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Competing Interests: Conflict of interest: A.V.R. is on the Scientific Advisory Board for CLEERLY. All other authors declared no conflict of interest.

The application of artificial intelligence (AI) to healthcare has garnered significant enthusiasm in recent years. Despite the adoption of new analytic approaches, medical education on AI is lacking. We aim to create a usable AI primer for medical education. We discuss how to generate a clinical question involving AI, what data are suitable for AI research, how to prepare a dataset for training and how to determine if the output has clinical utility. To illustrate this process, we focused on an example of how medical imaging is employed in designing a machine learning model. Our proposed medical education curriculum addresses AI's potential and limitations for enhancing clinicians' skills in research, applied statistics and care delivery.

Background: Pericoronary adipose tissue (PCAT) attenuation has been identified as a marker for cardiovascular risk. The effect of contrast enhancement on fat attenuation is unknown. We aim to compare precontrast coronary scans to postcontrast CCTA for quantification of pericoronary fat volume and attenuation., Methods: Thin slice pre- and post-contrast studies obtained at 120 kVp, heart rate <60, with no plaque or artifact in the right coronary artery (RCA) were selected. Analysis was limited to pixels -30 Hounsfield units (HU) to -190 HU and from 10 mm to 50 mm distal to the RCA origin at a radial distance equal to the vessel diameter. A subgroup with no plaque across all coronaries was also analyzed., Results: Of 119 study pairs, the average RCA diameter was highly correlated at 3.85 mm (postcontrast) and 3.84 mm (precontrast), r = 0.97, p < 0.0001. The mean attenuation of pre- and postcontrast images was also highly correlated at -87.02 ± 7.15 HU and -82.74 ± 6.54 HU, respectively (r = 0.65, p < 0.0001). Pericoronary fat volume in the -190 to -30 HU range was 396 mm³ lower in the post contrast versus pre-contrast, consistent with higher attenuation (less negative) voxels postcontrast (p < 0.0001). Inter- and intra-reader agreement ranged 95-100% and 90% for precontrast and 85-90% for postcontrast studies, respectively. Subgroup analysis revealed precontrast attenuation -85.59 ± 7.53 HU and postcontrast -82.21 ± 7.15 HU were highly correlated r = 0.67, p < 0.0001., Conclusion: Pericoronary fat enhances with iodinated contrast, potentially explaining some of its risk-predictive capabilities. Fat attenuation and volume can be reliably measured from precontrast calcium scans, with volume quantification showing particularly strong correlation. Excellent inter- and intra-reader agreement is also demonstrated., Competing Interests: Declaration of competing interest The authors have no conflicts of interest to report., (Copyright © 2020 Society of Cardiovascular Computed Tomography. Published by Elsevier Inc. All rights reserved.)

Most articles in 1995 demonstrated a sensitivity of 100% and specificities of 90% to 98% in detecting carotid bifurcation stenosis using a combination of MR angiography and ultrasonography, reserving conventional angiography for cases of disparate stenosis with the noninvasive studies. Multiple reviews concluded that noninvasive studies were less morbid than conventional angiography when taking into account the risks of conventional angiography and surgery as well as the morbidity associated with unnecessary surgery (false-positive noninvasive study) or withholding indicated surgery (false-negative noninvasive study). Thus, the current literature suggests that non-invasive studies have indeed reached a point where they can replace conventional angiography in the preoperative evaluation of carotid bifurcation stenosis. It is paramount that each imaging center review its own sensitivity and specificity in detecting surgically significant carotid stenosis with noninvasive studies. High sensitivity and specificity in diagnosing percent carotid stenosis with noninvasive studies can occur today with routine imaging equipment, but it takes a great deal of effort and dedication on the part of the imaging center.

Objective: The purpose of this prospective study was to compare MR angiography of the carotid artery from the aortic arch through the circle of Willis using maximum-intensity projection (MIP) and multiplanar reformation (MPR) images with intraarterial angiography in the depiction of extracranial carotid atherosclerosis., Subjects and Methods: The carotid arteries in 20 patients were studied with MR and intraarterial angiography. MR angiography included two-dimensional (2D) time-of-flight (TOF) sequences from the aortic arch through the skull base and three-dimensional (3D) TOF sequences centered at the carotid bifurcation and multiple overlapping thin slab acquisition (MOTSA) from the skull base to above the circle of Willis. Targeted MIP images of the 2D and 3D TOF MR angiograms through each carotid bifurcation were obtained. Last, MPR images of the 3D TOF MR angiograms at the obliquity that showed the greatest stenosis were obtained. All studies were reviewed in a double-blinded fashion by two neuroradiologists. Caliper measurements of MR angiograms and intraarterial angiograms were made by using North American Symptomatic Carotid Endarterectomy Trial criteria. Global MIPs of the aortic arch and common carotid arteries from the 2D TOF MR angiograms and targeted MIPs of the intracranial carotid arteries from the MOTSA MR angiograms were compared with the intraarterial angiogram and graded as normal, mild, moderate, severely stenotic, or occluded., Results: MPR of the 3D TOF MR angiograms was highly correlated with intraarterial angiograms for both observers 1 and 2 (0.94/p < .001, 0.96/p < .001 [Pearson correlation/p value]). No statistically significant difference between 3D TOF MPR and intraarterial angiography was seen with a paired t-test. With an alpha = 0.05 (5% probability of type 1 error), the power to detect a difference as small as +/- 5% stenosis between 3D TOF MPR and intraarterial angiogram was 80% for observer 1 and 90% for observer 2. Although both MIPs of the 2D and 3D TOF MR angiograms showed high Pearson correlation coefficients (0.83, 0.90) with intraarterial angiography, the paired t-test revealed a statistically significant difference in the estimation of carotid stenosis. Both observers thought the global MIPs of the 2D TOF MR angiogram allowed good to excellent visualization of the common carotid arteries. The aortic arch was seen in 70% of patients; most of the missed cases occurred early in our experience, when the 2D axial images were not placed sufficiently inferior to include the arch. No stenosis of the great vessel origins was seen in this study. All four stenoses of the intracranial internal carotid artery identified with intraarterial angiography were seen with the MOTSA MR angiogram but with a tendency to overestimate stenosis. Only one carotid siphon was thought to show severe stenosis on the MR angiogram. Intraarterial angiography showed a 50% stenosis., Conclusion: It is possible to image the entire carotid artery from the aortic arch through the circle of Willis with MR angiography in a clinically acceptable time. MPR of the 3D TOF MR angiogram reliably shows the percentage of carotid stenosis with no statistically significant difference compared with intraarterial angiography. The role of MR angiography in showing lesions in the circle of Willis or the aortic arch is promising, but the limited number of tandem lesions in this study makes it difficult to draw any conclusions.

Purpose: To assess perflubron for magnetic resonance (MR) imaging in the abdomen and pelvis in a multicenter trial., Materials and Methods: MR images were obtained in 127 subjects before and after ingestion of perflubron with T1-, proton-density-, and T2-weighted sequences at 0.38, 1.0, or 1.5 T. Postcontrast images were compared with baseline images, and percentage of additional bowel darkened, distinction of bowel from adjacent tissue, and change in image artifact were graded., Results: Perflubron increased the bowel darkening in over 92% of subjects with all sequences and field strengths. It improved definition of the left lobe of the liver and body and tail of the pancreas in 67%, 29%, and 42% of subjects, respectively, and of the uterus and bladder in 80% and 76%. Abnormal tissue was more conspicuous in 69% of subjects. Highest scores were achieved when the upper abdomen was imaged 5-30 minutes and the pelvis 10-40 minutes after ingestion. No image artifacts or side effects were attributed to perflubron., Conclusion: Perflubron is safe, and its efficacy was unaffected by pulse sequences, magnetic field strength, or time delay.

Because simultaneous noninvasive noncontrast imaging of cervical and cerebral vasculature and brain is possible with magnetic resonance angiography (MRA) and imaging (MRI), the following study was undertaken from July 1990 to January 1992. One hundred twenty-eight patients were examined with General Electric 1.5 Tesla MRI systems. Axially acquired volumetric three-dimensional time-of-flight MRA with 0.7 mm3 voxel size with regional maximum intensity projection after processing followed a two-dimensional time-of-flight localizing sequence. These two MRA sequences combined with spin-echo parenchymal brain MRI were compared with duplex scans, contrast angiograms, and surgical findings. Blinded readings by a radiologist and vascular surgeon allowed comparison of grades of luminal diameter narrowing (normal, mild, moderate, severe, and occluded) seen on MRA to be compared with those of Doppler and contrast angiography. Excluding 12 nondiagnostically imaged internal carotid arteries (10 MRA) and limiting duplex correlation to within 5 days of the MRA examination allowed critical appraisal of 182 internal carotid arteries. Exact correlation of grade of stenosis was obtained by the radiologist in 136 (74.7%) of 182 arteries and the surgeon in 138 (75.8%) of 182 arteries. Spearman rank correlation analysis found rank correlation coefficients of 0.88 (p < 0.001) and 0.83 (p < 0.001), respectively, for the radiologist and vascular surgeon. Disagreement one category apart was found by the radiologist in 35 studies (19.3%) and the surgeon in 28 studies (15.4%). Two or more grades of disagreement were found by the radiologist in 11 studies (6%) and the surgeon in 16 studies (8.8%). Contrast angiogram-MRA agreement was found in 86% of 36 internal carotid arteries. The degree of stenosis detected by MRA was concordant with surgical findings in 39 of 40 patients. Thus MRA emerges as a useful and accurate method of obtaining cerebrovascular evaluation in clinical practice.

Both time-of-flight and phase-contrast techniques are currently being employed in producing angiographic MR images of arteries and veins. Both two-dimensional and three-dimensional acquisition methods can be used to display projective vascular images with the maximum intensity projection algorithm, eliminating the need for image subtraction. MR angiography has expanded the armamentarium of MR imaging, especially in the head and neck, and should in some organ systems reduce the need for conventional x-ray catheter angiography.

A retrospective analysis of intracranial magnetic resonance (MR scans of 514 patients who underwent nonselective Gd-DTPA enhanced MR imaging was performed to determine the efficacy of this protocol for Gd-DTPA administration. This report reviews the frequency and clinical significance of abnormally enhancing areas that were entirely undetectable on precontrast images or would have been missed without the retrospective knowledge of enhancement. Fifty-seven patients (11% of the 514 patients studied) showed enhancing lesions, with 16 (3.1% of the total) of these patients demonstrating one or more lesions identifiable only on postcontrast images. Of those 16 patients, 8 had other focal abnormalities on precontrast studies, whereas the remaining 8 (1.6% of the total) had negative precontrast studies. The new diagnoses affected clinical management directly in five patients and in another nine contributed potentially significant information. Considerations regarding a selective versus nonselective protocol for Gd-DTPA administration for intracranial MR imaging and the use of clinical information to augment these protocols are discussed.

The reduced T2 (spin-spin) relaxation times (T2obs less than 200 ms) measured on pure fluids on our 0.35T magnetic resonance imagers stimulated an investigation into this phenomenon. The cause for the short T2obs of fluids was found to be translational molecular self-diffusion of hydrogen nuclei through the pulsed slice-selective magnetic gradient in the imagers. Similar reductions in biological tissue T2obs were also attributed to molecular self-diffusion.

Paramagnetic nitroxyl-containing compounds have been useful as contrast agents in magnetic resonance imaging (MRI) experiments in animals. Preliminary information on the metabolic fate, pharmacokinetic behavior, stability in tissues, and chemical reduction of two prototypic nitroxides, PCA and TES, is presented. In the dog TES was eliminated more rapidly than PCA. More than 80 % of the dose of both nitroxides was recovered in urine within 6 hours. Nitroxides were reduced in vivo to their corresponding hydroxylamines. No other metabolite was observed. Measured reducing activity in tissue homogenates was greater in liver or kidney than in brain, lung or heart. In each tissue PCA was more stable than TES. PCA was also more resistant to reduction by ascorbic acid at physiologic pH. These preliminary results favor the use of PCA, a pyrrolidinyl nitroxide, over TES, a piperidinyl nitroxide, for MRI contrast enhancement.

A unique occurrence of gastric carcinoma within a paraesophageal hernia is reported. The radiologist must carefully examine the distorted gastric anatomy to look for a second lesion.

Acute myocardial infarctions were produced in 11 dogs by ligation of the left anterior descending coronary artery. Twenty-four hours after ligation, 0.35 millimoles per kilogram of Gd-DTPA was injected intravenously, followed by cardiectomy either 90 seconds (3 dogs) or 5 minutes (5 dogs) later. The remaining 3 dogs had cardiectomy without injection of Gd-DTPA at 24 hours after coronary occlusion. The 3 dogs that did not receive Gd-DTPA had longer T1 and T2 relaxation times in infarcted myocardium than in normal myocardium, as measured by a 10.7-MHz magnetic resonance (MR) spectrometer. The T1 and T2 relaxation times of normal myocardium at 90 seconds postinjection of Gd-DTPA were significantly shorter (p less than 0.05) than those of the normal myocardium of animals that did not receive Gd-DTPA. At five minutes postinjection, significantly (p less than 0.01) greater T1 shortening was exhibited in the infarcted myocardium compared with adjacent normal myocardium in the dogs injected with Gd-DTPA. Thus, Gd-DTPA has differential and time-varying effects on relaxation times of normal and infarcted myocardium.

Complexes of paramagnetic ions that are tissue-, organ- or tumor-specific will supplement routine magnetic resonance imaging, help assess organ perfusion, and in some cases assess specific organ function. Studies are described in animals and man and the results suggest that dilute iron solutions may be useful for contrast-enhancement of the gastrointestinal tract; that ferrioxamine B, a stable ferric iron complex, appears to permit identification of focal blood-brain-barrier defects and to assess renal excretory function; and that gadolinium-DTPA can produce contrast-enhancement of a variety of lesions. In addition, gadolinium-DTPA can detect a breakdown in the blood-brain-barrier and can delineate functioning myocardium in the setting of acute ischemia.

Magnetic resonance (MR) imaging was performed in 10 children with intrathoracic disease. Entities imaged included mediastinal lymphoma and neuroblastoma, pulmonary metastases from hepatoblastoma and osteogenic sarcoma, hemorrhagic pleural effusion, and nonmalignant disease such as abscess, cystic fibrosis, and bronchogenic cyst. MR imaging was particularly valuable in delineating mediastinal and parenchymal masses from adjacent vascular structures without the need for contrast material enhancement. However, MR was insensitive to lesion calcification. Since pulmonary vessels give very low signal, differentiation of high-intensity pulmonary nodules from vessels is readily apparent. For similar reasons, MR was excellent in distinguishing endobronchial mucus plugs from peripheral pulmonary vessels.

The effects of an intravenously administered nitroxyl spin label (PCA) on the magnetic resonance imaging (MRI) appearance and relaxation times of acute canine myocardial infarctions were studied. Twenty-four hours after ligation of the left anterior descending coronary artery (LAD), animals were either sacrificed immediately (three dogs) or injected with 3.0 mmol/kg of PCA prior to sacrifice (six dogs). The PCA group dogs were sacrificed at either 5 minutes postinjection (three dogs) or 15 minutes postinjection (three dogs). Magnetic resonance imaging (0.35 T) using spin-echo techniques demonstrated high signal intensity in the infarct relative to normal myocardium in all three groups. In the control group, the T1 and T2 relaxation times were longer in infarcted compared with normal myocardium, but only the measure in T2 reached statistical significance (P less than .05). PCA produced infarct-avid T1 shortening in the six dogs that received it. Contrast in the group sacrificed at 15 minutes postcontrast administration was greater than that in the control group due to T1 shortening in the infarct. Thus, PCA produces differential effects on normal and infarcted myocardium. Between 5 and 15 minutes after IV administration, it causes greater changes in the infarct due to prolonged retention in this region.

The capability of magnetic resonance (MR) imaging for detecting aortic, iliac, and femoral stenoses and occlusions was evaluated. Multisection spin-echo studies at 0.35 tesla were obtained of the infrarenal aorta to the femoral bifurcation in 24 patients, all of whom had undergone intraarterial angiography within 14 days of imaging. Transaxial MR images were compared with the angiograms. Arterial stenoses and occlusions in these vessels detected by MR imaging correlated with angiographic findings in 91% of the instances. Protrusional atherosclerotic plaques and occlusions and stenoses in the aortoiliac region were demonstrated accurately on MR images; complications of previous vascular surgery, such as aneurysms at sites of previous anastomoses or endarterectomy, were also identified. Due to the limited spatial resolution, MR images failed to demonstrate some femoral stenoses. MR imaging may be used for evaluation of aortoiliac vascular disease and for follow-up study after surgical revascularization. However, the limited spatial resolution, noncomposite display of the aortoiliofemoral circulation, and lack of evaluation of peripheral runoff provided by current MR imaging techniques militate against its replacing angiography prior to vascular intervention.

Gadolinium (Gd)-DTPA complex was assessed as a nuclear magnetic resonance (NMR) contrast-enhancing agent by experimentally imaging normal and diseased animals. After intravenous injection, Gd-DTPA, a strongly paramagnetic complex by virtue of unpaired electrons, was rapidly excreted into the urine of rats, producing an easily observable contrast enhancement on NMR images in kidney parenchyma and urine. Spin-echo intensity of urine within the renal pelvis increased from 2263 to 4414 units; intensity of renal parenchyma increased from 2901 to 3893 after administration of 0.1 mmol/kg Gd-DTPA. Sterile soft-tissue abscesses demonstrated an obvious rim pattern of enhancement. A focus of radiation-induced brain damage in a canine model was only faintly detectable on spin-echo NMR images before contrast administration; after 0.5 mmol/kg Gd-DTPA administration, the lesion intensity increased from 3867 to 5590. In comparison, the normal brain with an intact blood-brain barrier remained unchanged in NMR characterization. Gd-DTPA is a promising new NMR contrast enhancer for the clinical assessment of renal function, of inflammatory lesions, and of focal disruption of the blood-brain barrier.

A piperidinyl nitroxide stable free radical derivative, TES, was tested as an NMR contrast enhancer of renal structures in normal animals and animals with experimentally induced unilateral renal ischemia, renal vascular congestion, and hydronephrosis. Physiologic measurements indicated that TES is rapidly excreted in the urine with a clearance rate equal to the glomerular filtration rate. Because the compound is strongly paramagnetic, it increases the observable NMR intensity within the kidneys and urine in relatively low doses (0.04 to 0.9 g/kg). TES-enhanced spin echo renal images clearly demonstrated the presence of disease and functional abnormalities in diseased kidneys. These abnormalities were either not evident or only indirectly suggested on nonenhanced NMR images.

Many MR artifacts are not understandable in terms of previous imaging experience. A variety of artifacts is presented here; their origins and elimination are discussed.

Contrast-enhancing agents for demonstrating abnormalities of the blood-brain barrier may extend the diagnostic utility of proton nuclear magnetic resonance (NMR) imaging. "TES," a nitroxide stable free radical derivative, was tested as a central nervous system contrast enhancer in dogs with experimentally induced unilateral cerebritis or radiation cerebral damage. After intravenous injection of TES, the normal brain showed no change in NMR appearance, but areas of disease demonstrated a dramatic increase (up to 45%) in spin-echo intensity and a decrease in T1 relaxation times. The areas of disease defined by TES enhancement were either not evident on the nonenhanced NMR images or were better defined after contrast administration. In-depth tests of toxicity, stability, and metabolism of this promising NMR contrast agent are now in progress.

Imaging by nuclear magnetic resonance (NMR) techniques has been shown to provide high-contrast resolution between soft tissues and characterization of normal and pathologic tissues by differences in magnetic relaxation times. The current study was designed to determine whether electrocardiogram (ECG)-gated NMR imaging of the canine heart in vivo could distinguish normal from infarcted myocardium without the use of intravenous paramagnetic contrast agents. Seven dogs were studied by ECG-gated NMR imaging in vivo (spin-echo technique) with a 0.35 Tesla superconducting magnet at 2 to 7 days after ligation of the left anterior descending coronary artery. In six of the seven dogs, signal intensity was increased in the anterior wall compared with the remainder of the left ventricle; this region of high signal intensity corresponded to the area of myocardial infarction demonstrated at postmortem examination. The signal intensity of the infarcted region was 66 +/- 27% greater than that of normal myocardium (p less than .01). The T2 (spin-spin) relaxation time was 69 +/- 3% longer in the infarcted myocardium as compared with normal myocardium (p less than .01). The NMR images from the seventh dog had uniform signal intensity throughout the myocardium of the left ventricle. An infarct was not evident on postmortem examination in this dog. Thus gated NMR imaging in vivo by the spin-echo technique displays acute myocardial infarctions as regions of high signal intensity without the use of contrast media. The infarct is characterized by a prolonged T2 relaxation time.

Nuclear magnetic resonance imaging of the hydrogen nucleus provides a unique noninvasive display of proton dynamics in biologic tissues and fluids as well as internal anatomy in a sectional imaging format. No ionizing radiation is utilized. Our experience with NMR imaging of the brain in 14 pediatric patients is presented and compared with computed tomography. The major advantages of NMR over CT include its greater sensitivity to blood flow, edema, hemorrhage, and myelinization and its lack of beam-hardening artifacts. In addition, the potential for tissue characterization exists by determination of T1 and T2 relaxation times and of mobile proton density. Disadvantages of NMR over CT include its failure to demonstrate calcification and bone detail and longer data acquisition times.

Superficial- and deep-tissue heating was measured in five dogs during high-specific-absorption-rate radiofrequency (RF) irradiation to see whether significant temperature changes could be produced by a 1.5-T clinical magnetic resonance imager. The RF power output employed was 6.3 times that required for routine imaging. Temperature probes were placed in both deep and superficial tissues, and temperatures were recorded before, during, and after exposure. In each dog, there was a linear temperature increase of several degrees during RF exposure; the maximal average change was 4.6 degrees C in the urinary bladder. The temperature increase was slightly greater in deep tissues than in superficial tissues. The calculated specific absorption rate, based on the temperature change, averaged 7.9 W/kg for all five dogs. These findings argue for continued caution in the design and operation of imagers capable of high specific absorption rates, particularly when they are used for imaging infants or patients with altered thermoregulatory capability.

A case of Horner syndrome occurring secondary to the high insertion of a chest tube is reported. Horner syndrome from this cause can easily be avoided. The tip of the chest tube should be kept at or below the third posterior rib unless the clinical situation dictates otherwise.

We examined the magnetic resonance properties of 12 paramagnetic piperidinyl nitroxyls in water and plasma solutions. Paramagnetic contributions to proton relaxation times were measured using 10.7 and 100 MHz spectrometers. Proton relaxation enhancement from nitroxyls increased with ascending molecular weight, in plasma solutions versus equimolar aqueous solutions, and with measurements at 10.7 MHz compared to 100 MHz. Relaxation rates were observed to approximately double at 10.7 MHz compared to 100 MHz and from water to plasma solutions. The data indicate that proton spin-lattice relaxation enhancement is magnetic field-dependent, and increases using nitroxyls of large molecular weight and with chemical substitutents that increase the microviscosity of solvent water molecules. The development of nitroxyls for diagnostic MRI will be aided by understanding these in vitro physical characteristics and trends.

Two dilute oral iron solutions, made from commonly available nonprescription dietary supplements, were found to enhance the gastrointestinal tract in nuclear magnetic resonance imaging of live rats and one human volunteer. The paramagnetic and pharmacologic properties of ferric ammonium citrate were more favorable than those of ferrous sulfate heptahydrate. The paramagnetic iron solutions shorten T1 and T2 relaxation times of water protons in the contrast media-filled gastrointestinal tract, producing easily observable change in NMR intensity. Because these iron solutions are available commercially and are known to be well tolerated, the clinical use of iron-containing NMR contrast agents for the gastrointestinal tract is feasible.

A case of anomalous inferior vena cava with azygos continuation first diagnosed by computed tomography (CT) and later confirmed by venography is presented. In addition to identifying the dilated azygos vein in the chest, CT can determine the direction of flow in the vein by means of a bolus contrast medium injection. Once this is established, a search for the etiology of the dilated vein can be directed to the appropriate anatomic area.

The origin of nuclear magnetic resonance signal is reminded in this paper. Different ways of increasing the contrast in magnetic resonance imaging are presented, especially modifications of tissues relaxation times by the use of paramagnetic ions or nitroxides. The potential of these substances is illustrated by several examples of application in the animal.

Tissue deposits of hemosiderin, a paramagnetic iron-protein complex, resulted in marked abnormalities of magnetic resonance (MR) spin-echo signal intensity within the viscera of three children with transfusional hemosiderosis and thalassemia major. In all patients the liver and bone marrow demonstrated abnormally low spin-echo intensities and the kidneys and muscles had abnormally high intensities. These observations correlate with in vitro MR observations of ferric (Fe+3) solutions, in which concentrations of ferric salts greater than 20 mmol yielded a low MR intensity signal and ferric concentrations less than 15 mmol yielded higher intensities than did water alone. MR imaging is sensitive to the tissue deposition of hemosiderin, and MR intensity appears to provide a rough measure of the amount of iron deposited.

A clinical dilemma in patients with cystic fibrosis is the determination of the nature of linear areas of decreased aeration in the lungs. It is difficult using chest roentgenograms or even computed tomography to differentiate atelectasis, mucoid impacted bronchi, or peribronchial inflammatory disease from normal pulmonary vascularity. Magnetic resonance imaging is a noninvasive sensitive means that provides the distinction. Pulmonary vessels are easily identified, because with the spin-echo sequence that we use, the rapidly flowing blood within the vessels has no signal intensity. In contradistinction, mucoid-impacted bronchi appear as high-intensity linear branching structures. Peribronchial inflammatory disease appears as curvilinear areas of high intensity, representing inflammatory edema, around central lucencies representing bronchi.

Chelation of the rare-earth element gadolinium (Gd) with diethylenetriaminepentaacetic acid (DTPA) results in a strongly paramagnetic, stable complex that is well tolerated in animals. The strongly paramagnetic gadolinium complex reduces hydrogen-proton relaxation times even in low concentrations (less than 0.01 mmol/L). The pharmacokinetic behavior of intravenously delivered Gd-DTPA is similar to the well known iodinated contrast agents used in urography and angiography; excretion is predominantly through the kidneys with greater than 90% recovery in 24 hr. The intravenous LD50 of the meglumine salt of Gd-DTPA is 10 mmol/kg for the rat; in vivo there is no evidence of dissociation of the gadolinium ion from the DTPA ligand. The combination of strong proton relaxation, in-vivo stability, rapid urinary excretion, and high tolerance favors the further development and the potential clinical application of gadolinium-DTPA as a contrast enhancer in magnetic resonance imaging.

By varying slice-selective gradients in successive data acquisitions, the first in vitro measurements of molecular self-diffusion coefficients were performed in a magnetic resonance imager at 0.35 Tesla. Reasonably accurate measurements were found by the MRI method in comparison with 2.3 T NMR spectrometer measurements on the same samples, and in comparison to reported literature values. Thus, in addition to T1, T2, mobile proton density, flow velocity, magnetic susceptibility, and chemical shift, molecular self-diffusion coefficients are now added to the list of biophysical parameters measurable by magnetic resonance imaging in the noninvasive characterization of biological systems.

Nitroxyl spin labels have been shown to be effective in vivo contrast agents for magnetic resonance imaging (MRI) of the central nervous system, myocardium, and urinary tract. A new pyrrolidine nitroxyl contrast agent (PCA) with better resistance to in vivo metabolic inactivation than previously tested agents was studied for its potential to enhance subcutaneous neoplasms in an animal model. Twenty-two contrast enhancement trials were performed on a total of 15 animals 4-6 weeks after implantation with human renal adenocarcinoma. Spin echo imaging was performed using a .35 T animal imager before and after intravenous administration of PCA in doses ranging from 0.5 to 3mM/kg. The intensity of tumor tissue in the images increased an average of 35% in animals receiving a dose of 3 mM/kg. The average enhancement with smaller doses was proportionately less. Tumor intensity reached a maximum within 15 min of injection. The average intensity difference between tumor and adjacent skeletal muscle more than doubled following administration of 3 mM/kg of PCA. Well-perfused tumor tissue was more intensely enhanced than adjacent poorly perfused and necrotic tissue.

Magnetic resonance imaging of the cardiovascular system offers great promise in the detection and characterization of the anatomic, physiologic, and biochemical consequences of atherosclerosis. This review will focus on the potential applications of MRI for evaluating atherosclerosis of the abdominal aorta and iliofemoral vessels.

Delineation of the gastrointestinal tract in magnetic resonance imaging (MRI) remains a problem. Ferric ammonium citrate is paramagnetic, producing a high MRI signal intensity by virtue of its spin-lattice (T1) relaxation rate enhancement properties. Water is diamagnetic, producing a low MRI signal intensity, especially with short TR and TE times. To compare efficacy for gastrointestinal contrast alteration, ferric ammonium citrate was administered to 18 patients and water was given to 10 patients. Spin-echo imaging at 0.35T was performed after administration of these agents. Ferric ammonium citrate produced high signal intensity within the esophagus, stomach, duodenum, and small intestine that aided in the differentiation of the gastrointestinal tract from adjacent tumors, vessels, and viscera. Delineation of the gut wall was superior using ferric ammonium citrate compared to that produced by water. Delineation of the margins of the pancreas, liver, and kidney from adjacent gastrointestinal tract was also better with ferric ammonium citrate. Optimal distinction between bowel and fat was better with water. Longer TE times (75 to 200 ms) may allow improved contrast between gut and intrabdominal fat using ferric ammonium citrate.