Proton MR Spectroscopy of Brain Abscesses

Letters to the Editor Letters to the Editor

Published ahead of print on June 25, 2010
doi: 10.3174/ajnr.A2159

American Journal of Neuroradiology 31:E67, September 2010
© 2010 American Society of Neuroradiology

E. Kapsalakia and K.N. Fountasa
aUniversity Hospital of Larissa
Departments of Diagnostic Radiology and Neurosurgery
School of Medicine
University of Thessaly
Larissa, Greece

We read with great interest the article by Pal et al.1 The authors reported on their experience with the application of proton MR (1H-MR) spectroscopy in the imaging evaluation of patients harboring intracranial abscess. In their retrospective study, they examined 194 patient charts and imaging studies, and they concluded that the detection of cytosolic amino acids is a strongindication of an abscess of pyogenic origin, without ruling out nonpyogenic etiology. In addition, they postulated that though the presence of acetate or succinate is highly suggestive of anaerobic bacterial pathogens, there are cases in which facultative anaerobic microbes may be implicated. We congratulate the authors on their well-organized large-scale study and their meticulous spectroscopic protocol. However, there are a few questions raisedby their article.

The role of 1H-MR spectroscopy in establishing the diagnosis of an intracranial abscess and differentiating it from other look-alike ring-enhancing lesions has been adequately explored in the literature.2–4 However, the sensitivity and the specificity of 1H-MR spectroscopy remain to be defined. It would be of great interest if the authors could provide their data regarding the sensitivity and specificity of 1H-MR spectroscopy in establishing the diagnosis of an intracranial abscess in their cohort. They reported in their article that 210 patients with the diagnosis of an intracranial abscess were examined, but they provided no data on how the diagnosis was established, the diagnostic accuracy of 1H-MR spectroscopy versus the conventional MR imaging, and its sensitivity and specificity in comparison with the biopsy results. In what percentage of their cases could 1H-MR spectroscopy accurately establish an abscess diagnosis along with conventional MR imaging? What was the respective percentage after applying diffusion-weighted imaging in their series? In how many cases was 1H-MR spectroscopy not possible due to technical limitations? Most interesting, the authors mentioned that in 56.2% of their patients, spectroscopic imaging was performed after systematic antibiotic initiation. Do they believe that antibiotic administration could alter the spectral characteristics of the studied abscesses?

Furthermore, there are no reports in their article regarding the methodology of spectroscopic analysis. Did the neuroradiologists review the spectroscopic studies in a double-blinded fashion? Were they aware of the biopsy results when performing their reviews, considering that this was a retrospective study? The authors reported no interobserver variation in the analyses of the obtained spectra between the 2 involved neuroradiologists. This finding is quite remarkable for spectroscopic analysis because numerous previously published series have reported lower interobserver agreement, even for conventional brain MR imaging.5

The authors mentioned that an aspiration biopsy was performed and an appropriate specimen was sent for cultures in all their cases. However, there are no details regarding the time interval between 1H-MR spectroscopy study and the surgical aspiration. This set of data may be of paramount importance because it is well known that spectroscopic characteristics of abscesses are rapidly evolving and continuously changing.3

There are several reports in the pertinent literature regarding the role of 1H-MR spectroscopy in monitoring the abscess response to antibiotic treatment.3 Have the authors any data regarding the role of 1H-MR spectroscopy in the evaluation of treatment response in their patients? We assume that in a retrospective study, there is a follow-up period long enough for evaluating this. Did the authors perform spectroscopy in any of their patients during antibiotic treatment?

The authors provide no data regarding the presence of lipids in their study. Did they detect lipids in all their cases? If not, what was the actual percentage of lipid presence in their cohort? It has been previously shown that the presence of lipidsmay be indicative of malignancy in ring-enhancing lesions.2,6 Although this finding remains controversial, the presence or absence of lipids and their diagnostic importance remains a black box for spectroscopy. We would like the authors to enlighten us regarding their findings on this subject.

We congratulate the authors again on their significant scientific contribution. We agree that 1H-MR spectroscopy constitutes a valuable diagnostic tool for intracranial abscesses, evaluating their evolution and treatment response. However, caution needs to be exercised in identifying the causative pathogen and categorizing abscesses based on the spectroscopic findings.

References

  1. Pal D, Bhattacharyya A, Husain M, et al. In vivo proton MR spectroscopy evaluation of pyogenic brain abscesses: a report of 194 cases. AJNR Am J Neuroradiol 2010;31:360–66[Abstract/Free Full Text]
  2. Lai PH, Ho JT, Chen WL, et al. Brain abscess and necrotic brain tumor: discrimination with proton MR spectroscopy and diffusion-weighted imaging. AJNR Am J Neuroradiol 2002;23:1369–77[Abstract/Free Full Text]
  3. Kapsalaki E, Gotsis ED, Fountas KN. The role of proton magnetic resonance spectroscopy in the diagnosis and categorization of cerebral abscesses. Neurosurg Focus 2008;24:E7[Medline]
  4. Fountas KN, Kapsalaki E, Gotsis ED, et al. In vivo proton magnetic resonance spectroscopy of brain tumors.Stereotact Funct Neurosurg 2000;74:83–94[CrossRef][Medline]
  5. Provenzale JM, Ison C, Delong D. Bidimensional measurements in brain tumors: assessment of interobserver variability. AJR Am J Roentgenol 2009;193:W515–22[Abstract/Free Full Text]
  6. Gotsis E, Fountas K, Kapsalaki E, et al. In vivo proton MR spectroscopy: the diagnostic possibilities of lipid resonances in brain tumors. Anticancer Res 1996;16:1565–67[Medline]

Reply

Published ahead of print on June 25, 2010
doi: 10.3174/ajnr.A2174

American Journal of Neuroradiology 31:E68, September 2010
© 2010 American Society of Neuroradiology

R.K. Guptaa
aDepartment of Radiodiagnosis
Sanjay Gandhi Postgraduate Institute of Medical Sciences
Lucknow, India

We greatly appreciate the interest shown by Kapsalaki and Fountas regarding our recent article “In Vivo Proton MR Spectroscopy Evaluation of Pyogenic Brain Abscesses: A Report of 194 Cases.”1 We will try to address the concerns raised by the authors about this study.

It is usually not possible to differentiate cystic intracranial mass lesions on conventional MR imaging. We have shown in our previous studies the sensitivity and specificity of proton MR (1H-MR) spectroscopy in the differentiation of cystic intracranial lesions,2–4 and we did not attempt to differentiate these abscesses from other cystic lesions by using conventional MR imaging. We have previously reported the role of 1H-MR spectroscopy in the etiologic characterization of the pyogenic brain abscess.5 The purpose of this study was to look for the sensitivity and specificity of the commonly encountered metabolites in the etiologic characterization of brain abscess. We restricted our discussion to only pyogenic abscess and 1H-MR spectroscopy.Kapsalaki and Fountas mention that the absence of amino acids seen in our study does not rule out the nonpyogenic etiology. However, we would like to reiterate that the absence of amino acids does not rule out a pyogenic etiology.1 Diagnosis of the brain abscess was based on the culture of the microbes on aspiration, excision, and repeated aspiration and follow-up on antibiotic therapy. In this study, 56% of the patients with brain abscess had taken antibiotics for a variable period before 1H-MR spectroscopy, and some of these abscesses were sterile on culture. We have already reported the effect of antibiotic therapy on the metabolite pattern in pyogenic abscesses.6 Most patients underwent surgical intervention within 24 hours of the MR imaging. Of 194 cases included in our study, 55 patients had lipids along with lactate, while 11 had only lipids. The spectroscopy data were analyzed by 2 neuroradiologists who were blinded to the microbial culture data, with no significant interobserver variation.

References

  1. Pal D, Bhattacharyya A, Husain M, et al. In vivo proton MR spectroscopy evaluation of pyogenic brain abscesses: a report of 194 cases. AJNR Am J Neuroradiol 2010;31:360–66. Epub 2009 Oct 1[Abstract/Free Full Text]
  2. Poptani H, Gupta RK, Jain VK, et al. Cystic intracranial mass lesions: possible role of in vivo MR spectroscopy in its differential diagnosis. Magn Reson Imaging 1995;13:1019–29[CrossRef][Medline]
  3. Shukla DA, Gupta RK, Roy R, et al. Prospective evaluation of in vivo proton MR spectroscopy in differentiation of similar-appearing intracranial cystic lesions. Magn Reson Imaging 2001;19:103–10[CrossRef][Medline]
  4. Mishra AM, Gupta RK, Jaggi RS, et al. Role of diffusion-weighted imaging and in vivo proton magnetic resonance spectroscopy in the differential diagnosis of ring-enhancing intracranial cystic mass lesions. J Comput Assisted Tomogr 2004;28:540–47[CrossRef][Medline]
  5. Garg M, Gupta RK, Husain M, et al. Brain abscesses: etiologic categorization with in vivo proton MR spectroscopy.Radiology 2004;230:519–27[Abstract/Free Full Text]
  6. Dev R, Gupta RK, Poptani H, et al. Role of in vivo proton magnetic resonance spectroscopy in the diagnosis and management of brain abscesses. Neurosurgery 1998;42:37–43[CrossRef][Medline]
Proton MR Spectroscopy of Brain Abscesses