G. P. Krestin

Magnetic resonance in the assessment of renal function

The kidneys are the most important organs to maintain homeostasis. In the assessment of renal functional disorders laboratory tests offer only indirect hints on location of the disease; radionuclide nephrography is hampered by low spatial resolution and radiologic methods provide only limited quantitative information. The MRI technique with fast pulse sequences and renally eliminated contrast agent has the capability of combining both anatomic and functional information. This article gives an overview on functional MRI of the kidneys with its possibilities and limitations. The clinical application of functional MRI allows a better understanding of some pathologic conditions such as urinary tract obstruction, renal insufficiency, effects of extracorporeal shock wave lithotripsy, different states of hydration, effects of drugs, vascular disorders, and effects of transplantation.

Detection of breast cancer with conventional mammography and contrast-enhanced MR imaging

The aim of this study was to compare the diagnostic performance of conventional mammography and dynamic contrast-enhanced fast 3D gradient-echo (GRE) MRI regarding the detection and characterization of breast lesions relative to histopathologic analysis and to assess the results of a combined evaluation of both methods. fifty consecutive patients with 63 histopathologically verified breast lesions underwent dynamic contrast-enhanced GRE MRI in addition to routine conventional mammography. All lesions were classified by both methods on a five-point scale as benign or malignant, and the results were correlated to histopathology. Conventional mammography and dynamic MRI yielded a sensitivity and specificity of 82 and 64 %, and 92 and 76 %, respectively. The difference between the results was statistically not significant (p > 0.05) with areas under the receiver-operating-characteristics curves of 0.807 for mammography and 0.906 for MR imaging. Combination of the results of both methods slightly increased the sensitivity for detection of breast cancer to 95 % but decreased specificity to 52 %. In this selected patient subset, including only patients referred for excisional biopsy, contrast-enhanced dynamic MRI proved more sensitive and specific than conventional mammography regarding the detection of malignancy. While a combination of both methods yields a slightly improved sensitivity, specificity is vastly reduced.

MR imaging of the augmented breast

Abstract. Mammographic evaluation of the augmented breast is challenging, since breast implants obscure significant amount of breast tissue while diminishing the effect of compression. Posttherapeutic scarring can make mammographic interpretation even more difficult. MRI has thus evolved into the modality of choice for diagnosing implant complications as well as detection of primary or recurrent breast cancer in these patient population. The present article attemps to give an overview of the MR findings of different breast augmentation and reconstruction techniques, i. e. prosthetic breast implants, breast reconstruction with autogenous tissue, free silicone injections and fat grafts, and their complications.

MRI of the uterus, uterine cervix, and vagina: diagnostic performance of dynamic contrast-enhanced fast multiplanar gradient-echo imaging in comparison with fast spin-echo T2-weighted pulse imaging

Abstract. The aim of this study was to compare the diagnostic performance of contrast-enhanced fast multiplanar gradient-echo (GRE) and T2-weighted fast spin-echo (FSE) image sets in the assessment of uterus, cervix, and vagina. Fast (up to 20 contiguous sections in 23 s) multiplanar GRE and FSE images of 45 patients referred for imaging of the female pelvis were evaluated retrospectively with regard to overall image quality and the ability to detect normal anatomic structures, as well as lesion conspicuity. Results were compared with histologic findings (n = 29) or clinical follow-up. Furthermore, a quantitative assessment of contrast-to-noise ratios among normal uterine and cervical structures as well as uterine lesions was performed for both sequences. On GRE images, uterine and cervical differentiation was best seen on the image sets acquired 15 and 60 s following contrast enhancement and results were significantly better compared with delayed images (p < 0.05). Delineation of the junctional zone was significantly (p < 0.05) better on FSE compared with GRE images; no significant difference was seen for the other anatomic structures. Overall image quality of GRE and FSE images was similar. Sensitivity for lesion detection based on both GRE and FSE images was 96 % with a sensitivity of 93 % for GRE, and 81 % for FSE images alone, respectively. Using the extended McNemar χ2 test, the difference in diagnostic performance between FSE and GRE revealed no significant difference, whereas the combination of both techniques performed better than FSE imaging alone (p < 0.05). The presented data suggest that dynamic contrast-enhanced GRE imaging should be part of an MR examination of the female pelvis. Combined GRE and FSE imaging provide an excellent sensitivity in the assessment of uterine and vaginal pathologies.

Semiautomated ROI analysis in dynamic MR studies. Part II: Application to renal function examination

Fast MR techniques and the application of water-soluble contrast agents allow the simultaneous examination of renal morphology and the functional aspects of glomerular filtration using bolus injections of Gd-DTPA. Spatial resolution is sufficient to resolve individual renal pyramids, but the quantitative examination of regions of interest (ROIs) is severely impeded by organ movements due to variations of the end-inspiratory position. A new image-processing scheme has been used and tested in 23 normal volunteers and patients. This scheme replaces a tedious frame-by-frame ROI analysis by positional correction of renal regions of all frames of the sequence such that the definition of the regions has to be performed only once. The signal intensities (SIs) of the local regions in each frame are used to compute statistics and to generate curves representing local temporal SI changes due to contrast agent excretion. The success rate of the procedure depends largely on the image quality and on the adherence to a proved acquisition protocol. The present article shows that the combination of MR and robust and reliable image-processing methods can be important for the highly automated analysis of a large number of images acquired as dynamic studies.

FDG PET-negative liver metastases of a malignant melanoma and FDG PET-positive Hurthle cell tumor of the thyroid

Abstract. A Hurthle cell tumor (oncocytoma) of the thyroid presented as a hypermetabolic focus in a fluorodeoxyglucose positron emission tomography (FDG PET) study which was performed as staging procedure in a patient with malignant melanoma. This finding led to the initial diagnosis of a metastasis. In contrast, multiple liver metastases, seen on MRI and sonography, did not show any increased FDG uptake. Cytology results of one liver mass confirmed a melanoma metastasis, and of the neck mass, a Hurthle cell tumor. The Hurthle cell tumor was, based on clinical evidence, thought to be benign. This is the first description of a FDG PET-positive benign Hurthle cell tumor, with FDG PET-negative liver metastases of a malignant melanoma, in the same patient.

Alterations in renal morphology and function after ESWL therapy: evaluation with dynamic contrast-enhanced MRI

Contrast-enhanced gradient-echo MRI was used to evaluate morphological and functional alternations in the kidneys after extracorporeal shock wave lithotripsy (ESWL). Dynamic MRI with a temporal resolution of 10 s per image was performed by repeated imaging in the coronal plane after administration of gadolinium-DTPA (0.1 mmol/kg) before and after ESWL for renal calculi in 25 patients. Before ESWL 22 patients had normally functioning kidneys, characterised by a marked decrease in signal intensity in the renal medulla 30–40 s after the onset of cortical perfusion. After ESWL 8 patients had functional abnormalities: in 2 cases the medullary signal decrease was disturbed throughout the whole organ, while 6 kidneys demonstrated regional loss of concentrating ability in the medulla. Morphological alterations (oedema with blurred contours and loss of corticomedullary differentiation; parenchymal haemorrhage and haemorrhage in a cortical cyst; subcapsular, perirenal and pararenal haematoma) were detected in 9 cases. Haemorrhage was encountered more often after administration of more than 2500 shock waves; however, no such correlation was seen in the kidneys with functional disturbances following ESWL therapy. MRI proved to be a sensitive method for the assessment of morphological and functional alterations after ESWL, but longer follow-up studies are required to identify the clinical impact of these early changes.

Assessment of renal artery stenosis by phase-contrast magnetic resonance angiography

Three-dimensional (3D) phase-contrast magnetic resonance angiography (MRA) and velocity-encoded cine magnetic resonance (VEC-MR) imaging were performed in 23 subjects to assess the severity of renal artery stenosis. MRA was used for detection of stenosis, demonstrating a sensitivity of 100% and a specificity of 80%; the severity of stenosis was overestimated in 33%. VEC-MR was used to quantify the renal flow oattern and was successful in 11 subjects. Mean blood flow of normal renal arteries (420 +- 107 ml/min) was significantly higher (P < 0.01) than mean blood flow of stenotic arteries (131 +- 46ml/min). The flow profile displayed both systolic and diastolic peaks in 75% of the normal arteries, while the flow in stenotic arteries showed only a single systolic peak in all cases. The systolic peak in stenotic arteries occurred significantly later (32 +- 3% of the period of one cardiac cycle) than in normal subjects (21 +- 7%) (P < 0.05). Phase-contrast MR is likely to gain considerable importance in the noninvasive aetection and quantification of renal artery stenosis.

Magnetresonanztomographie der abdominellen Gefäße

Die Magnetresonanztomographie ist ein fur jegliche Bewegung sehr empfindliches Verfahren. Dies hat einerseits den Vorteil, das sich fliesendes Blut und somit Gefase bereits ohne Verwendung eines Kontrastmittels kontrastreich vom umgebenden stationaren Gewebe abheben, andererseits hat die Bewegungsempfindlichkeit des Verfahrens auch zahlreiche, die Bildqualitat beeintrachtigende Artefakte zur Folge. Nicht zuletzt aus diesem Grunde eignete sich die kernspintomographische Bildgebung zunachst nur zur Untersuchung unbewegter Organe, so vor allem des ZNS und des Muskuloskeletalsystems. Wegen der atmungs- und blutflusbedingten Artefakte setzte sich die Methode im Thorax- und Abdominalbereich dagegen zunachst nur begrenzt durch. Die Einfuhrung artefaktkompensierender Verfahren und schneller Gradienten-Echo-(GE-)Sequenzen hat aber eine Neubewertung der MRT im Abdominalbereich notwendig gemacht.