Determinants of image appearance in contrast-enhanced magnetic resonance angiography. A review

The use of contrast agents in magnetic resonance (MR) studies of vascular pathology has permitted the exploration of regions that were heretofore poorly evaluated with conventional magnetic resonance angiography (MRA). An important feature of contrast-enhanced MRA (CE-MRA) is the very short acquisition times that are possible. The determination of the parameters to be used in a CE-MRA study rests on an understanding of the dynamics of the passage of the injected contrast agent and the response of the magnetization to the parameters of the MR imaging sequence. An overview of this interaction is presented.     

Comparison of gadolinium-DTPA and macromolecular gadolinium-DTPA-polylysine for contrast-enhanced pulmonary time-of-flight magnetic resonance angiography

RATIONALE AND OBJECTIVES: The authors investigated the enhancing effect of low-dose administration of the macromolecular, paramagnetic contrast medium gadolinium (Gd)-DTPA-polylysine (average molecular weight, 40,000-50,000 dalton [D]) compared with Gd-DTPA (molecular weight, 547 D) in time-of-flight magnetic resonance angiography of unilaterally damaged sheep lungs. MATERIALS AND METHODS: Thirteen heart-lung preparations were examined in the head coil of a 1.5-tesla imager (Magnetom SP, Siemens, Erlangen, Germany). The authors performed time-of-flight angiograms (coronal; repetition time, 35 mseconds; echo time, 6 mseconds; 20 degrees flip angle; pixel size 1.0 x 1.0 x 1.5 mm3) before and after application of the contrast agents. Gadolinium-DTPA-polylysine was used in a dose of 0.027 mmol/kg body weight while Gd-DTPA was injected in variable doses. RESULTS: After Gd-DTPA-polylysine, signal intensity increased by 118% in pulmonary arteries in healthy lungs and by 121% in damaged lungs (P < 0.001). In addition, the contrast-to-noise ratio measured between pulmonary arteries and perivascular parenchyma increased significantly (P < 0.01). On three-dimensional angiograms, two more generations of vascular branches could be detected. A dose of Gd-DTPA 6.1 times higher than the Gd-DTPA-polylysine dose was necessary to obtain the same contrast enhancing effect as Gd-DTPA-polylysine in healthy lungs. In damaged lungs, none of the administered doses of Gd-DTPA reached the average contrast enhancement of Gd-DTPA-polylysine. CONCLUSIONS: The authors' measurements demonstrate significant improvement of time-of-flight angiograms by low-dose administration of Gd-DTPA-polylysine.     

Contrast-enhanced three-dimensional magnetic resonance angiography of the splanchnic vasculature before and after caloric stimulation. Original investigation

BACKGROUND: Onychomycosis, a fungal nail infection, has become one of the most important dermatophytoses. Unfortunately, a predictably successful diagnostic approach to onychomycosis does not yet exist. OBJECTIVE: The purpose of this study was to develop a deoxyribonucleic acid (DNA)-based diagnostic method to improve the sensitivity and specificity of the detection and differentiation of the pathogenic fungi of onychomycosis. METHODS: We attempted to detect fungi in the nail using polymerase chain reaction (PCR) primer systems that were designed in conserved sequences of the small ribosomal subunit 18S-rRNA genes shared by most fungi, and differentiated between species by restriction enzyme analysis of the amplified product. RESULTS: Fragments of the gene coding for 18S-rRNA were amplified successfully from medically important fungi species, but not from normal nails. Restriction fragment length polymorphism patterns using HaeIII endonuclease were sufficiently different to allow the recognition of individual species. CONCLUSIONS: The PCR-restriction enzyme analysis method appears to be a more sensitive detection and identification technique for onychomycosis than conventional methods, and has considerable diagnostic value.     

Principles of magnetic resonance angiography

A strongly fluorescing 7-hydroxycoumarin (umbelliferone, U) oxidized in dilute (10 mumol/L-O, 1 mol/L) aqueous solution with CIO- or CIO- + H2O2 (but not with H2O2 alone) produces a strong chemiluminescence (CL). Light emission kinetics depends on the pH of solution (4.0-10.5) and the reaction has a low activation energy Ea = 31 +/- 2 kJ/mol (285-310 K). The spectrum covers the fluorescence of umbelliferone (400-550 nm, lambda max 460 nm). No red emission typical of 1 delta g, 1 sigma g, 1 sigma+g (O2)2 is observed either in the umbelliferone+CIO- or the umbelliferone +CIO- + H2O2 solution. The possible mechanism of CL and concomitant degradative oxidation of umbelliferone is discussed.     

Cerebrovascular applications of magnetic resonance angiography

Magnetic resonance angiography (MRA) has been applied to a variety of different manifestations of cerebrovascular disease to date. Practically, the limitations of these techniques must be taken into account such that the appropriate method is applied to answer a specific clinical question and the acquisition parameters are chosen to maximise the sensitivity and specificity of the study. Based upon these results, it must be decided whether the conventional parenchymal MR and MRA evaluations are sufficient in a particular setting or whether it is necessary to commit the patient to a more traditional, invasive angiographic study for a more thorough investigation. Presently, MR angiographic studies and flow measurement techniques serve to compliment the more traditional spin-echo evaluation of patients with small aneurysms, arterial and venous occlusions, vascular malformations and in some cases of neoplastic vascular invasion. With well-trained technicians and carefully prescribed protocols, this new information and improved diagnostic sensitivity can be used routinely with only a minor increase in patient exam time.     

Pulmonary veins: magnetic resonance angiography anatomy

OBJECTIVES: This study was undertaken to investigate the relation between dietary fat composition and adiposity in adult men. SUBJECTS: A sample of 128 male subjects who participated in Phase 2 of the Québec Family Study. DESIGN: The association between adiposity and total dietary fat intake (TFI), saturated fat intake (SFA), monounsaturated fat intake (MUFA) and polyunsaturated fat intake (PUFA) was analyzed in the overall sample. A comparison of body fatness was also performed between consumers of high (4th quartile) and low amounts (1st quartile) of TFI, SFA, MUFA and PUFA. RESULTS: Significant positive correlations were found between the percentage of dietary energy as total fat and body fatness. Men in the upper quartile of TFI displayed significantly more adiposity than those in the lower quartile. Significant differences were also observed when quartiles were established using SFA and MUFA. However, higher intakes of PUFA had no statistical effects on adiposity. CONCLUSION: These results confirm the notion that high fat diets might lead over time to excess body fat deposition. SFA and MUFA intake also seem to be predictors of actual adiposity markers while high PUFA intake seems to exert no effect on these markers.     

MR angiography of the peripheral vasculature

MR angiography (MRA) for the evaluation of peripheral arterial occlusive disease is a rapidly evolving technique. Recent prospective clinical trials have indicated that MRA may play an important role in the evaluation of patients with peripheral arterial disease. This article discusses the pertinent technical aspects and limitations of peripheral MRA as well as some of the clinical data available.     

CT angiography of the visceral vasculature

Gynecologic epithelial tumors can be grouped into two major categories depending on whether they are derived embryologically from squamous epithelium of the urogenital sinus or from müllerian ducts. Ovarian carcinomas appear morphologically similar to those arising in müllerian-derived organs, and molecular genetic defects present in tumors from these different sources appear to reflect more their histologic subtypes than their organ of origin. The possibility that ovarian epithelial tumors arise from remnants of müllerian ducts in the vicinity of the ovary therefore merits further investigation. Recent advances in our understanding of the state of clonality of various gynecologic tumors, of the influence of age and ovulatory activity on their genetic characteristics, and of their overall molecular genetic features, provide important clues about their initial underlying mechanisms. Novel strategies based on these advances are being tested for their potential utility in treating and monitoring gynecologic tumors.     

Three-dimensional ultrasound imaging of the vasculature

With conventional ultrasonography, the diagnostician must view a series of two-dimensional images in order to form a mental impression of the three-dimensional anatomy, an efficient and time consuming practice prone to operator variability, which may cause variable or even incorrect diagnoses. Also, a conventional two-dimensional ultrasound image represents a thin slice of the patients anatomy at a single location and orientation, which is difficult to reproduce at a later time. These factors make conventional ultrasonography non-optimal for prospective or follow-up studies. Our efforts have focused on overcoming these deficiencies by developing three-dimensional ultrasound imaging techniques that are capable of acquiring B-mode, colour Doppler and power Doppler images of the vasculature, by using a conventional ultrasound system to acquire a series of two-dimensional images and then mathematically reconstructing them into a single three-dimensional image, which may then be viewed interactively on an inexpensive desktop computer. We report here on two approaches: (1) free-hand scanning, in which a magnetic positioning device is attached to the ultrasound transducer to record the position and orientation of each two-dimensional image needed for the three-dimensional image reconstruction; and (2) mechanical scanning, in which a motor-driven assembly is used to translate the transducer linearly across the neck, yielding a set of uniformly-spaced parallel two-dimensional images.     

Gadolinium-enhanced magnetic resonance angiography of abdominal blood vessels

Contrast enhanced (CE) magnetic resonance angiography (MRA) provides high resolution angiograms within 20-40 sec. The technique is based on the acquisition of heavily T1-weighted three-dimensional (3D) gradient-echo data sets (FISP) with ultrashort echo-(< 2ms) and repetition times (< 5 ms) during arterial phase of an intravenously injected bolus of a T1-shortening agent such as Gd-DTPA. For MR-angiography of abdominal vessels CE-MRA is better suited than "time-of flight" (TOF) and phase-contrast (PC) MRA because motional artifacts can be obviated with breath-held acquisitions. We have optimised the technique and evaluated its potential for angiography of the abdominal aorta and its branches as well as the portal vein and its tributaries. Whilst CE-MRA provides reliable diagnostic accuracy in the aorta and the proximal sections of its branches, small peripheral arteries cannot be assessed accurately. The portal vein and its tributaries can often be depicted better with CE-MRA than with conventional angiography but, like conventional angiography, CE-MRA is hampered by slow and reversed flow, conditions under which TOF or "true FISP" MRA may perform bst. We have also investigated FLASH-echo-planar imaging (EPI) hybrid techniques, a further technical development which due to shorter acquisition times of 12-15 sec. allows semi-dynamic imaging of the arterial and venous phase and provide better vessel contrast due to the use of fat-suppression.     

Diagnosis of pulmonary embolism with magnetic resonance angiography

BACKGROUND: Diagnosing pulmonary embolism may be difficult, because there is no reliable noninvasive imaging method. We compared a new noninvasive method, gadolinium-enhanced pulmonary magnetic resonance angiography, with standard pulmonary angiography for diagnosing pulmonary embolism. METHODS: A total of 30 consecutive patients with suspected pulmonary embolism underwent both standard pulmonary angiography and magnetic resonance angiography during the pulmonary arterial phase at the time of an intravenous bolus of gadolinium. All magnetic resonance images were reviewed for the presence or absence of pulmonary emboli by three independent reviewers who were unaware of the findings on standard angiograms. RESULTS: Pulmonary embolism was detected by standard pulmonary angiography in 8 of the 30 patients in whom pulmonary embolism was suspected. All 5 lobar emboli and 16 of 17 segmental emboli identified on standard angiograms were also identified on magnetic resonance images. Two of the three reviewers reported one false positive magnetic resonance angiogram each. As compared with standard pulmonary angiography, the three sets of readings had sensitivities of 100, 87, and 75 percent and specificities of 95, 100, and 95 percent, respectively. The interobserver correlation was good (k=0.57 to 0.83 for all vessels, 0.49 to 1.0 for main and lobar vessels, and 0.40 to 0.81 for segmental vessels). CONCLUSIONS: In this preliminary study, gadolinium-enhanced magnetic resonance angiography of the pulmonary arteries, as compared with conventional pulmonary angiography, had high sensitivity and specificity for the diagnosis of pulmonary embolism. This new technique shows promise as a noninvasive method of diagnosing pulmonary embolism without the need for ionizing radiation or iodinated contrast material.     

Magnetic resonance angiography and magnetic resonance tomography in dissection of the vertebral artery

Vertebral artery dissection (VAD) is an important cause of posterior circulation stroke in young adults. Initial symptoms are often non-specific and diagnostic arteriography is not performed until neurological deficits are obvious. Since magnetic resonance tomography (MRT) is superior in the diagnosis of vertebrobasilar ischemia, we retrospectively analyzed the role of MRT and MR angiography (MRA) in the detection of dissections of the vertebral artery. Between 1989 and 1995 we identified 24 patients with a vertebral artery dissection and 1 patient with a basilar artery dissection (8 females and 17 males, 23-60 years of age, mean 41.2 years). The diagnosis of VAD (14 left VAD, 9 right VAD, 1 bilateral VAD, 1 basilar artery dissection) was established by specific arteriographical findings (DSA) or clinical and neuroradiological course. All patients underwent a combined MRT/MRA examination protocol at 1.5T that consisted of spin-echo imaging and time of flight MRA of the intra- and extracranial arteries using 2D Flash and 3D Fisp sequences. The MRT/MRA findings were correlated to DSA and ultrasound results. During the acute and subacute stage, MRT/MRA revealed abnormal findings in 21 of 22 dissected vessels (95.5%). There was one false-negative MRT/MRA in a patient with a V1 dissection (intimal flap without peripheral flow disturbances). In 7/22 VAD the MRT/MRA findings were rated specific (double lumen n = 1, mural hematoma n = 4, pseudoaneurysm n = 2). DAS was sensitive in 100% and ultrasound in 77.3%. Specific results were obtained by DSA in 8/ 22 VAD (36.4%) and in 7/22 VAD (30.4%) by MRT/MRA. When MRT/MRA and DSA results were combined, the specific findings increased to 43.5%. Follow-up examinations revealed recanalization in 52% of initially stenosed or occluded vertebral arteries; four patients developed a pseudoaneurysm, and two of them underwent ligation of the VAD. With this retrospective approach, we were able to show a high sensitivity of MRT/ MRA for the presence of disturbed flow in the dissected vertebral artery. The MRA projections tended to overestimate stenosis and were inferior to DSA in the appreciation of irregularities of the vessel wall. Identification of high-grade stenosis, especially in the presence of distal occlusion, was improved on the MRA source images. During the acute and subacute stage, the diagnosis of luminal thrombus can be difficult, because signal ambiguities exist between hemoglobin breakdown products and flow effects and adjacent fat tissues. The differentiation between luminal thrombus and mural hematoma requires interpretation of MRA source images, together with flow compensated spin-echo images. Additional fat suppressed images and flow presaturation may be required at the appropriate levels. The identification of mural hematoma is important, because this finding is considered specific and cannot be obtained with DSA. There is a complementary role of MRT/MRA and DSA for an improved overall specificity for vertebral artery dissection. A negative MRT/MRA result in a patient with appropriate symptoms, however, cannot exclude a dissection and should prompt DSA. On the other hand, a suggestive MRT/MRA result in the appropriate clinical context can replace DSA. The advantage of MRT/MRA is that the method offers a simultaneous diagnosis of posterior fossa ischemia and vertebral artery abnormalities. Therefore, MRT/MRA should be recommended in patients with suspected VAD and especially in those who have no definite neurological deficit. These patients will benefit greatly from early diagnosis and therapy. The fact that all our patients were diagnosed after neurological symptoms and that 64% of them have residual deficits gives an ethical and economical rationale for advocating early MRT/MRA in these patients.     

Clinical diagnostic valuation cine magnetic resonance coronary angiography

OBJECTIVES: This biomechanical cadaver study was performed to compare the fixation stability of a standard lateral condylar buttress plate with a similar condylar buttress plate with the distal screws locked to the plate. Then the study was repeated with six additional matched femoral pairs to compare the locked plate with a standard 95-degree blade plate. DESIGN: Six matched pairs of mildly osteopenic femurs were selected, and each side was assigned randomly to fixation with either a standard lateral condylar buttress plate or a modified lateral condylar buttress plate with locked distal screws. The experiment was repeated with six additional matched pairs of femurs instrumented with either a modified lateral condylar buttress plate with locked distal screws or a standard 95-degree blade plate. INTERVENTION: The femurs were instrumented, and a gap osteotomy was created at the distal femoral metaphysis. The instrumented femurs were then mechanically tested in axial compression and bending/torsional loading to determine fixation stability; then they were loaded at 1,000 newtons for 10(5) cycles and retested for stability. MAIN OUTCOME MEASUREMENT: The displacement across the osteotomy gap at 100-newton and 1,000-newton axial loads was measured directly for each specimen before and after cycling. In addition, resistance to displacement in bending/torsional loading (newtons/centimeter) was determined from load/displacement curves, before and after cycling. RESULTS: The locked buttress plate provided significantly greater fixation stability than the standard plate both before and after cycling in axial loading. The locked buttress plate also proved significantly more stable in axial loading than the blade plate both before and after cycling. CONCLUSION: A condylar buttress plate with locked screws is a valid concept for improving fixation stability.     

Three-dimensional magnetic resonance cholangiopancreatography for evaluation of obstructive jaundice

The purpose of this study was to assess the diagnostic value of magnetic resonance cholangiopancreatography (MRCP) in studying the anatomy, sites, and causes of obstructive jaundice. From September 1994 to May 1996 three-dimensional MRCP was performed on 31 patients with abdominal pain and obstructive jaundice with a fast spin-echo T2-weighted pulse sequence. The images were reconstructed using maximal intensity projection, AVERAGE and SURFACE algorithm processing techniques at a graphics workstation. All the reconstructed images were compared with those obtained using conventional cholangiographic techniques, such as endoscopic retrograde cholangiopancreatography, percutaneous transhepatic cholangiography and drainage, and intraoperative cholangiography. The patients' diagnoses included choledochal cyst (13), cholangiocarcinoma (five), choledocholithiasis (four), pancreatic head carcinoma (three), rhabdomyosarcoma (one), papillary Vater carcinoma (one), recurrent gastric carcinoma (one), ascaris (one), and biliary atresia (two). Extrahepatic biliary dilatation was present in all 13 patients with choledochal cyst; the pancreatic ducts and their entrance level to the common bile duct were observed in eight of these patients. The level of obstruction in patients with cholangiocarcinoma was well documented but the biliary tract of one patient with biliary atresia was not identified by MRCP. In one patient with biliary rhabdomyosarcoma, MRCP clearly delineated the extrabiliary extension of the tumor. In a patient with ascaris in the common bile duct an increase in signal intensity inside the digestive tract of the worm denoted fluid in its gut. Lithiasis was shown in all of the four patients with choledocholithiasis. Thus, MRCP is a useful tool in the assessment of biliary tract obstruction and its causes, and is a valuable addition to ultrasonography.     

Optimizing three-dimensional gadolinium-enhanced magnetic resonance angiography. Original investigation

RATIONALE AND OBJECTIVES: This primarily theoretical work examines three-dimensional gadolinium-enhanced magnetic resonance angiography f8p4Gd-MRA) with the goal of understanding how to achieve the best possible images with respect to signal to noise ratio (SNR) and k-space induced artifacts. Patient variables, contrast injection schemes, and pulse sequence parameters are considered for this purpose. METHODS: A theoretical analysis, including computer simulation, describes how contrast material injection profiles influence 3D Gd-MRA images, both in terms of intravascular signal and resultant artifacts. Further theoretical analysis of the spoiled gradient refocused pulse sequence describes how to maximize SNR. Clinical imaging complements computer modeling. RESULTS: Equations were derived relating contrast injection parameters and pulse sequence variables to SNR and artifacts. For present imaging equipment, administering contrast material over a duration of 60% to 80% of the total imaging time and using fractional echo techniques gives the best SNR without significantly sacrificing image quality. CONCLUSIONS: Three-dimensional Gd-MRA can be tailored to a specific clinical situation and imaging system through the use of proper breath-holding, bolus timing, Gd administration, and pulse sequence design.     

Pulmonary arteriovenous fistula: detection with magnetic resonance angiography

Pulmonary arteriovenous fistula are an uncommon disorder, and are most frequently congenital, usually then associated with hereditary hemorrhagic telangectasia (Rendu-Osler-Weber disease). We present, to our knowledge, the first case of a pulmonary arteriovenous fistula detected by gadolinium-enhanced pulmonary magnetic resonance angiography and confirmed by digital subtraction pulmonary angiography in a patient where the CT scan was unremarkable.