Ruptured vertebrobasilar aneurysms: frequency of the nonaneurysmal perimesencephalic pattern of hemorrhage on CT scans

Subarachnoid Hemorrhage of Unknown Cause: Distribution and Role of Imaging

Subarachnoid hemorrhage of unknown cause represents approximately 10% to 15% of nontraumatic subarachnoid hemorrhages. The key factors in determining the management strategy for a presumed nonaneurysmal subarachnoid hemorrhage are the distribution, location, and amount of subarachnoid blood. Hemorrhage distribution on computed tomography can be categorized as follows: perimesencephalic, diffuse, sulcal, and primary intraventricular. The extent of the workup required in determining the cause of hemorrhage depends on the distribution of blood. The authors review the potential causes, differential diagnoses, and acute and long-term follow-up strategies in patients with subarachnoid hemorrhage of unknown cause.

AH, R. PA. The Yield of Repeat Angiography in Angiography-Negative Spontaneous Subarachnoid Hemorrhage

Objective  Despite the technological advancement in imaging, digital subtraction angiography (DSA) remains gold standard imaging modality for spontaneous subarachnoid hemorrhage (SAH). But even after DSA, around 15% of SAH remains elusive for the cause of the bleed. This is an institutional review to solve the mystery, “when is second DSA really indicated?”

Methods  In a retrospective review from January 2015 to December 2017, we evaluated cases of spontaneous SAH with initial negative DSA with repeat DSA after 6 weeks to rule out vascular abnormality. The spontaneous SAH was confirmed on noncontrast computed tomography (NCCT) and divided into two groups of perimesencephalic SAH (PM-SAH) or nonperimesencephalic SAH (nPM-SAH). The outcome was assessed by a modified Rankin’s score (mRS) at 6 months postictus.

Results  During the study period, we had 119 cases of initial negative DSA and 98 cases (82.3%) underwent repeat DSA after 6 weeks interval. A total of 53 cases (54.1%) had PM-SAH and 45 cases (45.9%) had nPM-SAH. Repeat DSA after 6 weeks showed no vascular abnormality in 53 cases of PM-SAH and in 2 (4.4%) out of 45 cases of nPM-SAH. At 6 months postictus, all cases of PM-SAH and 93% of nPM-SAH had mRS of 0.

Conclusion  We recommend, a repeat DSA is definitely not required in PM-SAH, but it should be done for all cases of nPM-SAH, before labeling them as nonaneurysmal SAH. Although the overall outcome for nonaneurysmal spontaneous SAH is better than aneurysmal SAH, nPM-SAH has poorer eventual outcome compared to PM-SAH.

Meta-analysis of recent literature on utility of follow-up imaging in isolated perimesencephalic hemorrhage

OBJECTIVES

Isolated perimesencephalic subarachnoid hemorrhage is an uncommon, distinct subtype of subarachnoid hemorrhage with a more benign prognosis. A negative computed tomographic angiogram has been shown to be reliable in excluding aneurysmal rupture as the underlying etiology. However, some studies continue advocating for more imaging to determine a vascular cause in perimesencephalic subarachnoid hemorrhage. The objective of this study is to evaluate the evidence for use and utility of repeat angiographic imaging after a negative computed tomographic angiogram in patients with perimesencephalic subarachnoid hemorrhage.

PATIENTS AND METHODS

Retrospective institutional analysis of patients with perimesencephalic subarachnoid hemorrhage was performed from 2014 to 2017 for number and types of follow-up angiographic imaging studies performed. Updated meta-analysis of literature was performed from 2014 onwards to assess the utility of follow-up imaging after a negative initial angiographic study.

RESULTS

The institutional review revealed no utility of additional imaging after a negative computed tomographic angiogram in 6 patients with isolated perimesencephalic subarachnoid hemorrhage. Literature review and metaanalysis of 13 studies with 588 patients revealed a vascular etiology in 3 patients with isolated perimesencephalic subarachnoid hemorrhage from a single study- 2 aneurysms and 1 patient with vasculitis.

CONCLUSIONS

Use of repeat angiographic imaging after a negative computed tomographic angiogram for perimesencephalic subarachnoid hemorrhage patients remains not uncommon, despite previous meta-analysis. Review of the more recent literature is consistent with previously published meta-analysis and shows limited benefits despite frequent use. In patients with a strictly defined perimesencephalic subarachnoid hemorrhage pattern and clinical picture consistent with perimesencephalic subarachnoid hemorrhage, an initial negative computed tomographic angiogram should be adequate and repeated follow-up studies can be avoided.

The negative predictive value of CT angiography in the setting of perimesencephalic subarachnoid hemorrhage

Background

Perimesencephalic subarachnoid hemorrhage (PMSAH) is only rarely associated with a ruptured cerebral aneurysm and CT angiography (CTA) has very good sensitivity and specificity for aneurysm detection. The necessity for invasive imaging with digital subtraction angiography (DSA) is therefore debatable. We chose to assess the negative predictive value (NPV) of CTA in a series of patients with PMSAH treated at our institution over a 9-year period.

Methods

We retrospectively assessed the diagnostic yield of DSA after initial negative CTA in patients with a PMSAH pattern defined as blood centered anterior to the midbrain and/or pons within the pre-pontine or interpeduncular cistern with possible quadrigeminal or ambient cistern extension; possible extension into the basal parts of the sylvian fissures but not the lateral sylvian fissures; possible extension to the cisterna magna but not centered on the cisterna magna; and possible extension into the fourth ventricle and occipital horns of the lateral ventricles.

Results

Using this definition of PMSAH, of 72 patients, one patient showed a potentially significant finding on DSA that was not demonstrated on initial CTA (NPV 98.61% (95% CI 92.47% to 99.77%)). However, when cisterna magna extension was excluded from the definition of PMSAH, no false negative CTAs in 56 patients were encountered (NPV 100% (95% CI 93.56% to 100.00%)).

Conclusions

The NPV of normal CTA for an arterial abnormality in patients with PMSAH is high and our results therefore question the role of invasive imaging. The findings also suggest that a prospective study designed to clarify the necessity of performing DSA in this population would be feasible.

Evaluation of an anatomic definition of non-aneurysmal perimesencephalic subarachnhoid hemorrhage

Background and purpose

Perimesencephalic subarachnoid hemorrhage (PSAH) is not consistently defined in the existing literature. The purpose of this study was to test the inter-observer variability and specificity for non-aneurysmal subarachnoid hemorrhage (SAH) of an anatomic definition of PSAH.

Methods

Medical records of all patients who underwent catheter angiography for evaluation of non-traumatic SAH between July 2002 and April 2012 were reviewed. Patients with anterior circulation aneurysms were excluded. Three blinded reviewers assessed whether each admission CT scan met the following anatomic criteria for PSAH: (1) center of bleeding located immediately anterior and in contact with the brainstem in the prepontine, interpeduncular, or posterior suprasellar cistern; (2) blood limited to the prepontine, interpeduncular, suprasellar, crural, ambient, and/or quadrigeminal cisterns and/or cisterna magna; (3) no extension of blood into the Sylvian or interhemispheric fissures; (4) intraventricular blood limited to incomplete filling of the fourth ventricle and occipital horns of the lateral ventricles (ie, consistent with reflux); (5) no intraparenchymal blood.

Results

56 patients with non-aneurysmal SAH and 50 patients with posterior circulation or posterior communicating artery aneurysms were identified. Seventeen (16%) of the 106 admission CT scans met the anatomic criteria for PSAH. No aneurysm was identified in this subgroup. Inter-observer agreement was excellent with κ scores of 0.89–0.96 and disagreement in 2.8% (3/106) of cases.

Conclusions

Our anatomic definition of PSAH correlated with a low risk of brain aneurysm and was applied with excellent inter-observer agreement.

Use of Follow-Up Imaging in Isolated Perimesencephalic Subarachnoid Hemorrhage

Background and Purpose—

Multiple studies have shown that negative computed tomographic angiograms (CTAs) are reliable in excluding aneurysms in patients with isolated perimesencephalic subarachnoid hemorrhage (pSAH). We evaluate the use of digital subtraction angiography versus CTA for initial diagnosis and of angiographic follow-ups in patients with pSAH by performing an institutional analysis and a meta-analysis of literature.

Methods—

Retrospective institutional analysis of patients with pSAH was performed from 2008 to 2014. The number and types of follow-up imaging studies were tabulated. Initial and follow-up studies were evaluated by an experienced neuroradiologist for intracranial aneurysm. Meta-analysis of literature was performed to assess the use of initial digital subtraction angiography and of follow-up imaging.

Results—

Our institutional review revealed no additional use of initial digital subtraction angiography or of any angiographic follow-up after initial negative CTA in patients with pSAH on noncontrast CT. Meta-analysis of 40 studies yielded a total of 1031 patients. Only 8 aneurysms were first diagnosed on follow-ups (0.78%). Careful review showed that some of these aneurysms reported on follow-up are of questionable validity. Initial digital subtraction angiography and follow-up imaging after a negative initial CTA showed no statistically significant benefits.

Conclusions—

In patients meeting the strict imaging criteria of pSAH, initial negative CTA is reliable in excluding aneurysms. A critical review of the literature through meta-analysis shows no foundation for multiple follow-up studies in patients with pSAH.

Nonaneurysmal perimesencephalic subarachnoid hemorrhage: diagnosis, pathophysiology, clinical characteristics, and long-term outcome

Patients with nonaneurysmal perimesencephalic subarachnoid hemorrhage (NAPSAH) have no discernible source for the bleeding and generally are considered to have a benign condition. Correctly diagnosing these patients is essential because a missed aneurysm can have catastrophic consequences. Those presenting with NAPSAH have a low risk of complications and better outcome than patients presenting with aneurysmal subarachnoid hemorrhage; however, a limited body of literature suggests that not all of these patients are able to return to their premorbid functional status. Clinical screens of cognitive status, such as the mini-mental status examination, suggest good recovery of these patients, although these tests may lack sensitivity for identifying deficits in this patient population. More comprehensive neuropsychologic testing in some studies has identified deficits in a wide range of cognitive domains at long-term follow-up in patients with NAPSAH. Because these patients often do not lose consciousness (and thus do not have substantial transient global ischemia) and they do not undergo a procedure for aneurysm repair, the cognitive sequelae can be explained by the presence of blood in the subarachnoid space. NAPSAH presents an opportunity to understand the effects of subarachnoid blood in a clinical setting.

Repeated neurovascular imaging in subarachnoid hemorrhage when initial studies are negative

Approximately 15% of patients with non-traumatic subarachnoid hemorrhage have no causative lesion identified on their initial angiogram. We present two patients with non-traumatic subarachnoid hemorrhage with negative initial angiograms who were subsequently found to have small basilar perforator aneurysms on delayed neurovascular imaging. We discuss the possible mechanisms for false negative diagnostic cerebral angiograms. These patients support the current standard of care with repeat angiography in cases of subarachnoid hemorrhage when no causative lesion can be identified on initial neurovascular imaging.

Antithrombotic States and Outcomes in Patients With Angiographically Negative Subarachnoid Hemorrhage

BACKGROUND:

Antithrombotic states are encountered frequently, either because of medical therapy or by preexistent pathological states, and may affect the severity of hemorrhagic strokes such as angiographically negative subarachnoid hemorrhages.

OBJECTIVE:

To determine the effects of antithrombotic states on the outcomes of patients with angiographically negative subarachnoid hemorrhage by examining data pooled from 2 institutions.

METHODS:

This is a retrospective review of patients who experienced angiographically negative subarachnoid hemorrhage at 2 institutions over the past 5 years. The patients were grouped into those with and those without an antithrombotic state at time of hemorrhage and were stratified according to presentation, clinical grades, outcomes, need for cerebrospinal fluid diversion, and development of vasospasm. Computed tomography of the head was assessed for bleed pattern and modified Fisher grade. Patients were excluded if a causative lesion was subsequently discovered.

RESULTS:

There is a statistically significant association between antithrombotic states and poorer presentation, higher Hunt and Hess score, increased amount of subarachnoid hemorrhage, higher modified Fisher grade, increased incidence of vasospasm, hydrocephalus, and poor outcomes as assessed by modified Rankin scale (P < .001). Patients with an antithrombotic state experience worse outcomes even with adjustment for the amount of hemorrhage as assessed by modified Fisher grade (P < .001).

CONCLUSION:

Patients in an antithrombotic state presenting with angiographically negative subarachnoid hemorrhage present with inferior clinical scores, diffuse hemorrhage patterns, and worse modified Fisher grades and have worse outcomes.

Inter- and intraobserver agreement in CT characterization of nonaneurysmal perimesencephalic subarachnoid hemorrhage

BACKGROUND AND PURPOSE

The perimesencephalic pattern of SAH as seen on unenhanced CT is associated with significantly better outcomes when compared to an aneurysmal pattern of SAH. The aim of this study was to determine the degree of inter- and intraoberver agreement for characterization of the NAPH as seen on unenhanced CT.

MATERIALS AND METHODS

We retrospectively reviewed the CT scans of 37 patients with spontaneous SAH, all of whom had undergone CT within 24 hours of onset of headache symptoms. All patients had undergone conventional cerebral angiography to confirm or exclude aneurysms or other vascular pathology. All 37 cases were angiographically confirmed nonaneurysmal SAHs. Four readers with neuroradiology subspecialty training independently evaluated CT images to characterize the hemorrhage pattern as compatible with the well-described NAPH. Each reader performed a second reading session blinded to the initial readings. The first and second sets of readings were performed approximately 4 months apart. Inter- and intraobserver agreement for characterization of the NAPH was determined by using the kappa statistic.

RESULTS

Of the 37 angiographically confirmed nonaneurysmal SAHs, there was unanimous agreement as to the hemorrhage pattern in 29 (78%) cases and disagreement in 8 (22%) cases. Overall, intraobserver agreement was good (kappa = 0.80). Interobserver agreement was also good (kappa = 0.79).

CONCLUSIONS

Overall, inter- and intraobserver agreement for the NAPH was good. There was, however, a level of disagreement among observers, thus suggesting that clinicians should be cautious when deciding whether to pursue follow-up imaging.

Multislice CT angiography in the selection of patients with ruptured intracranial aneurysms suitable for clipping or coiling

Introduction

We sought to establish whether CT angiography (CTA) can be applied to the planning and performance of clipping or coiling in ruptured intracranial aneurysms without recourse to intraarterial digital subtraction angiography (IA-DSA).

Methods

Over the period April 2003 to January 2006 in all patients presenting with a subarachnoid haemorrhage CTA was performed primarily. If CTA demonstrated an aneurysm, coiling or clipping was undertaken. IA-DSA was limited to patients with negative or inconclusive CTA findings. We compared CTA images with findings at surgery or coiling in patients with positive CTA findings and in patients with negative and inconclusive findings in whom IA-DSA had been performed.

Results

In this study, 224 consecutive patients (mean age 52.7 years, 135 women) were included. In 133 patients (59%) CTA demonstrated an aneurysm, and CTA was followed directly by neurosurgical (n = 55) or endovascular treatment (n = 78). In 31 patients (14%) CTA findings were categorized as inconclusive, and in 60 (27%) CTA findings were negative. One patient received surgical treatment on the basis of false-positive CTA findings. In 17 patients in whom CTA findings were inconclusive, IA-DSA provided further diagnostic information required for correct patient selection for any therapy. Five ruptured aneurysms in patients with a nonperimesencephalic SAH were negative on CTA, and four of these were also false-negative on IA-DSA. On a patient basis the positive predictive value, negative predictive value, sensitivity, specificity and accuracy of CTA for symptomatic aneurysms were 99%, 90%, 96%, 98% and 96%, respectively.

Conclusion

CTA should be used as the first diagnostic modality in the selection of patients for surgical or endovascular treatment of ruptured intracranial aneurysms. If CTA renders inconclusive results, IA-DSA should be performed. With negative CTA results the complementary value of IA-DSA is marginal. IA-DSA is not needed in patients with negative CTA and classic perimesencephalic SAH. Repeat IA-DSA or CTA should still be performed in patients with a nonperimesencephalic SAH.

Non-Aneurysmal Perimesencephalic Subarachnoid Haemorrhage with Associated Pontine Haemorrhagic Infarction

Non-aneurysmal perimesencephalic subarachnoid saemorrhage is characterized by an accumulation of blood in the perimesencephalic and prepontine cisterns identified on sectional imaging together with persistently negative cerebral angiography. Magnetic resonance imaging usually contributes no further information on the possible cause of the bleed but may occasionally show further features including associated parenchymal infarcts such as the pontine haemorrhagic infarct seen in the case described here.

Comparison between perimesencephalic nonaneurysmal subarachnoid hemorrhage and subarachnoid hemorrhage caused by posterior circulation aneurysms

OBJECT

Some authors have questioned the need to perform cerebral angiography in patients presenting with a benign clinical picture and a perimesencephalic pattern of subarachnoid hemorrhage (SAH) on initial computerized tomography (CT) scans, because the low probability of finding an aneurysm does not justify exposing patients to the risks of angiography. It has been stated, however, that ruptured posterior circulation aneurysms may present with a perimesencephalic SAH pattern in up to 10% of cases. The aim of the present study was twofold: to define the frequency of the perimesencephalic SAH pattern in the setting of ruptured posterior fossa aneurysms, and to determine whether this clinical syndrome and pattern of bleeding could be reliably and definitely distinguished from that of aneurysmal SAH.

METHODS

Twenty-eight patients with ruptured posterior circulation aneurysms and 44 with nonaneurysmal perimesencephalic SAH were selected from a series of 408 consecutive patients with spontaneous SAH admitted to the authors' institution. The admission unenhanced CT scans were evaluated by a neuroradiologist in a blinded fashion and classified as revealing a perimesencephalic SAH or a nonperimesencephalic pattern of bleeding. Of the 28 patients with posterior circulation aneurysms, five whose grade was I according to the World Federation of Neurosurgical Societies scale were classified as having a perimesencephalic SAH pattern on the initial CT scan. The data show that the likelihood of finding an aneurysm on angiographic studies obtained in a patient with a perimesencephalic SAH pattern is 8.9%. Conversely, ruptured aneurysms of the posterior circulation present with an early perimesencephalic SAH pattern in 16.6% of cases.

CONCLUSIONS

This study supports the impression that there is no completely sensitive and specific CT pattern for a nonaneurysmal SAH. In addition, the authors believe that there is no specific clinical syndrome that can differentiate patients who have a perimesencephalic SAH pattern caused by an aneurysm from those without aneurysms. Digital subtraction angiography continues to be the gold standard for the diagnosis of cerebral aneurysms and should be performed even in patients who have the characteristic perimesencephalic SAH pattern on admission CT scans.

Causes and management of thunderclap headache: a comprehensive review

BACKGROUND

Thunderclap headache (or sudden severe headache) is an uncommon type of headache. Recognition and accurate diagnosis of this headache are important, because there is often a serious underlying brain disorder.

SUMMARY

In this article, causes and management of thunderclap headache are discussed. In the primary care setting, there is a serious cause in one third of patients, but in the hospital setting, up to two thirds of patients have a serious underlying brain disorder. Clues in history and physical examination can point to a possible serious underlying cause of thunderclap headache, such as subarachnoid hemorrhage, intracranial hematoma, or cerebral venous thrombosis. The remaining patients with thunderclap headache, however, have a primary headache disorder, such as migraine or (less frequently) tension headache with an unusual sudden onset, exertional headache, coital headache, cough headache, or cluster headache. The concept of thunderclap headache as a distinct clinical entity is discussed, with implications for its evaluation. Present radiological techniques are reviewed with regard to their diagnostic utility in detecting a serious brain disorder.

CONCLUSIONS

Thunderclap headache is an uncommon type of headache, and a serious underlying cause should be excluded.

Diagnosis of perimesencephalic nonaneurysmal subarachnoid hemorrhage with computed tomography

OBJECTIVE

In 4% to 31% of patients with acute subarachnoid hemorrhage (SAH), no underlying cause is identified. Blood is restricted to the perimesencephalic cisterns in about two thirds of these patients. These patients are identified as having perimesencephalic nonaneurysmal subarachnoid hemorrhage (PNSAH), a syndrome based on the interpretation of computed tomography (CT) findings on admission, with an excellent prognosis, far better than other patients with SAH with or without an aneurysm. However, the diagnosis is subject to interrater variability, and differentiation between PNSAH and ruptured supratentorial aneurysm by means of CT has not been investigated. Therefore we investigated the validity of prediction of PNSAH with CT scan.

METHODS

A total of 303 consecutive patients, admitted within 72 hours after the initial bleeding with symptoms and signs of spontaneous SAH, which was confirmed by CT or spectrophotometric analysis of the cerebrospinal fluid (CSF) in whom either a selective 4-vessel angiogram (n = 293) or postmortem examination (n = 10) was performed, were studied.

RESULTS

We found an interrater agreement for the diagnosis perimesencephalic nonaneurysmal subarachnoid hemorrhage of 93% and a kappa value of 0.65. Sensitivity was 30% for observer 1 and 46% for observer 2. The positive predictive value was 73% for observer 1 and 76% for observer 2. Among those diagnosed as having PNSAH, we found vertebrobasilar artery aneurysms in 19% (both raters) and internal carotid artery aneurysms in 5% (observer 1) and 8% (observer 2).

CONCLUSION

We conclude that PNSAH can be distinguished on CT in the majority of patients; however, the angiographical management in PNSAH should not differ from other SAHs.

Prognostic value of intraventricular blood in perimesencephalic nonaneurysmal subarachnoid hemorrhage

INTRODUCTION

Perimesencephalic nonaneurysmal subarachnoid hemorrhage (PNSH) is a distinct type of subarachnoid hemorrhage with a characteristic bleeding pattern, and an excellent clinical outcome. However, the clinical course of patients presenting with a perimesencephalic nonaneurysmal bleeding pattern and intraventricular blood has not yet been investigated.

MATERIALS AND METHODS

In this retrospective study we describe the cisternal blood distribution and the clinical course of 34 PNSH patients with (10 patients) and without (24 patients) intraventricular blood.

RESULTS/DISCUSSION

Patients without intraventricular blood were in good clinical condition on admission; the in-hospital course was uneventful except for acute hydrocephalus, which was detected in two cases and improved spontaneously in both patients. All patients resumed their previous lifestyles. In contrast, two patients with PNSH and intraventricular blood were drowsy and had focal neurologic deficits on admission. Four patients developed an acute hydrocephalus, and two of these patients needed ventricular shunting. There was evidence for delayed cerebral ischemia in one of these patients. Outcome was excellent in eight cases, and one patient was moderately and one patient was severely disabled at dismissal from the hospital. Rebleeding did not occur in all patients, and repeated four-vessel angiography did not reveal the source of bleeding in any of our patients.

CONCLUSION

Our data indicate that the presence of intraventricular blood may be a good indicator for the development of acute hydrocephalus in PNSH. Moreover, our results suggest that PNSH patients with ventricular blood may have a higher complication rate and a poorer outcome compared with PNSH patients without intraventricular blood.

Perimesencephalic hemorrhage. Exclusion of vertebrobasilar aneurysms with CT angiography

BACKGROUND AND PURPOSE

It is important to recognize a perimesencephalic pattern of hemorrhage in patients with subarachnoid hemorrhage (SAH), because in 95% of these patients the cause is nonaneurysmal and the prognosis is excellent. The purpose of this study was to investigate whether CT angiography can accurately exclude vertebrobasilar aneurysms in patients with perimesencephalic patterns of hemorrhage and therefore replace digital subtraction angiography (DSA) in this setting.

METHODS

In 40 patients with posterior fossa SAH as shown on unenhanced CT, 2 radiologists independently evaluated unenhanced CT for distinguishing between perimesencephalic and nonperimesencephalic pattern of hemorrhage and assessed CT angiography for detection of aneurysms. All patients subsequently underwent DSA or autopsy.

RESULTS

Observers agreed in 38 of 40 patients (95%) in differentiating perimesencephalic and nonperimesencephalic patterns of hemorrhage on unenhanced CT. On the CT angiograms, both observers detected a vertebrobasilar aneurysm in 16 patients and no aneurysm in 24 patients. These findings were confirmed by DSA or autopsy. No patients with a perimesencephalic pattern of hemorrhage were found to have an aneurysm on either CT angiography or DSA.

CONCLUSIONS

Good recognition of a perimesencephalic pattern of hemorrhage is possible on unenhanced CT, and CT angiography accurately excludes and detects vertebrobasilar aneurysms. DSA can be withheld in patients with a perimesencephalic pattern of hemorrhage and negative CT angiography.

Imaging of neurological emergencies: trauma, hemorrhage, and infarction

In summary, the radiologist plays a critical role in the diagnosis of acute neurological conditions, such as traumatic brain and spinal injuries, intracranial hemorrhage, and acute ischemia/infarction, which may require emergent therapy. CT is the imaging modality of choice in such conditions and may serve to direct appropriate subsequent studies. The radiologist must be aware of the variable clinical presentations of many of these entities, and he or she must be familiar with and diligent in his or her search for the early, and often subtle imaging findings of these often life-threatening conditions.