Hepatic hemangiomas: evaluation by magnetic resonance imaging and technetium-99m red blood cell scintigraphy.

A study was performed to evaluate and compare the sensitivity of magnetic resonance imaging (MRI) and radionuclide blood-pool scanning in the detection of hepatic hemangiomas. All patients had known hemangiomas which were first detected on either ultrasound or computed tomography. Sixteen patients with a total of 23 lesions were investigated. Eleven patients had both MRI and blood-pool scans performed. In the group studied by both modalities, 18 lesions were detected ranging in size from 1 to 11 cm. All lesions were detected by both techniques. However, two of the 18 lesions had an atypical appearance on MRI. Our experience to date indicates that the anatomic location and specific diagnosis of hemangiomas can be made with a high degree of certainty when both MRI and blood-pool scanning techniques are utilized.

Accurate demonstration of hepatic infarction in liver transplant recipients.

Serial HIDA scanning has proven to be a valuable tool in the postoperative management of liver transplant patients. Previous reports have documented its efficacy in detecting biliary leakage, abscess, and rejection. We have also found HIDA scanning to be a sensitive method for detecting early hepatic infarction before ultrasonographic changes occur. Two cases are presented to demonstrate the characteristic findings seen with hepatic infarctions.

Aliasing artifacts in MR imaging.

Aliasing artifacts occur in the phase encoding direction when the dimensions of the imaged object exceeds the field of view. Signal generated from outside the field of view appears as a superimposed object at the opposite edge of the image. Increasing the field of view, changing the gradient axes relative to the patient, or use of surface coils can reduce aliasing and are parameters which are controlled by the radiologist/technologist. The manufacturer may provide software packages which exploit two additional strategies, either limiting the volume of the patient from which the MR signal is acquired as in Inner Volume Imaging or display of resolution unless the number of phase encoding steps is increased at a cost of increased acquisition time. The radiologist may in some clinical situations choose to tolerate aliasing in favor of improved resolution in the area of interest and decreased acquisition time.

Hepatic abscess in liver transplantation. Accurate diagnosis and treatment.

Serial HIDA scanning was performed on a patient following liver transplantation. During the patient's course he developed biliary obstruction that manifested as a photopenic region in the liver, on the HIDA scan which filled in on the delayed views. The patient subsequently developed a region in the superior portion of the right lobe of the liver that did not fill in with activity on delayed views. The patient was experiencing low-grade fevers and was clinically believed to have either an abscess or an episode of rejection. A gallium scan was performed revealing a photopenic defect in the same region as the HIDA. Because of the clinical suspicion of abscess, a percutaneous transhepatic drainage study was performed, revealing a large abscess cavity in the suspect area within the liver. Following the drainage the patient did well. This case illustrates the usefulness of serial HIDA scanning in patients who have received liver transplants. It also is important to note that the gallium scan was negative in this hepatic abscess. In the authors' opinions, the finding of an intrahepatic fluid collection in a septic patient that does not fill with activity on the HIDA study, should be considered the source of the infection, until proven otherwise, and should be drained regardless of the findings of other studies.

Nonsurgical repositioning of central venous catheters.

Long-term central venous catheters are placed for total parenteral nutrition and/or chemotherapy. These catheters are placed surgically and fixed to the subcutaneous tissues. Complications include infection, venous thrombosis, and mechanical problems. The authors developed a method to percutaneously reposition displaced central venous catheters. The procedure is performed with fluoroscopy and modified angiographic techniques. Fifteen patients underwent a total of 17 procedures. The initial success rate was 76%; the final success rate was 88%. Many central venous catheters can be salvaged with this low-morbidity procedure, which negates the need for surgical intervention.

MR of craniopharyngiomas: tumor delineation and characterization.

MR imaging and CT (with and without contrast enhancement) were performed in 20 patients with an established or clinically suspected diagnosis of craniopharyngioma. Fifteen had biopsy-proven craniopharyngioma and five had presumed craniopharyngioma based on clinical and CT findings. In two cases MR was superior to contrast-enhanced CT in demonstrating the tumor. A variable appearance on T1-weighted MR images reflected the pathologic appearance of craniopharyngiomas. High intensity on T1-weighted images corresponded to high cholesterol content or presence of methemoglobin. MR was the preferred method in the evaluation of tumor extent, especially in the cavernous sinus and posterior clival region. CT was superior to MR in detecting the presence of calcification, which with the clinical history correctly suggested the diagnosis of craniopharyngioma. Both MR and CT studies are desired initially to establish the diagnosis and to evaluate tumor extent. MR was the preferred method in detecting the presence of recurrent tumor.

Boundary artifact due to truncation errors in MR imaging.

A boundary artifact in MR images due to truncation of the infinite Fourier series necessary to encode tissue discontinuities was investigated by using doped water phantoms and normal volunteers. All images were obtained on 0.3-T permanent and 0.6-T superconducting MR imagers with varying phase and frequency sampling rates. The artifact appeared in both the phase and frequency encoding direction as parallel lines or ringing adjacent to borders or tissue discontinuities. This was unlike motion artifacts, which occur predominantly in the phase direction, and chemical shift misregistration errors, which are most pronounced in the frequency direction. Increasing the sampling frequency from 128 to 512 resulted in higher frequency ringing and more rapid drop-off in amplitude. Low-pass digital filtering also decreased the ringing at the expense of fine detail. The truncation of the infinite Fourier series necessary to encode edges to the 128-512 terms used for most MR imaging produces the artifact. It is important to recognize this common artifact and not mistake it for patient motion or disease.

Magnetic resonance imaging artifacts: mechanism and clinical significance.

Many types of artifacts may occur in magnetic resonance imaging. These artifacts may be related to extrinsic factors such as patient motion or metallic artifacts; they may be due specifically to the MR system such as power gradient drop off and chemical shift artifacts; they may occur as a consequence of general image processing techniques, as in the case of truncation artifacts and aliasing. Change in patient position, pulse sequence, or other imaging variables may improve some artifacts. Although reduction of some artifacts may require a service engineer, the radiologist has the responsibility to recognize MR imaging problems. The radiologist's knowledge of MR imaging artifacts is important to the continued maintenance of high image quality and is essential if one is to avoid confusing artifactual appearances with pathology.