MRI Web Clinic — May 2009

Incidentaloma
David S. Levey, M.D.

Clinical History: A 64 year-old male complains of back pain extending into the left leg. (1a) A left lateral T2-weighted sagittal and (1b) and T1-weighted axial image at the level of the L4-5 disc are provided. What are the findings? What is the diagnosis?

1a
1b
Figure 1:

(1a) A left lateral T2-weighted sagittal and (1b) and T1-weighted axial image at the level of the L4-5 disc.

Findings

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Figure 2:

The left of midline sagittal T2-weighted image reveals lobulated soft tissue signal intensity masses (arrows) in the retroperitoneal region.

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Figure 3:

On the T1-weighted axial view, abnormal soft tissue (arrow) is redemonstrated to the left of the iliac arteries, anterior to the psoas muscle (asterisk).

Diagnosis

Incidental finding of extensive retroperitoneal lymphadenopathy. A subsequent biopsy indicated metastatic colon carcinoma.

Introduction

An incidental finding, often referred to as an “incidentaloma” both in the literature and in the daily lexicon of radiologists, is a finding found by coincidence while examining a patient for another clinical symptom or suspicion1. A more magnetic resonance imaging (MR) specific definition defines an incidentaloma as a disease or physical condition found as a secondary by-product of capturing the necessary volume of tissue within the field of view of the MR examination.

The nature of incidental imaging findings varies greatly in terms of size, location and clinical import on the patient. An incidental finding may have no clinical impact on the patient whatsoever, or as in the current case, may be life-threatening. These findings, such as an incidental adrenal mass on a lumbar MRI (4a,5a), may need further work up if size or other criteria are met2. Other medically significant findings may need to be documented by the radiologist, but immediate action may not be taken, such as is the case with incidental identification of asymptomatic gallstones on a lumbar spine MRI (6a,7a).

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Figure 4:

A T1-weighted coronal scout examination from a lumbar spine MRI reveals a heterogeneous mass superior to the right kidney, in the expected location of the adrenal gland.

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Figure 5:

A T2-weighted axial image confirms the large adrenal mass. In light of the areas of fat signal intensity within the lesion, it is likely benign (such as a myelolipoma), an assumption that could be confirmed with dedicated adrenal MR, including the use of out of phase gradient echo images.

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Figure 6:

A proton density-weighted axial image from a patient undergoing lumbar MR for back pain demonstrates a large gallstone (arrow).

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Figure 7:

Numerous additional smaller, dependent gallstones (arrows) are seen on a more inferior T2-weighted axial image. The non-dilated pancreatic bile duct (arrowhead) is also visible.

Case Examples

Incidentalomas may be seen on virtually any type of orthopaedic or spine MR examination and may involve any osseous or soft-tissue structure within the field of view. They can be quite subtle or extremely large. When at the periphery of the field of view, or when isointense to normal structures, they can be extremely challenging to identify, particularly in a high volume practice. The following cases are presented to demonstrate incidental findings that have been seen in our busy, MR focused practice.

 

8a
8b
Figure 8:

A 50 year old man fell and presented with the history of buttock and hip pain. (8a,8b) Axial fat-suppressed proton density-weighted images from a subsequently obtained hip MR reveal an enlarged, heterogeneous prostate gland. An anteroinferiorly projecting mass extends beyond the capsule ( arrowheads, 8a), with probable invasion of the bladder wall. On 8b, the right seminal vesicle appears dilated (arrows), likely related to invasion from this probable prostate carcinoma.

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Figure 9:

A hip and pelvic MR were performed on a 66 year old female with a history of breast cancer and pain in sacrum and right hip. The patient's pain is explained by the edema and linearity (arrowheads) within the right sacral ala on (9a, left) T1 and (9a, right) fat-suppressed T2-weighted coronal images, compatible with an insufficiency fracture. A 2.2 cm low signal intensity uterine leiomyoma (arrows) is incidentally noted, with mild mass effect on the superior bladder.

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Figure 10:

Lumbar spine MR performed on a 25 year old male complaining of increasing back pain for 7-8 months. The (10a) T2-weighted sagittal image left of midline reveals a massive, lobulated, fluid signal paraspinal lesion (arrowheads).

 

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Figure 11:

On the (11a) T2-weighted axial image, renal parenchyma from the right kidney courses towards the midline (arrows). The fluid signal intensity lesion (arrowheads) is again seen, and is bordered by a thin rim of renal parenchyma. Overall, the appearance is typical for a horseshoe kidney. Patients with this congenital abnormality are prone to renal obstruction, which obviously has occurred within the left collecting system in this patient.

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Figure 12:

(12a) Multiple T2-weighted sagittal images from a 45 year old woman who presented with neck pain and a history of "rule out herniated disc". A bilobed soft tissue mass (arrows) is noted within the left side of the prepontine cistern, extending inferior to the left temporal lobe within the middle cranial fossa. The lesion is relatively subtle, as it is isointense to normal brain. Subsequent contrast enhanced cranial MR confirmed the abnormality, and the patient underwent craniotomy and tumor resection, yielding a benign schwannoma.

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Figure 13:

A 64 year-old male underwent left knee MR for persistent pain. On the (13a) T1-weighted sagittal view, increased signal intensity is evident within the popliteal artery (arrowheads). Though prominent, a similar appearance may be seen normally due to flow artifact.

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Figure 14:

A corresponding (14a) fat-suppressed proton density-weighted coronal image, however, also reveals high signal intensity within the popliteal artery (arrowheads). Surrounding soft-tissue edema is present, and at this point, the overall appearance is not typical for artifact. A subsequent arterial duplex exam confirmed the diagnosis of popliteal artery thrombosis.

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Figure 15:

A 44 year-old male presents with "pain and limited range of motion in the right shoulder". Rounded soft tissue signal intensity lesions (arrows) are evident in the right axillary region on a (15a) oblique T1-weighted coronal image.

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Figure 16:

The extent of the abnormality (arrows) is better seen on the corresponding (16a) T2-weighted sagittal image. Subsequent biopsy revealed non-Hodgkin lymphoma.

Discussion

The frequency, site and multiplicity of incidentalomas have been studied in many settings, with one of the most extensive studies performed at the University of California San Diego on 1192 consecutive patients who underwent whole body electron beam computed tomography (CT) screening3. Spinal lesions, followed by vascular, lung, kidney and hepatic incidental findings were most common, and 86% of patients had at least one abnormality on their whole body CT. In musculoskeletal MR imaging, the majority of incidental finding are related to the osseous structures, both spinal and peripheral, as hemangiomata of the vertebral bodies, enostoses (bone islands) and cysts dominate the landscape. The preponderance of spine and sacrum, pelvic and hip exams, however, allow inclusion of a large volume of urologic (kidney, adrenal, bladder, prostate) and female gynecologic (uterus, cervix, vulvar, adnexal) tissues in the field of view, so abnormalities in these regions are commonly encountered as well.

Incidentalomas may be encountered in the form of a developmental variation, such as in the case of the horseshoe kidney (10a,11a) , where a perturbation of fetal development lends itself to variant anatomy. An incidentally recognized finding may not be part of the main study, but rather seen only on a rudimentary scout examination (4a), used for slice selection and centering of the body part to examined, and typically not well optimized for either spatial or contrast resolution. Incidental findings can be obscured by motion, pulsation and magnetic susceptibility artifact, and image distortion can occur in relation to technical factors, such as magnetic field strength and field of view constraints, which may be particularly severe at the periphery of a given image.

Rarely, an incidental finding may harbor its own incidental finding (17a,18a), as in the following case of patient with an unsuspected pregnancy found to be in breach and harboring a nuchal cord. This finding was important to verify and record, as fetuses in breech position and with a nuchal cord are more likely to be delivered via caesarean section4.

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Figure 17:

A 17 year-old female presents after a fall on the sacrum during a basketball game. An intrauterine pregnancy (arrows, 17a) is discovered on the sacral MR, and is in the breech position.

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Figure 18:

(18a) Sequential sagittal images reveal the umbilical cord (arrowheads), seen as two en face, punctate flow voids (arrowheads), encircling the fetal neck.

Conclusion

Incidental findings are an important subset of abnormalities that may be found on any MR imaging study, be they central and obvious, or tiny and peripheral. Proper knowledge of this issue sets the tone for a careful search pattern. Though some findings are of minor significance, others have great medical import, and recognition of the abnormality in such cases can have a dramatic impact on patient care.

References

1 Westbrook JI, Braithwaite J, McIntosh JH. The outcomes for patients with incidental lesions: serendipitous or iatrogenic? AJR 1998; 171:1193-1196.

2 Mitchell IC, Nwariaku FE. The Oncologist 2007; Vol. 12, No. 2: 168-174.

3 Furtado CD, Aguirre DA, Sirlin CB, et al. Whole-body CT screening: spectrum of findings and recommendations in 1192 patients. Radiology 2005 ; 237 (2): 385-94.

4 Ogueh O, Al-Tarkait A, Vallerand D, Rouah F, Morin L, Benjamin A, Usher R. Obstetrical factors related to nuchal cord. Acta Obstetricia et Gynecologica Scandinavica 2006; Volume 85, Number 7: 810-814.

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