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CHAPTER 9 Diagnostic Imaging in the Neonate
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using US to explore the ocean floor of the North
Atlantic. Medical ultrasonography has its roots in
sound navigation and ranging (sonar) developed
during World War II.
In medical ultrasound, a transducer (essentially
a piezoelectric crystal) converts electrons into a
mechanical vibration that creates high-frequency
sound waves within the body. The same transducer
serves as both the transmitter of the sound wave
and the receiver of the reflected sound. Within the
body, these high-frequency sound waves propagate
through the soft tissues until they meet a reflective
surface that reflects some of those sound waves back
to the transducer. The percentage of the sound beam
reflected relates to the difference in the acoustic
impedance of the material being evaluated. When
the acoustic impedances of materials are similar, as is
the case with the musculature of the abdominal wall
(e.g., liver, kidney), most of the sound is transmitted,
and a small percentage is reflected at each interface.
As the sound wave travels through the abdominal
wall to the liver, the abdominal wall–liver interface
reflects a portion of the beam and transmits most
of the sound through the liver to the liver–kidney
interface. The small difference in acoustic impedance between the liver and kidney causes reflection
of some of the beam and transmission of most of
it to the posterior abdominal wall. This allows the
visualization of multiple interfaces that are deeper
than the first structure encountered. If the velocity
of the sound beam in tissue is known, the distance
to the reflective surface can be estimated by measuring the time it takes for the pulse to travel the
distance to and from the object imaged.
Most of the tissues in the body have similar
acoustic impedances; however, air has extremely
low impedance, and bone has extremely high
impedance. This means that there is a big dif-
ference in the acoustic impedance between these
substances and the organs most commonly imaged.
For this reason, both bone and air reflect nearly all
of the sound that reaches them. This is why a coupling gel is used on the skin surface to eliminate
the air gap between the transducer and the skin.
This also explains why imaging through the liver
gives a good acoustic window to deeper structures, but bowel gas obscures imaging lower in the
abdomen. For ultrasonographic imaging of the
brain in a neonate, the anterior fontanel serves
as the acoustic window because the bone of
the skull acts as a reflective surface that limits
through-transmission of US to deeper structures. The mastoid fontanel serves as a window
to the posterior fossa. Bulk fluids within the body,
such as urine in the urinary bladder, bile in the
gallbladder, or cerebrospinal fluid in the ventricles,
have no internal interfaces and therefore are seen
as solid black on conventional ultrasonography.
Cysts have a sharp posterior wall and have increased
through-transmission because the sound wave penetrates the fluid without any reflections to block
transmission of the sound.
Doppler ultrasonography takes advantage of
the physical principle that the US reflection from
a moving object distorts the wavelength, with the
distortion related to the velocity of the object being
measured. This is the principle that causes the pitch
of a train’s whistle to change from high to low as
the train passes an observer. It is the same principle
used by radar guns to monitor the speed of a car or
to measure the velocity of a pitcher’s fastball. In fact,
this same principle is responsible for the “red shift”
observed by astronomers in determining that we
live in an expanding universe. The Doppler evaluation in medical ultrasonography uses the distortion
of the wavelength caused by moving red cells to
identify flowing blood.
One of the major advantages of US imaging
is the lack of ionizing radiation. Although most
diagnostic imaging utilizes low doses of ionizing
radiation, any radiation exposure is a concern and
should be avoided when possible. The portability
of US equipment has made it a valuable adjunct to
diagnostic imaging in the neonatal intensive care
setting.
Clinical Utility in the Neonatal
Intensive Care Setting
Ultrasonography has had a major effect on the
evaluation of the neonatal brain. Most of the
early work focused on intracranial hemorrhage,
which was a common occurrence in preterm neonates. Even though the incidence has decreased,
intracranial hemorrhage remains an issue for
which US imaging is extremely well suited.
Ultrasound equipment has improved tremendously,
and with the addition of color and pulse Doppler
technology, great strides have been made in the
refinement and sophistication of intracranial imaging. Numerous complex structural abnormalities
can be recognized, and screening for developmental
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abnormalities can largely be accommodated with
cranial ultrasonography. Because bone reflects most
of the sound, limiting through-transmission, the
open fontanel is the window to the brain. As the
fontanel closes over time, ultrasonography becomes
less and less useful for intracranial imaging.
Renal imaging offers another major role for
ultrasonography in the neonatal unit. The kid-
neys are well visualized ultrasonographically, either
through a posterior approach or, more commonly,
by using the liver or spleen as soft-tissue acoustic
windows to the kidneys. US is an excellent way
to evaluate hydronephrosis, which is now fre-
quently picked up on routine prenatal evaluations.
US has a role in the evaluation of a jaundiced
patient because it is ideal for evaluating cystic
structures, such as the gallbladder, and can readily
identify dilated biliary ducts. Jaundice caused by
biliary obstruction from a choledochal cyst, for
instance, can be diagnosed readily with US. Because
of the reflectivity of bone and bowel gas, US imaging is much more effective in the upper abdomen,
in which the liver and spleen serve as the acoustic
windows, or in the pelvis, in which the urinary
bladder can function as the window.
Although US is limited by bone, it has a signif-
icant role in the evaluation of the hips in the
neonate. Because the capital femoral epiphysis of the
newborn is cartilage, the hip can be well imaged in a
neonate. Maternal estrogen causes ligamentous laxity.
This changes significantly during the first weeks of
life; therefore, the accuracy of hip ultrasound
examinations improves after the first 3 to 4 weeks
of life. Ultrasonography is very good for the detec-
tion of developmental dysplasia of the hip and can be
used to evaluate the degree of femoral head coverage,
the acetabular angle, and any instability of the hip.
The use of US to provide the localization
of central lines is becoming common practice.
The types of lines frequently used for infants in the
NICU include umbilical venous catheters (UVCs),
umbilical arterial catheters (UACs), and PICCs.
Many complications can occur if the tips of these
lines are not in the correct position, including
pleural effusions, cardiac arrhythmias, cardiac
tamponade, thrombosis, liver hematoma, liver
injury, and portal hypertension. The gold stan-
dard for assessing the position of lines and tubes has
been radiographs of the chest, abdomen, or both.
The use of US has many advantages over radiography, including real-time assessment of the
line; no radiation exposure; the ability to place
and manipulate the catheter under US guidance;
reduced insertion time; and fewer malpositions,
manipulations, and complications.* Bedside US
can be accomplished with appropriate training.
Numerous recent studies have suggested that the
use of US for line placement has good sensitivity
and specificity compared with radiographs.
†
Focused Discussion: Cranial
Ultrasonography
Ultrasonography is an ideal tool for evaluating
the brain in a newborn. In general, an ultrasonographic examination is the first step in the imaging
evaluation for any neurologic question. A routine
screening US is recommended at between 7
and 14 days of age in all premature infants
born under 32 weeks’ gestation and should be
repeated at 36 to 40 weeks of gestational age.32
Structural abnormalities, intracranial hemorrhage,
sequelae of anoxic or ischemic events, and infection are all well assessed via ultrasound. The most
common approach is through the anterior fontanel,
but additional information can be gained with axial
imaging through the squamosa of the temporal
bone. The posterior fossa can be evaluated through
the posterior lateral fontanel. Familiarity with the
normal anatomy is essential. Coronal and parasagittal views are obtained. Normal structures can be
easily recognized; their absence or deformity can
define developmental abnormalities of the brain.
The ventricular size and configuration are assessed.
Characteristic ventricular configurations can define
lobar or semilobar holoprosencephaly. In addition,
the ventricular configuration can suggest septo-optic dysplasia or agenesis of the corpus callosum. The
corpus callosum can be visualized directly; abnormalities of the corpus callosum are commonly associated with Chiari malformation and other structural
abnormalities of the brain, such as Dandy-Walker
malformation. Dilation of one or more of the ven-
tricles can be an indication of a pathologic condition. An obstruction of the flow of cerebrospinal
fluid (CSF) in the region of the Sylvian aqueduct
manifests with a disparity in ventricular size. The
lateral and third ventricles are enlarged, whereas the
fourth ventricle remains normal in size. Dilation of
*
References 2, 10, 12, 16, 17, 18, 19, 23.
†
References 2, 12, 16, 17, 18, 19, 24, 28.
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one of the lateral ventricles, especially when associated with an area of porencephaly, is indicative of an
in utero destructive event. Seizures or apnea may
indicate an anoxic or ischemic event in a neonate. Certain structural abnormalities can suggest a
specific diagnosis; for instance, periventricular nodules and cortical tubers define tuberous sclerosis. US
examination is less sensitive than computed tomography (CT) and magnetic resonance imaging (MRI)
in defining subtle areas of gray-matter heterotopia
or focal pachygyria, examples of developmental
abnormalities associated with seizures. US imaging
is very sensitive to intracranial hemorrhage, and
areas of increased echogenicity can be demonstrated in areas of edema. Intracranial hemorrhage
is an important concern in a premature neonate (Fig.
9.8) and is classified into four grades; each grade is
prognostically significant.34 Grade I hemorrhage
usually has a good outcome, whereas the prognosis with grade IV hemorrhage is frequently poor.
Grade I hemorrhage is confined to the germinal
matrix in the caudothalamic groove. This is
the last fetal germinal matrix to mature and is
prone to hemorrhage in preterm babies. Grade
II intracranial hemorrhage has intraventricular
blood. Grade III hemorrhage is associated with
ventricular dilation as the intraventricular clot
enlarges the lateral ventricles. Grade IV hemorrhage is defined by parenchymal extension. It
has been hypothesized that grade IV hemorrhage
may be the result of venous infarction that occurs
from obstruction of the septal veins by the swollen
germinal matrix hemorrhage. White-matter injury
of prematurity (periventricular leukomalacia) is a
consequence of anoxic or ischemic injury to the
brain that manifests as increased echogenicity in
the deep periventricular white matter of the centrum semiovale. This may progress to cavitation
and is then called cystic leukoencephalomalacia. US
can detect changes of leukoencephalomalacia, which
is usually apparent within 2 weeks of birth.
33
COMPUTED TOMOGRAPHY
CT was initially developed in 1972 in Middlesex,
England, by Electric and Musical Industries (EMI),
an industrial research company. After signing the
Beatles in 1962, the company sold its computer
business; however, it kept a researcher named
Godfrey Hounsfield and funded his independent
research through the revenue generated by the
Beatles’ success. Hounsfield imagined that he could
determine what was inside a box by taking x-rays
of the box at all angles. He then worked to build a
computer that could reconstruct a slice of an object
from the data of these x-rays acquired at various
angles. He shared the Nobel Prize in Physiology
and Medicine with Allan Cormack, who developed
the theoretical mathematics for the invention of CT.
A B
FIGURE 9.8 Coronal (A) and parasagittal (B) ultrasound images from a cranial ultrasound study demonstrate an echogenic clot (arrows)
within the dilated right ventricle. The intraventricular clot with ventricular dilation defines a grade III hemorrhage.
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Early CT scanners produced images by acquiring
x-ray data in multiple positions in an axial plane,
moving to a different level between acquiring
slices. The initial EMI scanner solved mathematical equations representing the attenuation of the
different x-ray beams, like a giant Sudoku puzzle.
The complexity of mathematics involved as well as
limitations in computers at the time contributed to
the limited speed of the scanner; it took approximately 10 minutes to acquire and reconstruct each
slice, and the resolution was limited to an 80 × 80
matrix. For comparison, today’s fastest dual-source
CT scanners can acquire 64 slices in about 0.07
second in the fastest scanning mode with a matrix
of 1024 × 1024.
On modern multidetector helical CT scanners,
image acquisition takes place as the patient is carried on a mobile table through a rotating gantry
containing an x-ray source on the opposite side
of the gantry from x-ray detectors. A ring of data
is acquired as the patient is moved through the
gantry; the path of image acquisition is in effect a
helix, resembling a coiled spring, hence the term
helical or spiral CT. Multidetector arrays of 4, 8, 16,
64, and even 320 elements allow rapid acquisition
of multiple slices in a single rotation of the tube.
This renders high-resolution isovoxel data sets (each
volume element has the same width, height, and
depth). Because the data are acquired in a continuous helix in an isovoxel data set, as if peeling an
apple from top to bottom in one peel, the data can
be reconstructed in any plane as well as rendered in
three-dimensional (3D) format, just as you could
imagine putting an apple peel back together to look
like an intact apple. Reconstruction of the acquired
data has undergone and continues to undergo revisions. In the most commonly employed method,
filtered back-projection lines of gray representing
the attenuation of the patient are layered on top of
each other to create images, with different filters
applied to accentuate different aspects of the image.
This renders a cross-sectional slice that can show
all of the structures within that slice. For instance,
a slice through the upper abdomen may show the
liver, spleen, pancreas, both kidneys, and the spine,
each separated by a plane of fat and each with a
subtly different density.
New technologies on recently introduced CT
scanners include volumetric scanning, dual-source
and dual-energy CT scanners, and technology
designed to reduce the radiation dose to patients
by limiting the patient’s exposure during the
scan or allowing for improved reconstruction of
scans obtained at lower radiation doses. Volumetric
scanning achieved with up to 320 detectors can
allow for an area up to 16 cm to be scanned in a
single rotation of the scanner gantry. Dual-source,
dual-energy scanners can leverage two tubes and
two detector arrays, either for an increased scanning speed of up to 0.07 second for 64 slices with
both tubes at the same energy or improved tissue
differentiation using different energies. Given the
recent attention to radiation effects from CT
in both the medical literature and the lay press,
there has been a renewed focus on technology
to reduce the dose in CT scans. This includes
both improving the mechanics of the CT scanners
themselves as well as developing more advanced
reconstruction algorithms that allow scans obtained
at lower doses to produce excellent-quality images.
These recent advances in CT technology
have resulted in spectacular images and an
explosion in CT utilization. The radiation dose
in CT, however, is significantly higher than
that in routine radiography. CT now accounts
for more than 60% of the radiation exposure
from medical imaging in the United States.
Trailing this growth in the utilization of CT has
been greater understanding of and concern for the
effects of ionizing radiation in medical imaging.
Evaluation of the risk associated with the low doses
of ionizing radiation used in medical imaging is a
complex and evolving topic. The majority of studies estimating risk from low doses of radiation have
been based on models in which survivors of large
known doses of radiation, such as from the atomic
bombs in Japan, have been followed longitudinally
to assess the risk of developing cancer. These models assume that by extrapolating the effects of large
radiation doses to low doses, the risk for low doses
can be assessed in what is called a linear no-threshold model. Although the accuracy and assumptions
in this type of model have been questioned, with
some investigators claiming that there is in fact no
risk, or even benefit, with low doses of radiation,
more recent studies have suggested that there is
a small increased risk of cancer associated with
clinical medical imaging.
5,6,13
A retrospective
cohort study that looked at the incidence of
leukemia and brain tumors in pediatric patients
who had undergone CT demonstrated an association between the radiation received from
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CT scans and leukemia and brain tumors.25
Although there was a significant increase in relative risk suggested in this study, the absolute
risk remained low.
Medical research suggests that the radiation
dose currently used in diagnostic CT is associated
with an increase in the risk for radiation-induced
malignancy. Neonates, infants, and children are
more susceptible to the effects of radiation than
adults. This stems from the fact that for a given
exposure to radiation, the smaller the patient,
the greater is the effective dose because more of
the radiation penetrates the patient. Also, longer
life expectancy puts younger patients at an increased
risk because there is a longer period of time for a
radiation-induced complication to develop.
Ionizing Radiation in Perinatal
Medicine
The effects of ionizing radiation on the fetus
of a pregnant patient also present complex and
important concerns. Although the use of ionizing
radiation generally should be avoided in patients
who are pregnant, there still exist indications for
which the benefit of performing the examination
may outweigh the risk to the fetus. Additionally, on
rare occasions, a CT may have been performed on a
patient who was not known to be pregnant before
the examination. Although a comprehensive analysis of the risk and radiation exposure of a pregnant
patient undergoing CT should be considered in
consultation with a medical physicist, general guidelines have been provided by the American College
of Radiology in conjunction with the Society for
Pediatric Radiology, as briefly described in the following paragraphs:
Before conception, there has been no documented genetically heritable risk in the human
population. In the first 2 weeks after conception,
the only potential risk is felt to be the loss of
pregnancy. Doses associated with radiographic
procedures have not been clearly associated with
increased risk, although this is difficult to determine
because approximately half of all conceptions are
lost in this period, often without recognition of the
loss by the woman.
Radiation exposure between 2 and 15 weeks
after conception has more complex risk implications. In general, radiologic procedures outside of
the abdomen and pelvis, including the head, neck,
1
and chest, should result in only a very low dose to
the fetus from scatter radiation. In a patient who
is known to be pregnant, the study should be
optimized to limit the dose to the fetus even
further. When imaging of the abdomen and pelvis
is indicated or has been performed, consideration of
the risk to the fetus takes on greater importance. In
centers that carefully manage their CT radiation
dose, as do most children’s hospitals, the dose
to the fetus is thought to be below the level
associated with any developmental abnormality.
However, before counseling takes place, verification
of the dose by a qualified medical physicist is recommended. If the dose is determined to be low,
the majority of the risk associated with radiation
exposure to the fetus is a small increased risk
of cancer in later life, and termination of pregnancy would not be indicated. Doses associated
with increased developmental disorders are
uncommon in routine practice and usually occur
in circumstances that have important implications for the pregnancy, such as in patients who
require complex cardiology or interventional
radiology procedures.
The effects of radiation exposure to the fetus
more than 15 weeks after conception are even
smaller, with risk to the developing nervous system
occurring only at very high doses, usually beyond
what would be typically encountered even with
multiple radiology procedures. Therefore, after
2 weeks’ gestation, the predominant concern
with low-dose diagnostic imaging is the small
increase of developing cancer over a lifetime.
Although concern for the deleterious effects
of radiation is important, it should always be
viewed in the context of the patient as a whole.
CT can be a powerful tool in evaluating the
pediatric patient. Caution is the key: (1) image
only when indicated; (2) limit the scan to the
region of concern; and (3) be cognizant of dose
and use as low peak kilovoltage (kVp) and
milliampere-second (mAs) as possible, and use
dose-reduction technologies while maintaining
diagnostic-quality examinations.
Clinical Utility in the Neonatal
Intensive Care Setting
Cranial imaging is the most common use of
CT in most neonatal intensive care settings.
CT adds significant specificity to the abnormalities
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recognized with ultrasonography. Concern about
ionizing radiation and the fact that CT equipment is
generally not portable makes obtaining a CT more
difficult than obtaining a sonogram. CT is more
accurate in assessing the nature of extra-axial
fluid collections and is very helpful in further
defining structural abnormalities of the brain,
particularly those associated with the abnormal
distribution of gray or white matter. It is also
very good for evaluating intracranial hemorrhage
and infection. Exquisite bone detail defines craniofacial anomalies, choanal atresia and stenosis, and
abnormalities of the petrous bone associated with
hearing loss.
Focused Discussion: Computed
Tomography Angiography of the Chest
Chest CT angiography (CTA) is becoming
much more frequent in the NICU. CTA is often
used to evaluate abnormalities detected during
intrauterine US, chest x-ray, and echocardiogram
examinations, such as congenital lung lesions and
potential surgical lesions such as vascular rings identified on chest x-ray films.
Through carefully controlled administration
of intravenous (IV) contrast, CTA can supplement, and in many cases replace, the need for
traditional catheter angiography.15 Uses include
describing the vascular supply to congenital lung
lesions such as bronchopulmonary sequestrations,
defining vascular and airway anatomy for surgical
planning in patients with vascular rings, and surgical
planning in patients with complex cardiac disease.
Congenital lung lesions are a spectrum of developmental abnormalities of the lung that include
bronchial atresia, congenital pulmonary airway
malformations, and bronchopulmonary sequestration, among others. Often suggested on prenatal
sonography and, in some cases, characterized on
fetal MRI, CTA is often used in the postnatal
period to further characterize the lesion and
plan surgical intervention.11 Bronchial atresia can
be diagnosed on CTA when an atretic bronchus
is identified, often in association with a dilated,
mucus-filled segment or “bronchocele/mucocele”
distal to the atretic airway. Although often found
in isolation, studies have identified bronchial atresia
in up to 77% of other congenital lung lesions.20
Cystic pulmonary airway malformations (CPAMs)
have been traditionally defined by the size of the
7,11
cysts within the lesion, with type 1 having cysts
greater than 2 cm, type 2 with cysts less than 2
cm, and type 3 appearing solid; more recent alternative classification systems have been proposed,
and the pathology of type 3 has been disputed.
CPAMs can have a multilobar and rarely bilateral
distribution. CTA can define the cyst size and
distribution as well as provide a roadmap for
surgical planning. Rare malignant potential for
type 1 CPAMs has been reported, as has recurrent
infection. Bronchopulmonary sequestrations were
traditionally defined as portions of the lung with
a systemic arterial supply and then further characterized as either extralobar or intralobar. Extralobar
sequestrations most often appear as a wedge-shaped
mass with systemic arterial supply with a pleural
margin, often on the left and below the otherwise
normally formed lung. Intralobar sequestrations
are thought to be developmental abnormalities of
the lung composed of isolated nonfunctional lung
tissue, sometimes accompanied by cystic change
within an otherwise-normal lobe. CTA provides
excellent identification and characterization of both
the vascular supply and associated parenchymal
change in these lesions.
Although vascular rings are often initially
suggested on chest x-ray or upper GI, CTA and
MR angiography have largely replaced other
techniques in the characterization of vascular
7,14
rings.
CTA has the advantages of easy availability,
improved depiction of the airway and lung parenchyma, and short scanning times, which in some cases
can eliminate the need for sedation. Disadvantages
include the use of ionizing radiation and iodinated
contrast. MR angiography is a viable alternative in
some cases in which the need for a detailed evaluation of the airway and lung parenchyma justifies the
need for sedation.
In embryonic development, the ventral and dorsal aorta are connected by six pairs of aortic arches.
Normally there is regression of some of these arches
to result in the normal left-sided aortic arch. The
sixth arch forms the ductus arteriosus and proximal
right and left pulmonary arteries. Failure of regression or inappropriate development of these primitive vascular structures leads to the development of
vascular rings and slings.
Vascular rings are formed when vessels, either
patent or atretic, encircle the trachea and esophagus, with the potential to result in esophageal
or tracheal compression. The most common
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A B
FIGURE 9.9 Axial (A) and volume-rendered three-dimensional (B) images of a computed tomography angiogram of the chest demon-
strate a double aortic arch with hypoplastic left arch (long arrow, A and B) with atresia of the posterior aspect of the left arch (short arrow,
B). The volume-rendered image (viewed from behind) better demonstrates the spatial relationships of the arches to each other as well as to
the encircled trachea (star).
215
symptomatic vascular ring is a double aortic arch
(Fig. 9.9).14 In most cases the right arch is dominant
and higher than the left, although the arches can
be the same size and one arch can be atretic. The
second most common vascular ring is a right-sided
aortic arch with aberrant left subclavian artery and
left-sided ligamentum arteriosum. Less common
vascular rings include left-sided aortic arch with
aberrant right subclavian artery and right-sided ligamentum arteriosum and right-sided aortic arch with
mirror-image branching and left-sided ligamentum
arteriosus as well as circumflex aortas. In several of
these conditions, the sidedness of the ligamentum
arteriosus or other atretic structures determines
if a complete vascular ring is present. The atretic
structure itself usually cannot be visualized. Instead,
the presence of a dimple on the aorta or pulmonary
artery, diverticulum of the subclavian artery, or position
of the descending aorta on the side opposite the arch
(also referred to as the three “Ds”) determines the
position of the atretic segment.
Ultrasonography remains the first-line diagnostic tool for the evaluation of the kidneys,
liver, and spleen, but when a pathologic condition of the abdomen is a concern and a good
acoustic window for US imaging is not available,
CT is frequently the examination of choice. CT
can be performed with significantly less sedation than that necessary for MRI. CT eliminates
many of the artifacts, including those of vascular
flow, respiratory motion, and even bowel peristalsis,
that limit the utility of MRI. Skeletal lesions are
well visualized with CT. Ultrasonography is the
method of choice in the evaluation of congenital hip dysplasia, but CT can be very helpful
in evaluating the position of the femoral heads
after reduction and treatment of congenital hip
dysplasia when the patient is immobilized in a
plaster cast.
CT can be very helpful in identifying the organ
of origin of a specific pathologic condition.14 This
is, of course, the first step in narrowing a differential
diagnosis. The addition of IV contrast can define
the presence and extent of tumor and infection
or abscess. CT can also evaluate for adenopathy, as
well as the presence of tumor thrombus in renal
arteries and the inferior vena cava, which may
affect the surgical approach to renal and hepatic
neoplasms. The findings on CT often lead to a
specific diagnosis. The multiplanar, cross-sectional
rendition of anatomy, which allows structures to be
distinguished from one another, and the improved
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contrast resolution make CT a useful tool when
clinically indicated.
Focused Discussion: Intracranial
Blood
Noncontrast CT is extremely sensitive and
specific for the detection and localization of
intracranial blood (Fig. 9.10). Blood from acute
hemorrhage has a density on noncontrast CT
(measured in Hounsfield numbers) higher than any
normal structure except bone and calcium. The
increased contrast resolution of CT allows the differentiation of gray matter from white matter and
lends itself to a detailed structural evaluation of the
brain. The ventricles are low in density (0 Hounsfield
units, equal to water), the white matter is denser, and
the gray matter is even more dense still, followed by
blood from acute hemorrhage and, finally, calcium
and bone. Blood from acute hemorrhage is visu-
alized as white as soon as a clot is formed, and
this high density will slowly decrease over time.
For instance, the blood in a subdural hematoma
after 2 to 3 weeks will become lower and lower in
density until it is indistinguishable from water and
CSF. MRI can differentiate blood from a chronic
subdural hematoma for a longer time than CT
because the protein within a chronic subdural
hematoma modifies the signal on MRI for an
extended period. Although all intracranial blood
does change density over time, the compartment in
which the blood is found affects the rate of change
to some extent. Therefore, the timing of an event
responsible for the blood cannot be precisely
determined based on density alone.
The location of the blood is the next issue. CT is
the most accurate imaging method for detection
of subarachnoid blood. The presence of subarach-
noid blood postpartum is common, even after a
relatively nontraumatic birth. Unfortunately, on rare
occasions, subarachnoid blood can cause vasospasm
of vessels near the skull base, which can result in
relative ischemia or hypoperfusion of the peripheral
cortex. Areas of edema can be detected by looking
for the loss of the normal gray–white differentiation
or by finding a focal area of brain edema characterized by relatively low density resulting from the
addition of low-density water to an otherwise-normal area of the brain.
The shape of a collection of blood is important in
evaluating intracranial hemorrhage. Subarachnoid
FIGURE 9.10 A single axial image near the vertex demonstrates a high-den-
sity lenticular mass (arrows) in the extra-axial space over the right cerebral
cortex. The lenticular configuration is that of an epidural fluid collection and the
high-density characteristic of blood from a chronic hematoma.
blood assumes a configuration that follows the arachnoid space. Therefore, it is most frequently seen in the
suprasellar cistern, the ambient cistern, the Sylvian
fissure, or the interhemispheric fissure, or layering
on the tentorium. The most sensitive locations for
identifying subarachnoid blood are in the region of
the quadrigeminal plate cistern, the posterior aspect
of the third ventricle, and the interpeduncular cistern.
Subdural hematomas occur most frequently over the
convexities or along the interhemispheric fissure.
Those over the convexity can be differentiated from
epidural hematomas by their crescentic configuration
as opposed to the lenticular configuration of an epidural hematoma. The dura is the periosteum of the
inner table of the skull; therefore, an epidural hematoma is limited by the adhesion of the periosteum to
the skull and hence the lenticular configuration. This
also explains why epidural hematomas are most often
associated with higher-pressure arterial bleeding and
why subdural hematomas frequently are associated
with venous bleeding. Another key to differentiating
the compartment is the relationship to cranial sutures.
An epidural hematoma will not cross a suture
line because of the anatomic limitation of the
dura by the suture. A similar limitation by the
dural attachment at suture lines helps distinguish
a cephalohematoma from a caput succedaneum.
The direct sagittal imaging of MRI has revealed the
high prevalence of subdural blood in the posterior
fossa.
CHAPTER 9 Diagnostic Imaging in the Neonate
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217
MAGNETIC RESONANCE
IMAGING
Background
MRI is a modality that images protons or hydrogen ions within the body. The rapid development
of magnetic resonance was a result of, in part, the
transfer of sophisticated reconstruction algorithms
used in CT and the computer power developed in
other fields, such as the 3D graphics used in animation, cartography, and seismology. These technologic
advances allow tremendous amounts of information
to be manipulated quickly enough to make image
reconstruction a reality. MRI is essentially hydro-
gen imaging, and because the human body
is 98% water, much hydrogen is available to
image.
MRI is performed by placing a patient in a strong
magnetic field, which varies slightly from the head
to the foot. Each proton acts as a small magnet, and
just as the needle on a compass orients itself in one
direction when placed next to a magnet, the protons
in the body align when placed into a strong magnetic
field. This alignment of protons is essential to create
an environment that has a net electromagnetic field.
Without the alignment of protons by the magnetic
field, the random orientation of protons would have
no measurable net field effect when stimulated and
therefore would create no signal to image.
Once the patient is in the magnetic field, a
radiofrequency (RF) pulse is delivered. In current
imaging systems, the pulse wave has the frequency
of an FM radio wave. Less than 1 in 1 million
hydrogen ions will absorb any energy, and only certain RFs will allow the transfer of energy from the
RF pulse to a hydrogen ion.
An analogy of this energy transfer can be seen
on a schoolyard playground. Visualize a child on a
swing. When pushing the swing in rhythm or resonance with the natural frequency of the motion of
that swing, the swing will absorb the energy, and the
child will swing higher and higher with each push.
This natural rate of harmonic motion depends on
the length of the rope on the swing and the mass
of the swing and the child. If you were to push at
a rate that was not synchronous with the swing’s
natural rhythm, pushing would not allow the energy
to assist in propelling the swing higher and higher,
and in fact, you would disrupt the normal rhythm
of the swing.
In a famous TV commercial for Memorex in the
1970s, the playback of Ella Fitzgerald’s voice caused
a goblet to break, demonstrating the absorption of
resonance frequency by the crystal in the goblet.
The absorbed energy caused the goblet to shatter.
In MRI, the FM RF energy is used to stimulate
hydrogen ions or protons in the body.
Because the field strength of the magnet used
for imaging varies slightly from one end of the
patient to the other and the resonance frequency
depends on the field strength of the magnet, one
can selectively stimulate various locations within
the patient. By changing the RF slightly, a different specific group of protons is stimulated. Protons
stimulated by an RF pulse absorb energy and move
to an unstable higher-energy state. They give up
the absorbed energy as an RF pulse or “echo” of
the pulse they received. The echo is received by an
antenna, just as with a radio receiver, and converted
to an image. The signals or echoes received are the
result of T1 and T2 relaxation times, which are simply physical parameters that describe the environmental interactions that influence the signal released
from a proton. Such spin-echo pulse sequences are
frequently used sequences in routine MRI.
An analogy of a spinning top can help to explain
the T1 and T2 relaxation times that result in spinecho imaging. Each hydrogen ion has a dipole
moment (a positive pole and a negative pole)
and therefore acts like a small magnet within the
powerful magnetic field of the imaging magnet.
These protons spin or precess with a precessional
frequency that is related to the field strength of the
magnet. Electromagnetic energy can be transferred
to these protons if the energy is delivered at the resonance frequency. Once an RF pulse of resonance
frequency is delivered, a small number of protons
will absorb this energy and move to a higher-energy state. The T1 relaxation time reflects the time
it takes for these excited protons to give up their
higher energy and return to baseline.
T2 relaxation times relate to a second parameter of physical interactions. Although the protons
are rotating at a frequency proportionate to the
magnetic field in which they exist, they are not in
phase. In other words, there is no net direction of
polarity from all of these spinning magnets. Once
the RF pulse perturbs or stimulates these protons,
they begin to spin synchronously and therefore create a net magnetic field. This spinning net magnetic
field generates an electromagnetic wave that can be
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picked up by the RF antenna of the imaging system as an “echo” of the original RF pulse delivered.
(The principle of a spinning magnet inducing an
electromagnetic pulse is the basis for the turbines of
hydroelectric generators.)
Because its immediate electromagnetic environment affects each proton differently, these protons
will remain synchronous in their precession for a
very short period. As they move out of phase or
synchrony, the net magnetic field that was created
dissipates; therefore, the signal received by the RF
antenna diminishes. The T2 relaxation time indicates the time it takes the protons to go from a state
of synchronous rotation, when maximal signal is
created, to random, out-of-phase precession, with
zero net magnetic field and hence no signal. The
requirement of a net magnetic field to create a signal
is used to evaluate flowing blood without the need
for contrast. An RF pulse saturates the protons in
the field being imaged. The saturated blood within
the vessels of that field flow out of the field and are
replaced with nonsaturated blood from an adjacent
slice. Consequently, there is no signal from the vessel
containing the blood flowing perpendicular to the
slab being imaged.
Diffusion-weighted sequences have proven to
be very sensitive in defining neonatal pathology.
Diffusion weighting is the most sensitive technique
in the identification of early anoxic-ischemic injury.
Diffusion weighting takes advantage of the random
Brownian motion of molecules in fluid and the
restriction of Brownian motion by anatomic barriers or edema. Water within the ventricular system
will diffuse homogeneously in all directions (i.e.,
no restricted diffusion) and will be a low signal
on a diffusion-weighted sequence and high signal
(white) on an apparent diffusion coefficient (ADC)
map. An acute or subacute infarction, for instance,
will cause restricted diffusion in the affected region
as fluid rushes into cells due to failure of the cells’
homeostatic mechanisms, such as ion pumps. The
restricted diffusion caused by acute cell death will
be a high signal on the diffusion sequence and a
low signal on the ADC map. Late findings after
infarction will demonstrate facilitated diffusion with
a high signal on both the diffusion and ADC map
as the infarcted cells burst and therefore no longer
restrict the movement of water molecules. Diffusion
tensor imaging measures diffusion in at least six
planes simultaneously. From that data, a map of the
magnitude and direction of water movement can
be generated. The axons within white-matter tracts
will allow diffusion in the direction of the axon but
will restrict diffusion in any direction other than
the course of the axon. This allows one to map the
white-matter tracts and has been studied in relation
to developmental abnormalities in the brain, the
relationship of intracranial neoplasm to white-matter tracts, and the plasticity of the developing brain
in response to injury.
The key feature of the physics of MRI is that
images are acquired without ionizing radiation,
which is particularly important in pediatrics. No
known harmful effect of either magnetic exposure
or RF exposure at the levels used in MRI has been
observed. However, MRI is still relatively new,
and one should be cautious in using MRI for
fetal and newborn imaging. Energy deposition is
a concern, and protocols have been established
that limit patient exposure. Another concern is
the effect a magnetic field might have on electronic
instrumentation, such as pacing devices and metallic
surgical clips. The torque on metallic implants can
be quite high, but this is rarely of clinical significance. However, the artifact caused by the disturbance of the magnetic field can be significant. The
most important and real safety concern is that of the
magnetic-field attraction of ferromagnetic material.
Pens, stethoscopes, or even oxygen canisters can act
as projectiles when inadvertently brought too close
to a magnetic field.
The main drawback of current MRI technology is the time it takes to acquire an image.
Motion-free imaging is necessary for optimal
image quality, and because image acquisition in
MRI takes minutes, sedation frequently is necessary. Respiratory and cardiac gating can help for
physiologic motion, but even physiologic motion
can be problematic.
Focused Discussion: Practical
Considerations
The physics of MRI is complex, and multiple
variables influence the signal received (Fig. 9.11).
These influences variably affect the T1 and T2
relaxation times in spin-echo imaging. Imaging
sequences tend to be called T1 or T2 sequences,
depending on which physical parameter has the
most influence on the appearance of the image. A
helpful simplification of spin-echo imaging is that
in T1-weighted spin-echo sequences, fluid is black,
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