphy. Time-intensity curves obtained from infarcted and noninfarcted regions were correlated with coronary anatomy and left ventricular function. Two perfusion patterns were observed in infarcted regions by comparison with the normal myocardial pattern. All patients but 1 had persistent myocardial hyperenhancement within the infarcted region up to 10 minutes after contrast. In 10 patients, this hyperenhanced region surrounded a subendocardial area of decreased signal at the center of the infarcted region associated with coronary occlusion at angiography, Q waves on ECG, and greater regional dysfunction by echocardiography. Moreover, the extent and location of the MRI abnormalities correlated well with the extent and location of the fixed single-photon emission computed tomography thallium defects.

CONCLUSIONS

Large human infarcts, associated with prolonged obstruction of the infarct-related artery, are characterized by central dark zones surrounded by hyperenhanced regions on MRI. Conversely, reperfused infarcts with less regional dysfunction have uniform signal hyperenhancement. The MRI hyperenhanced segment correlates well with the fixed scintigraphic defect in patients with acute myocardial infarction.

A magnetization-driven gradient echo pulse sequence for the study of myocardial perfusion

A T1-weighted imaging pulse sequence for contrast-based studies of myocardial perfusion is presented and evaluated in phantoms and in vivo. The sequence is similar to spoiled gradient-recalled echo sequences except that nonselective preparatory RF pulses drive magnetization to steady state prior to image acquisition. Steady state is thus obtained in both tissue and blood resulting in a stable, homogeneous, and dark pre-contrast baseline. Tip angles and timings are chosen so that pixel intensity approximates a linear relation to 1/T1. The dynamic range of signal response to contrast agent concentration is greater than that of an inversion-recovery fast low angle shot sequence. The sequence proposed should be useful for myocardial perfusion studies.

A phased array coil for human cardiac imaging

A prototype cardiac phased array receiver coil was constructed that comprised a cylindrical array and a separate planar array. Both arrays had two coil loops with the same coil dimensions. Data acquisition with the cylindrical array placed on the human chest, and the planar array placed under the back, yielded an overall enhancement of the signal-to-noise ratio (SNR) over the entire heart by a factor of 1.1-2.85 over a commercially available flexible coil and a commercially available four-loop planar phased array coil. This improvement in SNR can be exploited in cardiac imaging to increase the spatial resolution and reduce the image acquisition time.

Three-dimensional myocardial deformations: calculation with displacement field fitting to tagged MR images

PURPOSE

To reconstruct three-dimensional (3D) myocardial deformations from orthogonal sets of parallel-tagged magnetic resonance (MR) images.

MATERIALS AND METHODS

Displacement information in the direction normal to the undeformed tag planes was obtained at points along tag lines. Three independent sets of one-dimensional displacement data were used to fit an analytical series expression to describe 3D displacement as a function of deformed position. The technique was demonstrated with computer-generated models of the deformed left ventricle with data from healthy human volunteers.

RESULTS

Model deformations were reconstructed with a 3D tracking error of less than 0.3 mm. Error between estimated and observed one-dimensional displacements along the tags in 10 human subjects was 0.00 mm +/- 0.36 (mean +/- standard deviation). Robustness to noise in the tag displacement data was demonstrated by using a Monte Carlo simulation.

CONCLUSION

The combination of rapidly acquired parallel-tagged MR images and field-fitting analysis is a valuable tool in cardiac mechanics research and in the clinical assessment of cardiac mechanical function.

Pancreatic adenocarcinoma: CT versus MR imaging in the evaluation of resectability--report of the Radiology Diagnostic Oncology Group

PURPOSE

To compare findings with computed tomography (CT) and magnetic resonance (MR) imaging in pancreatic adenocarcinoma and to determine optimal pulse sequences for MR imaging.

MATERIALS AND METHODS

CT scans and MR images were compared of 189 adult patients with known or suspected adenocarcinoma of the pancreas. Levels of confidence were correlated with surgical and pathologic results.

RESULTS

The accuracy of CT was 0.73 and of MR imaging was 0.70. The negative predictive value of CT was 0.28 and of MR imaging was 0.23. The positive predictive value of CT was 0.89 and of MR imaging was 0.88. Gradient-echo and T1-weighted spin-echo sequences ranked equally in evaluation of vascular invasion, T1-weighted spin-echo sequences were preferred for assessing lymphadenopathy, and T2-weighted spin-echo sequences were preferred for detecting hepatic metastases.

CONCLUSIONS

Cross-sectional imaging modalities are useful in the identification of unresectable pancreatic carcinoma. CT is recommended for initial imaging assessment.

Variability in the size of individual airways over the course of one year

Several studies have examined the reproducibility over time of standard clinical measurements of pulmonary airways parameters, such as FEV1 and FVC. These studies indicate fairly consistent reproducibility of such measurements over periods of months and years. Although such traditional pulmonary function tests can provide quantitative measures of lung function, they do not provide any insight into the local or regional variation of individual airways. Therefore, in a longitudinal study in dogs, we directly measured the baseline size (cross-sectional area) of the same individual canine airways multiple times over a 1-yr period, using high-resolution computed tomography (HRCT). We compared the variability of individual airway size both within and between subjects to the various sizes of airways and to seasonal differences in size. Five anesthetized, intubated, and ventilated dogs were studied between five and eight times over a 15-mo period. HRCT scans were acquired at end expiration with the dogs at FRC on all occasions. All individual airways showed considerable variability in size over time as measured by the coefficient of variation (CV) (range: 9.2 to 69.3). The sizes of individual airways within a dog were seen to change by as much as twofold over time. Some dogs also showed significantly greater overall variability in airway size than did other dogs (p < 0.01), but this variability was related neither to the size of the airway (p = 0.34) nor to the season. These findings indicate a substantial local and regional variability in airway caliber that would not be detectable with conventional pulmonary function tests.(ABSTRACT TRUNCATED AT 250 WORDS)

Digital chest radiography: comparison of unprocessed and processed images in the detection of solitary pulmonary nodules

PURPOSE

To compare the accuracy with which simulated solitary pulmonary nodules can be identified on digital images of the chest that are unprocessed, processed with adaptive spatial filtering, or processed with global filtering.

MATERIALS AND METHODS

Six experienced chest radiologists evaluated 408 test radiographs (136 from each of the three types, half with simulated nodules) and judged whether a nodule was present. Data from the 2,448 observations were evaluated by means of a receiver operating characteristic curve with location methods.

RESULTS

Accuracy was significantly better with the adaptive filter technique than with the global technique (P < .05), and it was better with adaptive filtering than with no processing in the detection of pulmonary nodules in the mediastinal-subdiaphragmatic areas (P < .05). No significant difference was found between no processing and global filtering.

CONCLUSION

Adaptive filtration is superior to global filtration in the identification of solitary pulmonary nodules and is superior to no processing in nodules projected over the radiopaque areas of the thorax on digital images.

Multi-shot EPI for improvement of myocardial tag contrast: comparison with segmented SPGR

To assess the potential value of multi-shot EPI relative to segmented k-space SPGR for myocardial tagging, we measured tag contrast for both sequences in a phantom and human study and compared it with theoretical predictions. In the human heart, EPI tag contrast was three times that of SPGR at the end of systole. Tag duration was lengthened with EPI to at least 600 ms. In addition, the entire heart was examined in a total of 32 heartbeats with EPI versus 152 heartbeats with SPGR.

Visualization of airway obstruction in vivo during pulmonary vascular engorgement and edema

Although pulmonary vascular engorgement has often been hypothesized to decrease airway caliber, leading to airway obstruction in asthma, direct evidence for this hypothesis is lacking. In the present study, we used high-resolution computed tomography to directly measure the changes in the caliber and wall thickness of conducting airways after volume loading with normal saline (NS) and homologous blood. Five anesthetized dogs received 0.2 mg/kg of atropine followed by either three sequential fluid challenges of 50 ml/kg of NS or two sequential challenges of 25 ml/kg of blood. Several weeks later, the same dogs received the other fluid challenge. Volume loading with 150 ml/kg of NS decreased the average airway luminal area to 68 +/- 3% (+/- SE) of baseline. Concomitantly, airway wall thickness increased to 150 +/- 6% of baseline. Volume loading with 50 ml/kg of blood decreased the average airway luminal area to 81 +/- 2% of baseline. Concomitantly, airway wall thickness increased to 108 +/- 2% of baseline. Therefore, for comparable changes in pulmonary vascular pressure, an infusion of NS caused a significantly greater decrease in airway luminal area and a larger increase in airway wall thickness than an infusion of blood. This suggests that the presence of edema fluid in or immediately surrounding the airway wall acts to decrease the airway lumen. However, since the degree of airway narrowing was only moderate, even with a most extreme fluid load, it seems unlikely that airway wall thickening or edema could be a primary cause of conducting airway obstruction in patients with asthma or impaired left ventricular function.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of myocardial water exchange on T1 enhancement during bolus administration of MR contrast agents

Interpretation of first-pass myocardial perfusion studies employing bolus administration of T1 magnetic resonance (MR) contrast agents requires an understanding of the relationship between contrast concentration and image pixel intensity. The potential effects of myocardial water exchange rates among the intravascular, interstitial, and cellular compartments on this relationship are controversial. We directly studied these issues in isolated, nonbeating canine interventricular septa. Myocardial T1 was measured three times/s during bolus transit of intravascular (albumin-Gd-DTPA and polylysine-Gd-DTPA) and extracellular (gadoteridol) contrast agents. For polylysine-Gd-DTPA, the peak changes in myocardial 1/T1 (delta R1) scaled nonlinearly with perfusate contrast concentration whereas a linear relationship would be expected for fast water exchange among the vascular, interstitial, and cellular compartments. For all agents, the peak delta R1 were much smaller than the values expected on the basis of fast myocardial water exchange. The data demonstrate that in isolated myocardial tissue, myocardial T1 enhancement during bolus administration of contrast can be strongly affected by myocardial water exchange for both intravascular and extracellular MR contrast agents.

MR imaging of the breast in patients with silicone breast implants: normal postoperative variants and diagnostic pitfalls

Because of the wide variety of types of silicone breast implants, modifications, and surgical procedures, a number of normal variants and potentially misleading appearances can be encountered on MR images of breast implants. Familiarity with these findings is essential to proper interpretation of these images. The purpose of this essay is to illustrate the overlap in appearances of MR images in different clinical scenarios. When confusing findings are present, confirmation of the surgical history can usually clarify the matter.

Spontaneous airways constrict during breath holding studied by high-resolution computed tomography

Airway constriction during a breath hold could not be examined previously using standard methods. We used high-resolution computed tomography (HRCT) in vivo to assess the temporal changes in airway area and the effects of a deep inspiration with and without vagal suppression. Five dogs were anesthetized, intubated, and their lungs ventilated with 100 percent oxygen. Fifteen HRCT slices were obtained at functional residual capacity (FRC) either immediately after stopping ventilation at end expiration after either a tidal volume breath or three deep inspirations. Subsequently the dogs were given atropine, 0.2 mg/kg, and the scans were repeated. The cross-sectional areas of 33 airways ranging in size from 1.6 to 9.7 mm in diameter were measured. Airways were separated in three groups based on size: small (< 3 mm in diameter); medium (3 to 6-mm in diameter); and large (> 6 mm in diameter). The small, medium, and large airways showed a spontaneous constriction over time to 49 +/- 8 percent, 83 +/- 4 percent, and 82 +/- 4 percent of initial airway size, respectively (p < 0.01), (p < 0.0001). The deep inspiration caused an initial dilation only in the smallest airways to 133.3 +/- 4 percent. The subsequent constrictions were even greater than after the tidal volume breath averaging 67 +/- 15 percent, 61 +/- 6 percent, and 60 +/- 9 percent of initial airway area in the small, medium, and large airways, respectively (p = 0.001). Atropine caused an average increase in baseline airway area of 115 +/- 5 percent and 121 +/- 6 percent after a tidal volume breath and deep inspiration, respectively, compared with the preatropine controls, with no difference between the three groups. Atropine also completely abolished the spontaneous airway constriction observed after either a tidal volume breath or a deep inspiration in all three groups equally. In conclusion, using direct airway imaging in vivo, we found that airways spontaneously constrict during a prolonged expiratory pause, and a deep inspiration significantly augments this airway constriction. These responses are mediated via vagal afferent pathways, likely arising from progressively decreasing slow-adapting receptor activity.