738 SECTION VI Pediatric Critical Care Neurologic postprocessing of the data can produce three dimensional (3D) images of the vessels Magnetic Resonance Imaging Magnetic resonance imaging (MRI) uses a[.]
738 S E C T I O N V I Pediatric Critical Care: Neurologic A C B D • Fig 61.3 Intracranial hemorrhage secondary to a ruptured aneurysm Two-year-old male presented with vomiting that was assumed to be secondary to an unwitnessed trauma while playing with a sibling Coronal image from a noncontrast computed tomography (CT) scan of the head (A) demonstrates subarachnoid blood in the left sylvian fissure associated with a rounded, more focal density (arrow) Coronal image from a noncontrast CT head scan (B) performed the next day after the patient became acutely less responsive, hypertensive, and bradycardic demonstrates a left frontoparietal intraparenchymal hemorrhage with intraventricular extension (arrowheads); a rounded, more focal density is again seen in the sylvian fissure (arrow) Coronal image from a CT angiography head scan (C) demonstrates an aneurysm (circle) of the M3/M4 segment of the left middle cerebral artery The patient underwent emergent hemicraniectomy and clipping of the aneurysm Axial image from a noncontrast CT head scan (D) obtained days after surgery demonstrates wedge-shaped hypodensity in the left middle cerebral artery distribution consistent with cytotoxic edema secondary to an acute/subacute infarction postprocessing of the data can produce three-dimensional (3D) images of the vessels Magnetic Resonance Imaging Magnetic resonance imaging (MRI) uses a high-field-strength magnet in combination with radiofrequency pulses to produce images based on the nuclear resonance of hydrogen protons in tissue, primarily in water and fat MRI provides the most sensitive measure of most (but not all) central nervous system (CNS) pathologies However, imaging requires patient transport to the scanner, increased examination complexity, potential safety concerns related to the strong magnet (typically 1.5 or 3.0 Tesla), and longer imaging time versus CT scan MRI provides superior tissue characterization because multiple sequences are obtained, each optimized to evaluate certain tissue types or pathologic processes (1–10 minutes required per sequence) MRI is particularly useful in the evaluation of the posterior fossa (brainstem and cerebellum) and spinal cord, where “beam hardening” due to bone results in considerable artifact on CT, although this problem has become less of an issue with modern multidetector CT scanners Safety associated with MR scanning is an important issue MRI-compatible monitoring and life support systems must be used Patients need to be screened for any internal hardware or implants that would preclude scanning Electronic devices such as pacemakers are rendered dysfunctional by the changing magnetic field occurring during MRI; hence, patients with such devices generally should not undergo MRI, though MRI may be feasible with some devices in certain situations Other medical devices, such as cochlear implants, can also limit the feasibility of MR imaging (further details are available at http://www.mrisafety com) Many contemporary programmable shunts and vagal nerve CHAPTER 61 Neuroimaging stimulators can be scanned but require reprogramming immediately afterward Injuries and deaths have been reported because of the failure to recognize these dangers Most MRI sequences require imaging times in the range of minutes and are susceptible to motion degradation Though newer sequences have increased tolerance for (minor) motion, for patients who are unable to cooperate, deep sedation or anesthesia is often required “Fast and feed” approaches taking advantage of postprandial somnolence are often used successfully in young infants, thereby avoiding sedation It also is important to screen the renal function of patients who might get an MRI because gadolinium-based contrast may need to be used Gadolinium has been implicated in the development of nephrogenic systemic fibrosis in some patients with compromised renal function, which is not uncommon in the ICU setting.9 Small amounts of gadolinium can deposit in the body without proven clinical adverse effects; however, ongoing investigation is merited.10 MRI exploits nuclear characteristics of hydrogen protons principally in tissue water, referred to as the T1 (longitudinal relaxation) and T2 (spin-spin relaxation) of the tissue An imaging sequence includes both T1 and T2 components to create an image; however, an individual sequence can be formulated to be predominately one or the other—that is, T1 weighted or T2 weighted (often referred to as simply T1 or T2 sequences) Because most pathobiology results in some disturbance of water, these imaging parameters turn out to be sensitive to many pathologies On T1-weighted images, water—such as CSF in the ventricles or vitreous in the globes—is dark This water is bright on T2-weighted images One point to be aware of is that, for the most part, the signal intensity of any tissue being imaged on MRI is relative (unlike the absolute density measure in CT) Hence, the detection of pathology is dependent on visualizing differences in intensity of one tissue relative to another Fluid-attenuated inversion recovery (FLAIR) is a variation of T2-weighted imaging in which the CSF is specifically rendered dark or suppressed to enhance visibility of hyperintense pathology in parenchyma, especially adjacent to normal CSF spaces Fat is hyperintense on T1-weighted sequences and is also fairly bright on fast T2-weighted sequences, which is possible with most modern-day scanners In some situations it is advantageous to suppress the brightness of fat, which can be specifically done at the cost of increased scan time Gradient-recalled echo (GRE) and susceptibility-weighted (SWI) MRI sequences are often used for detection of hemorrhage, although these sequences generally also result in more artifacts The appearance of acute blood on CT is fairly straightforward, appearing hyperdense initially (because of the extraction of serum), becoming isodense by about week, and appearing hypodense thereafter The appearance of blood on MRI is more complex Although it can be detectable for a much longer period on MRI, it can be difficult to appreciate hyperacute hemorrhage (approximately ,12 hours old) with MRI, especially SAH Overall, the detection of calcium and hyperacute blood can be difficult by standard MRI CT is often better in this regard, though newer sequences, SWI in particular, are sensitive IV contrast is also used in MRI, with similar application with CT In this case the contrast is a gadolinium chelate that shortens the T1 relaxation time of nearby water The result is that the water near gadolinium is hyperintense on T1-weighted images This effect can be used to detect BBB disruption, which is seen in various pathologies, including ischemia, inflammation/infection, and status epilepticus Malignant and some benign brain tumors also demonstrate enhanced capillary permeability with enhancement 739 following contrast MRI with gadolinium is about an order of magnitude more sensitive than CT using iodinated contrast in detecting BBB disruption Some areas of the brain lack the BBB, including the choroid plexus, pituitary stalk, the median eminence, and the pineal gland; thus they enhance normally Diffusion-weighted imaging (DWI) has assumed an invaluable role, particularly in the intensive care setting This sequence is now integrated into most brain MRI examinations It is particularly sensitive to areas where brain parenchyma has sustained acute cytotoxic injury—most commonly, but not limited to, ischemia DWI is sensitive to the motion of water at the molecular level.11 Detecting this molecular-level motion requires strong, fast gradients that have become standard on modern MRI scanners to allow sufficiently rapid scanning to effectively “freeze” the gross tissue motion that normally swamps this molecular-level motion Different cerebral pathologies produce different types of edema (e.g., cytotoxic, vasogenic, and interstitial) Evaluation of the molecular (Brownian) motion of water turns out to be a sensitive measure of some pathologic states, the most significant application currently being cytotoxic edema associated with the detection of ischemia.12 Cytotoxic edema causes “restriction” of water diffusion with a decrease in the apparent diffusion coefficient (ADC) This is seen as hypointensity on the calculated ADC map image but is more easily appreciated as hyperintensity on the DWI “trace” image In contrast to cytotoxic edema, vasogenic and interstitial edema result in “increased” water diffusion and therefore an increase in the ADC Care must be taken in interpreting the DWI trace images because they have some T2 weighting, and “T2 shine-through” can result in a focus of increased signal on DWI trace images that does not represent true restricted diffusion To confirm true restricted diffusion indicating cytotoxic edema, the calculated ADC map images should be reviewed for corresponding dark signals Also, the hyperintensity on DWI trace images may pseudonormalize for a period of several days following the ictus because of the development of associated vasogenic edema Restricted diffusion due to infarct will generally resolve over the course of to weeks; this evolution in diffusion signal allows new infarcts to be distinguished from older lesions It also should be noted that restricted diffusion is not limited to ischemia and can be seen in other acute cerebral pathologies, primarily myelin vacuolization, as can be seen in acute demyelination, infection, and highly cellular brain tumors Advanced Magnetic Resonance Imaging Techniques Newer techniques in MRI, such as MR perfusion, MR spectroscopy (MRS), diffusion tensor imaging, and intracranial vessel wall imaging, are providing new information that is being increasingly used in the management of critically ill patients.13 Perfusion MR is used in the evaluation of cerebral perfusion The most widely available techniques use rapid scanning with a bolus of IV gadolinium contrast to measure first-pass changes in signal intensity These techniques give a relative measure of perfusion but not quantitative flow This relative measure can be used to detect diminished cerebral blood flow in one area compared with another Newer noncontrast MRI techniques, such as arterial spin labeling, generally have lower signal-to-noise ratio (sensitivity) but are improving and potentially quantifiable There is some early experience with arterial spin labeling in pediatric stroke cases.14,15 Conventional MRI is based on the signal intensities derived from the hydrogen bound to oxygen in water and, to a lesser extent, the hydrogen bound to carbon in fat With the appropriate 740 S E C T I O N V I Pediatric Critical Care: Neurologic A B • Fig 61.4 Bilateral fusiform aneurysms of the cavernous carotid arteries Two-year-old female with multiple congenital aneurysms throughout the body Axial (A) image and superior maximum intensity projection image (B) from a time-of-flight magnetic resonance angiogram demonstrate fusiform dilation of the bilateral cavernous carotid arteries (A, arrows) software, MR can be used as a “probe” for hydrogen bonded to other molecules (termed MR spectroscopy) The sensitivity is sufficient to detect metabolite molecules in the millimolar range, although at a much lower resolution than that used to detect water for imaging The nonwater molecules are most commonly reported as ratios of signal peaks that correspond to one of several molecules In the brain the most common metabolite peaks detected are N-acetyl aspartate, creatine, choline, and—in some physiologic states—lactate The MRS signal is either obtained from a single voxel (usually several milliliters in volume; voxel is the volume element of the image, which equals pixel depth) or multiple voxels (as small as 1–2 mL in volume) Multivoxel MRS can be used to make low-resolution images using a technique termed chemical shift imaging for some of the more prevalent metabolite peaks The utility of MRS is still in evolution, and newer variants of MRS are constantly being evaluated Detection of lactate to evaluate newborn ischemia, some metabolic diseases, and brain tumors is one of the ongoing applications and areas of research MRS also has a role in imaging children with mitochondrial disorders16 and has been applied in creatine deficiency syndromes as two examples of clinical applications.17 Magnetic Resonance Angiography Fluid motion within tissues can be used by MRI to image flow in vessels and CSF This imaging can be accomplished with and without contrast, although contrast techniques are generally more sensitive for vascular flow, with some newer contrast angiography sequences also providing temporal/flow information The MR angiogram can be tailored for artery (MRA) or vein (MRV) visualization, primarily based on flow direction, and is usually most effective if the area of interest can be narrowed—for example, the circle of Willis—although newer techniques have greatly increased the area that can be covered in one scan The maximum resolution of MRA of slightly under mm is generally less than CTA, where the maximum resolution can be under 0.25 mm.18 In addition to the source images (the thin angiographic sections obtained during the scan), various reconstructed images can be obtained, including 3D rotational reformations derived from maximum intensity projection reconstructions (Figs 61.4 and 61.5) and surface renderings, which also can be viewed from multiple projections Time-resolved MRA techniques (such as timeresolved angiography with interleaved stochastic trajectories), which used the first pass of gadolinium-administered IV, have begun to make inroads in providing temporal information on vascular lesions previously available only with catheter angiograms, though the latter remain the gold standard Many clinical cerebral arterial questions, however, are now answered with MRA, and MRV of the superficial and deep venous systems has largely replaced diagnostic catheter venograms The main indication for MRV is to evaluate for dural venous sinus and cortical vein thrombosis This may be done either without (i.e., time-of-flight and phase contrast MRV) or with contrast (see Fig 61.5) MRV with contrast is essentially a volumetric 3D gradient recalled echo T1-weighted sequence obtained after the administration of IV contrast In a recent study by Sari et al., MRV with contrast had a higher sensitivity, specificity, and accuracy in detecting dural venous sinus and cortical vein thrombosis compared with conventional MR sequences and phase contrast MRV.19 Catheter Angiograms Although significant progress has been made in cerebral vascular evaluation with both CTA and MRA, catheter angiography remains the gold standard for most CNS arterial imaging when indicated Catheter angiography also provides temporal/flow data, such as appreciation of early venous drainage with arteriovenous malformations Angiography is basically a rapid series of radiographs obtained during injection of iodinated contrast directly into the arteries or veins being imaged Most angiography units produce digital images using digital subtraction (hence the term digital subtraction angiography)—that is, images in which the background “mask” has been subtracted, leaving an image primarily of the contrast-filled vessels High-end modern-day units use biplane technology that permits acquisition of digital CHAPTER 61 Neuroimaging A 741 B • Fig 61.5 Occlusive thrombosis of the left sigmoid sinus and internal jugular vein in a 17-year-old female with acute suppurative sinusitis Anterior maximum-intensity projection image (A) from a time-of-flight magnetic resonance venography demonstrates lack of flow-related enhancement on the left sigmoid sinus and proximal internal jugular vein (circle) suggestive of occlusive thrombus; compare with the normal flowrelated enhancement on the right (arrows) (B) Coronal T1 postcontrast image demonstrates lack of contrast enhancement in the left sigmoid venous sinus (circle) and corroborates the diagnosis of thrombosis; compare with the normal intraluminal contrast enhancement on the right (arrow) angiographic images from multiple projections using a singlecontrast injection This technology also can be used to derive 3D rotational angiograms, images that can be further postprocessed to obtain volume-rendered and surface-shaded images Catheter angiography requires transport to and patient support in the angiography suite Vascular access for arterial studies is usually through the femoral artery and generally has a small but “nonzero” risk of vessel injury, including dissection and embolization In very young patients, injury to the femoral artery is of greater concern—although any acute risk to the limb is extremely uncommon, relative diminished leg growth in some cases has been reported Risks of neurologic complication following cerebral angiography are small but exist The incidence of permanent defects in diagnostic angiograms using modern techniques is typically reported in the range of 0.5% to 0.7%.20,21 Therapeutic endovascular procedures carry higher risks but generally are in lieu of riskier neurosurgical procedures or, at times, are the only or preferred avenue of treatment Catheter-based endovascular procedures in children have mostly been limited to the treatment of arteriovenous malformations, fistulas, and aneurysms However, advancements in adult ischemic stroke intervention using mechanical thrombectomy for revascularization are being investigated in pediatric stroke.22 Myelography Myelography involves radiographs or CT of the spine following opacification of the subarachnoid space by intrathecal injection of iodinated contrast, most commonly injected at the lumbar level The myelogram has almost completely been replaced by MRI, especially in the pediatric population, because MRI can depict both extrathecal encroachment and intrathecal masses as well as evaluate signal characteristics of the spinal cord itself and any intramedullary mass High-resolution myelographic type contrast can be achieved using MRI with 3D constructive interference in steady-state and volume T2 sequences to investigate pathologies for which detailed anatomic information is needed, such as with spinal nerve root avulsion and cranial nerve pathologies Exceptions for which imaging may revert to CT include the inability to obtain an MRI either because of local field disruption, most commonly from ferromagnetic spinal rods, or because of MRI incompatibility due to safety issues (e.g., a pacemaker) CT provides better results than MRI in the evaluation of the bony spinal column, particularly in patients with trauma, whereas MRI is used in patients with trauma to evaluate soft tissues in the spinal column, primarily the spinal cord Nuclear Medicine Nuclear medicine studies involve the administration (injection, ingestion, or inhalation) of a very small amount of radioactively labeled substance (i.e., a radiopharmaceutical) and imaging the localization of the radiopharmaceutical with a gamma camera Tc-99m hexamethylpropyleneamine oxime (Tc-99m HMPAO) and Tc-99m ethyl cysteinate dimer are the preferred radiopharmaceuticals for evaluating both cerebral blood flow and perfusion and may be used to support a clinical diagnosis of brain death (Fig 61.6) Tc99m HMPAO single-photon emission computed tomography (SPECT) may be used to monitor for cerebral perfusion defects in patients with SAH who are at risk for vasospasm Tc99m HMPAO SPECT with acetazolamide (Diamox), a carbonic anhydrase inhibitor, may be used to evaluate cerebrovascular reserve capacity in patients with chronic cerebral ischemia, such as moyamoya vasculopathy A nuclear medicine CSF shunt study may be used in patients with suspected CSF shunt malfunction (such as obstruction of the proximal or distal catheter or valve malfunction) A small amount of radiopharmaceutical is instilled in the shunt reservoir and a combination of dynamic and static images is obtained to follow the distal flow of the radiotracer 742 S E C T I O N V I Pediatric Critical Care: Neurologic A B • Fig 61.6 Lack of cerebral and cerebellar blood flow on Tc-99m hexamethylpropyleneamine oxime (Tc- 99m HMPAO scan)—an ancillary test for brain death Two-year-old male with a history of mitochondrial cytopathy underwent a Tc-99m HMPAO brain scan after cardiopulmonary arrest Anteroposterior (A) and lateral (B) images of the head, neck, and chest demonstrate radiotracer accumulation in the soft tissues of the face, including the nose (“hot nose sign,” circle in A), lungs, and liver There is no radiotracer accumulation in the cerebrum or cerebellum (“empty skull sign”) Findings are consistent with lack of blow flow to the brain that, in the appropriate clinical setting, is suggestive of brain death Neurologic exam performed both before and after the brain scan demonstrated no evidence of brainstem function A 2D Echo B LT SAG • Fig 61.7 Germinal matrix hemorrhage in a premature infant Coronal (A) and left parasagittal (B) ultra- sound images through the anterior fontanelle demonstrate dilated lateral ventricles with intraventricular extension of clot on the left (arrows) and periventricular hemorrhagic venous infarction on the right (arrowheads) consistent with a grade hemorrhage on the left and grade hemorrhage on the right (some radiologists would assign this hemorrhage an overall grade of 4) Hypoxic Ischemic Injury and Germinal Matrix Hemorrhage in the Neonate In the premature infant, ultrasound is the primary modality for detection, grading, and follow-up of germinal matrix hemorrhage (GMH) and its complication, hydrocephalus (Fig 61.7) GMH is graded as to depending on the presence of intraventricular hemorrhage, ventricular dilation, and hemorrhagic parenchymal venous infarction Ultrasound may be used to assess for white matter injury, including periventricular leukomalacia (PVL), especially the cystic form However, MRI will be more sensitive for noncystic PVL In premature infants, MRI performed at about the third week of life has been reported as predictive of final MRI findings seen at term.23 Routine screening cranial ultrasound has been recommended for all infants younger than 30 weeks’ gestation between day and day 14, optimally repeated at 36 to 40 weeks’ gestation.24 Understanding of hypoxic ischemic encephalopathy (HIE) is complicated by the developmental status of the baby (for a detailed discussion of HIE in children, see Chapter 65) The pattern of injury seen is determined by the characteristics of the insult and the maturational state of the brain Metabolic demands and regions of selective vulnerability evolve during development Grayscale ultrasound, which is widely used for intracranial imaging in the neonate, is relatively insensitive to acute changes associated with HIE.25 Brain edema and hemorrhage are both echogenic on ... MRI CT is often better in this regard, though newer sequences, SWI in particular, are sensitive IV contrast is also used in MRI, with similar application with CT In this case the contrast is... clinical applications.17 Magnetic Resonance Angiography Fluid motion within tissues can be used by MRI to image flow in vessels and CSF This imaging can be accomplished with and without contrast, although... nearby water The result is that the water near gadolinium is hyperintense on T1-weighted images This effect can be used to detect BBB disruption, which is seen in various pathologies, including