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Neuroradiology Case of the Week
Case 367
December 2008
Rajiv Mangla, MD and Per-Lennart Westesson, MD, PhD, DDS
Clinical
Presentation: Aim: To discuss the conventional and advanced imaging in differentiation of various etiologies of brain abscesses.
Imaging Findings:
Patient 1: A 58-year-old female presented with facial droop. T1 post-contrast images reveal an Irregular thick rim enhancing lesion in right frontal lobe (Fig. 1). Diffusion images show significant restriction in the cystic portion of the lesion (Fig. 2). MR spectroscopy with TE of 35 and 144 (Figs. 3A & B) shows peaks suggestive of acetate at 1.96, the alanine at 1.47, lipid lactate at 1.3, and amino acids at 0.9 show inversion on 144 TE. Perfusion image shows mild increase in rCBV (relative cerebral blood volume) in the wall, otherwise the lesion shows predominantly low rCBV (Fig. 4).
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| Figure 1: Post-gadolinium T1 axial image. |
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Figure 2: DWI image shows restricted diffusion in the lesion.
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| Figure 3A&B: In vivo proton MR spectra (A) (1500/35) shows signal resonance of AA at 0.90 ppm, Lac at 1.33 ppm, and Ac at 1.92 ppm. Spin-echo MR spectra (B) (2000/144) for resonance of AA, Lac, and alanine (1.47 ppm) signals shows phase reversal that is suggestive of J coupling.
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Figure 4: rCBV map shows predominantly low perfusion in the wall but there are few areas of increased perfusion also seen.
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Patient 2: A 24-year-old American-Asian male with history of lung tuberculosis, now presents with headache and vertigo. T1 post-contrast images show a multilobulated contrast enhancing lesion in pons (Fig. 5). No restriction of diffusion is seen on DWI (Fig. 6) MR spectroscopy with TE of 35 (Fig. 7A) and 144 (Fig. 7B) shows only a prominent lipid peak . No evidence of amino acid, lactate or acetate peak is noted. Small NAA and choline peaks are seen, most likely due to smearing from normal parenchyma. Small focal areas of increase rCBV are noted on the PWI (Fig. 8).
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| Figure 5: Post-gadolinium T1 axial image shows a multilobulated enhancing lesion in the pons. |
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Figure 6: DWI image shows no restriction of diffusion in the lesion.
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| Figure 7A&B: In vivo proton MR spectra (A) (1500/35) shows signal resonance of lipid at 1.33 ppm. Spin-echo MR spectra (B) (2000/144) shows no phase reversal suggesting absence of lactate. No AA, Ac, and alanine are visualized.
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Figure 8: rCBV map shows few areas of increased perfusion in the wall of the lesion while the core of the lesion shows low perfusion.
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Patient 3: A 61-year-old diabetic male presents with diabetic ketoacidosis and right-sided weakness. T1 pre-contrast image shows mild hemorrhage within the lesions (Fig. 9A). T1 post-contrast image shows only mild incomplete peripheral enhancement (Fig. 9B). DWI images show restriction only in the periphery of the lesion (Fig. 10). MR spectroscopy shows a lactate peak at 1.3 and a very small amino acid peak (Figs. 11A & B). The rCBV of the lesion is noted to be lower than normal brain parenchyma (Fig. 12).
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Figure 9A & B: Pre-contrast (A) T1 image shows hyperintensity suggestive of methemoglobin. Postcodes (B) image shows only mild enhancement in the rim.
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Figure 10: DWI image shows heterogeneous areas of diffusion in the lesion with wall showing more restriction of the diffusion.
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| Figure 11A&B: In vivo proton MR spectra (A) (1500/35) shows signal resonance of AA at 0.90 ppm, Lac at 1.33 ppm. Spin-echo MR spectra (B) (2000/144) for resonance of AA and Lac signals shows phase reversal that is suggestive of J coupling.
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Figure 12: rCBV map showing predominantly low perfusion.
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Diagnosis: Brain Abscesses:
Patient 1: Pyogenic Brain Abscess.
Patient 2: Tuberculomas of the Brainstem
Patient 3: Fungal Abscess
Discussion: It is difficult to differentiate various types of abscesses on conventional imaging, but there can be some clues. Intracavitary projections without contrast enhancement are found more in fungal abscess. Overall enhancement is less in fungal abscess. MR spectroscopy and diffusion have been found to be effective in differentiating between tubercular fungal or pyogenic abscesses [1]). Pyogenic abscesses are typically found to have amino acid, acetate and succinate peaks. Tubercular abscesses have a predominant lipid peak with absence of amino acid, acetate and succinate peaks. Fungal abscess can have amino acid peak like pyogenic abscesses but usually do not have acetate or succinate peak. Additional trehalose peaks between 3.6 and 3.8 has been reported in the literature [2]. There is also a possibility of differentiating anaerobic from aerobic or sterile brain abscesses on the basis of metabolite pattern observed on MR spectroscopy [3]. Acetate and succinate have been found to be more common in abscess with obligate anaerobes. Also, anaerobe abscesses have been found to have lower latate/amino acid ratio as compared to other abscesses.
Significant restriction of diffusion has been found in the cavity of pyogenic abscess as seen in our case (Patient 1) [4]. Tubercular abscess has also been found to have low ADC [5]. The appearance of the abscess has been found to be dependent on the stage of the abscess. The heterogeneity on diffusion-weighted images with areas of increased and decreased diffusion has been found in cerebritis and early capsular stage of fungal abscess [6]. In addition, even in the mature stage, restricted diffusion is more common in the wall and projection of the fungal abscess while the core of the abscess might not show restricted diffusion. On the other hand, in pyogenic and tubercular abscesses the wall as well the cavity show restricted diffusion. High ADC in abscesses can be seen after surgical drainage of pus and or medication. This may be the cause of high ADC found in our case of tubercular abscess (Patient 2), since the patient was already taking anti-tubercular medication for his lung tuberculosis.
The relative cerebral blood volume (rCBV) in the wall of abscesses has been found to be low and has been found to be useful in differentiation of pyogenic brain abscess from cystic brain tumors, making it a strong additional imaging modality in the early diagnosis of these two entities. Tubercular abscess can have high rCBV in the periphery and can mimic high grade gliomas on perfusion imaging [7]. Fungal abscesses have been found to have low perfusion, as is also seen in our case (Patient 3).
In conclusion, advanced MR imaging in addition to conventional imaging is not only useful in differentiating brain abscesses from other tumors of the brain, but also can be helpful in predicting the pyogenic, fungal or tubercular etiology of the abscesses.
References:
- Luthra G, Parihar A, Nath K, et al. Comparative evaluation of fungal, tubercular, and pyogenic brain abscesses with conventional and diffusion MR imaging and proton MR spectroscopy. AJNR Am J Neuroradiol. 2007 Aug;28(7):1332-8. [PubMed]
- Himmelreich U, Dzendrowskyj TE, Allen C, et al. Cryptococcomas distinguished from gliomas with MR spectroscopy: an experimental rat and cell culture study. Radiology. 2001 Jul;220(1):122-8. [PubMed]
- Garg M, Gupta RK, Husain M, et al. Brain abscesses: etiologic categorization with in vivo proton MR spectroscopy. Radiology. 2004 Feb;230(2):519-27. [PubMed]
- Ebisu T, Tanaka C, Umeda M, et al. Discrimination of brain abscess from necrotic or cystic tumors by diffusion-weighted echo planar imaging. Magn Reson Imaging. 1996;14(9):1113-6. [PubMed]
- Gupta RK, Prakash M, Mishra AM, et al. Role of diffusion weighted imaging in differentiation of intracranial tuberculoma and tuberculous abscess from cysticercus granulomas-a report of more than 100 lesions. Eur J Radiol. 2005 Sep;55(3):384-92. [PubMed]
- Gaviani P, Schwartz RB, Hedley-Whyte ET, et al. Diffusion-weighted imaging of fungal cerebral infection. AJNR Am J Neuroradiol. 2005 May;26(5):1115-21. [PubMed]
- Batra A, Tripathi RP. Perfusion magnetic resonance imaging in intracerebral parenchymal tuberculosis: preliminary findings. J Comput Assist Tomogr. 2003 Nov-Dec;27(6):882-8. [PubMed]
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