Readily available in most settings, computed tomography (CT)
maintains its dominant role in the evaluation of patients with
acute stroke. Despite the robust capability of magnetic resonance
imaging, CT is the primary modality used in the triage of patients
in whom thrombolytic intervention is contemplated. CT angiography
(CTA) and perfusion CT have augmented the utility of CT in this
setting, offering information useful in therapeutic planning.
Routine CT
Decisions about the appropriateness of thrombolytic therapy in
patients with acute stroke are based largely on the findings of
unenhanced CT (Table 1). CT assessment remains the gold standard
for detecting the presence of hemorrhage. Despite the superior
sensitivity of diffusion-weighted MRI (DW MRI) to the presence of
acute stroke, lack of immediate, 24-hour availability limits
utilization in most clinical settings. Due to its importance in
therapeutic decision-making, it is critical that the reader is
sensitive to the often-subtle changes that characterize acute
infarction on CT (Table 2, figure 1).
CTA
CT angiography is employed increasingly in the evaluation of
patients with acute stroke to characterize the presence and level
of vascular thrombosis.
1
At many institutions, a central arterial occlusion will be treated
with intra-arterial thrombolytic therapy. CTA studies that are
normal or that indicate the presence of a peripheral branch
occlusion will be treated intravenously or not at all.
CTA studies are feasible at all hours using modern operator
interfaces that allow rapid, semi-automated, online creation of
overlapping limited volume maximum intensity projections (OLIVE
MIP), also known as sliding MIP or MPVR (Figures 2 and 3). Without
requiring close physician supervision, the scanning technologist
can render these images in minutes while not interrupting the flow
of patient scanning in busy departments. Automated volume rendering
engines available with some CT systems add ready access to
three-dimensional depiction.
Perfused blood volume
Perfused blood volume (PBV) studies can be created at the
scanning console from the source images used to create the CT
angiogram. CTA source images are reformatted (if necessary) into
3-mm slices at 1.5-mm intervals and viewed at windows and levels
typical for routine head CT. These images of the "blood pool" are
more sensitive to the presence of infarcted brain tissue than
unenhanced CT (Figure 4) and correlate better with eventual infarct
size.
2
Perfusion imaging
Perfusion studies are obtained by monitoring the first pass of a
standard iodinated contrast agent through the cerebral vasculature
(Figure 5). The contrast bolus causes a transient rise in
attenuation proportional to the amount of tracer in a given region
(Figure 6). Integration of data over the first pass of the contrast
agent allows creation of maps of brain perfusion. Commercially
available perfusion CT software provides information about
parameters such as cerebral blood volume (CBV), mean transit time
(MTT), and cerebral blood flow (CBF) in a clinically feasible time
frame. Determination of CBV is augmented by normalization for the
pixel value of venous blood. The MTT calculation uses a complex
deconvolution algorithm to adjust for the finite arrival time of
arterial blood (Figure 7). The net result is an accurate, xenon
equivalent, calculation of CBF (CBV/MTT) processed in a nearly
fully automated fashion on the scanner console.
3,4
Perfusion techniques are utilized for the evaluation of acute
and subacute stroke, offering the most sensitive measure of the
extent of brain tissue under ischemic conditions. In the acute
setting, the deficit on a perfusion study is often greater than
that seen on studies such as unenhanced CT, PBV CT, and DW MR
imaging. A reduction in CBF is a typical accompaniment of acute
stroke. Depending on severity, this will manifest as a compensatory
increase or a resultant decrease in CBV, as well as a regional
prolongation of MTT and time to peak (TTP). Outside of the setting
of acute stroke, perfusion imaging can yield useful information
about the functionality of collateral circulation and thus the
significance of vascular occlusive disease (Figure 8).
Quantitative assessment of CBF yields information about brain
tissue viability and hemorrhagic risk, which is critical in
thrombolytic therapy decision-making. Subtracting the volume of
brain with restricted diffusion from the perfusion-indicated volume
of tissue under ischemic conditions yields the commonly accepted MR
paradigm for tissue at risk for extension of infarction. The CT
parameter that best estimates infarction volume is not clear and is
under active study. It is possible that the deficit on PBV CT or
the region of uncompensated reduction in CBV may best correlate
with the volume of infarcted brain.
Summary
CT maintains its preeminent role in the evaluation of patients
with symptoms suggesting acute stroke. CT angiography, perfused
blood volume CT, and, recently, first pass perfusion CT techniques
have significantly augmented the role of CT for these patients by
offering structural and functional information critical in
therapeutic decision-making. The multitasking, automated
functionality of modern, user-friendly scanner interfaces
facilitates performance of advanced postprocessing techniques even
in the busiest department.
REFERENCES
1. Tanenbaum LN, Verro P, Borden NM, et al. The role of CT
angiography in acute ischemic stroke: A prospective study. Poster
presented at the 52nd Annual Meeting of the American Academy of
Neurology. April 29-May 6, 2000; San Diego, CA.
2. Verro P, Tanenbaum LN, Fonzetti P, et al. CT perfusion
deficits in acute cerebral ischemia predict brain at risk.
Presented at the 52nd Annual Meeting of the American Academy of
Neurology. April 29-May 6, 2000; San Diego, CA.
3. Cenic A, Nabavi DG, Craen RA, et al. Dynamic CT measurement
of cerebral blood flow: A validation study.
AJNR Am J Neuroradiol
.
1999 20:63-73.
4. Nabavi DG, Cenic A, Craen RA, et al. CT assessment of
cerebral perfusion: Experimental validation and initial clinical
experience.
Radiology
. 1999;213:141-149.
TECHNICAL ISSUES
Multislice CT Urography: Analysis of Technique
Jeffrey D. McTavish, MD and Paul M. Silverman,
MD
CT is widely used in the radiologic evaluation of the kidneys
and urinary collecting system. It has almost completely replaced
intravenous urography (IVU) in evaluating renal masses, infection,
trauma, and stone disease. However, the initial image-based
evaluation of the hematuria in many centers still relies on IVU.
This is because, until recently, a CT technique for the evaluation
of the urothelium had not been satisfactorily developed.
Technical Advances with MDCT
The primary reason that CT has not replaced IVU for evaluation
of the urothelium is its lower spatial resolution, particularly in
non-axial planes. Multidetector CT (MDCT) offers many technical
advances that could enhance the capability of CT to evaluate the
urinary collecting system. The ability of MDCT to acquire multiple
channels of data simultaneously permits a thinly collimated
acquisition to be obtained through the entire abdomen in a single
breath-hold. This results in near isotropic voxels and improved
spatial resolution, relative to conventional spiral CT, in
non-axial planes. When acquired during the excretory phase, the
entire collecting system and ureter can be imaged with one
acquisition.
MDCT Urography: Spatial Resolution
To optimize spatial resolution, collimation should be as narrow
as possible, in the range of 1 to 1.25 mm. This is obtainable using
most current MDCT scanners in a single breath-hold lasting 30 to 35
seconds. This allows thinner reconstructed slices than was possible
with conventional spiral CT for improved in-plane resolution and
multi-planar reconstructions.
MDCT Urography: Opacification
As with conventional IVU, visualizing the intrarenal collecting
systems (IRCS) and ureters is dependent on opacification and
distention. A fundamental problem in CT urography is that, due to
peristalsis, it is difficult to obtain a single acquisition during
which all segments are opacified and distended. A multiple
acquisition approach is not feasible with CT urography due to the
high radiation dose that would result.
Many techniques have been described that attempt to optimize
imaging the opacified ureters and collecting system with CT
urography. The effect of a supplemental saline infusion following
the injection of contrast medium has been evaluated, as this
increases the volume of fluid presented to the urinary collecting
system in an attempt to improve distention.
1
This technique has demonstrated improved opacification of the
distal ureters.
The effect of patient position on opacification has been
examined, though the reports are conficting. An early study, using
conventional spiral CT, found that the prone position resulted in
improved opacification of the distal ureters, which are the most
difficult ureteral segment to opacify.
2
A later report, using MDCT, did not confirm this benefit of prone
imaging.
1
Other methods of CT urography have been described, including a
two-part CT acquisition, with upper tract imaging using an
abdominal compression device, and lower tract imaging after the
device is removed.
2
MDCT urography has also been described using a special CT table-top
apparatus that can obtain both nephrographic-phase CT images and
pyelographic-phase radiographic images without moving the patient.
3
However, this special apparatus is not commercially available at
this time.
Study Interpretation
There are many display methods used in the interpretation of CT
urography. Studies can be interpreted using the source axial images
alone, which can be time-consuming due to the large number of
images. Furthermore, as the ureter is primarily imaged in
cross-section, subtle areas of narrowing and unopacified segments
are difficult to appreciate. This problem can be overcome by using
multiplanar reconstruction (MPR), which displays axial, sagittal,
and coronal planes simultaneously. Using this method, the optimal
plane can be chosen for each segment of the urinary collecting
system and ureter. A third method of display is the maximum
intensity projection (MIP), which can display the entire urinary
collecting system and ureters in a single image. This technique
requires some manual post-processing to remove bony structures, and
may overestimate areas of narrowing and fail to identify areas that
are faintly opacified. A final display method is volumetric
reconstruction. This method offers many advantages, such as a
single image display, good sensitivity to faintly opacified
segments, and reliable depiction of narrowed segments. The primary
drawback to this display method is the manual post-processing
required, which can be time-consuming.
Summary
While technical advances have led to CT being the examination of
choice in most anatomic renal pathology evaluation, the
satisfactory evaluation of the urinary collecting system with CT,
other than for stone disease, has been elusive. Utilizing MDCT
urography, multiple techniques have been described that will
display the opacified urinary collecting system reliably. Due to
its unique ability to evaluate both the renal parenchyma and
urinary collecting system, CT urography may provide a "one-stop"
evaluation of the kidneys and urinary collecting system. Further
study is warranted to evaluate its sensitivity and specificity in
the detection of urothelial lesions.
REFERENCES
1. McTavish JM, Jinzaki M, Zou KHS, Silverman SG. Multidetector
CT urography: Analysis of techniques and comparison with IVU
[abstract]. Radiology. 2000;217(P):225.
2. McNicholas MM, Raptopoulos VD, Schwartz RK, et al. Excretory
phase CT urography for opacification of the urinary collecting
system. AJR Am J Roentgenol. 1998; 170:1261-1267.
3. Vrtiska TJ, Rochester MN, King BF, et al. CT urography:
Description of a novel technique using a uniquely modified
multidetector-row CT scanner [abstract]. Radiology.
2000;217(P):225.
PRACTICAL ISSUES
The Utility of an Iso-osomolar Contrast Material for CT
Angiography: A New Application
Robert D. Bloch, MD; Eric Hoffer, MD; Theodore Dubinsky,
MD; Matthew Vaughn, MD; Glen Ross, RT
As CT evolves to new levels of speed with multidetector scanning
and CTA finds its place in the diagnosis of vascular disease, it
becomes apparent that the quality of examinations (i.e., their
sensitivity and specificity) will drive the future uses of CT in
vascular imaging. The parameters we can modify presently are the
collimation of the beam and table pitch, the optimization of
contrast timing (scanning at peak contrast bolus), and the
increasing ability to rescan large areas while contrast is still
present. In this milieu of new CT protocols, an improvement may
have been missed.
Nonionic iso-osmolar contrast has been available for a number of
years now, being driven by the idea that it may be less nephrotoxic
(with less osmolar shock to the kidney). The original intent of
this contrast was to minimize patient pain, especially during
angiographic runoff angiograms where the poor circulation of the
patients made this exam excruciating. As no advantage for CT
scanning was shown, Visipaque (Amersham Health, Princeton, NJ) was
rarely used in this arena. However, on a theoretical level, there
are several aspects of this type of contrast that make it a
possibly ideal vascular CT agent:
1. The iso-osmolarity lends to less fluid shift in the first
pass through the circulation and may lead to a higher retained
density on subsequent circulatory passes.
2. The extremely high viscosity of Visipaque compared with other
agents should lend itself to more sluggish flow on both arterial
and venous sides.
Together, these two properties should lead to a prolonged first
pass through the circulation.
3. In addition, a longer peak phase should be more "forgiving"
for the technologist who mistimes the peak of the contrast, and
should allow scanning of longer distances in the body, or allow
higher resolution scanning at a greater branch order of detail in
small areas, compared with other agents.
We have seen the following:
* Better branch-order resolution for pulmonary embolism (PE) in
peripheral pulmonary branches. We believe Visipaque improves the
level to which we can exclude PE with confidence by one branch
order. This study is ongoing (Figure 1).
* In pulmonary CTA, there is retention of 40% of the peak
intravascular contrast density after 3 minutes with 180 cc of
Visipaque administered intravenously. The same dose of Omnipaque
(Amersham Health, Princeton, NJ) results in retention of only 25%
of peak contrast. This implies a longer circulatory half-life of
the Visipaque "density," which results in longer vascular
opacification (Figure 2). Our preliminary work also shows slower
opacification in vascular masses and tissues. If this is confirmed,
Visipaque has multiple characteristics for an ideal vascular
contrast agent.
Other findings we have noted:
1. Aortic endografts and endoleak identification: Evidence is
preliminary but we are noting small endoleaks at a higher
frequency, especially from lumbar branches, and we have seen no
aneurysms grow that didn't have identifiable endoleaks on a
Visipaque CT scan, which is an issue in current follow-up (jet on
aneurysms that grow no endoleak). Feeding branches to Type II
endoleaks are identified and seen on a 3D workstation as a
continuous vessel whose origin can be defined when Visipaque is
used, which allows preprocedural planning for the interventionalist
(Figure 3).
2. Lumbar and intercostal arteries (2- to 3-mm in size) are
being imaged consistently (Figure 4).
3. Accurate depiction of small accessory arteries in
preoperative CTA is improved, probably due to the longer
intravascular phase of the contrast (Figure 4).
4. Preoperative aortography for stentgraft planning sometimes
provides better information than angiography, including
identification of whether the inferior mesenteric artery flow is
from the superior mesenteric artery (origin occluded) or is from
the origin of the inferior mesenteric artery from the aorta (Figure
4).
5. Cerebral aneurysms are being imaged consistently, including
3rd and 4th order intracranial vessels (Figure 5).
Future uses of Visipaque as a longer-acting contrast might
include:
1. GI bleeding--with fast scanning and repeat scanning, the
addition of Visipaque might permit CT scans for GI bleeds to become
a reality. The currently accepted sensitivity for bleeding is 0.1
cc/minute for red-cell scanning, and 1 cc/minute for angiography.
The sensitivity of CT scanning with Visipaque might well be between
these two values.
2. Assessing the microvasculatity of previously considered
"avascular" masses.
3. Assessing for DVT after pulmonary CTA.
Conclusion
While the improved CT quality we have seen can not be attributed
to low osmolar nonionic contrast alone, when taken in conjunction
with technological improvements in multidetector CT, this
represents a phenomenal step forward in CT vascular imaging. This
portends a major role for CT in the future of diagnostic vascular
imaging as well as preprocedural planning for the interventionalist
and the surgeon.
Medical Economics
HIPAA + Technology = Randall Lindner and Paul M.
Silverman, MD
Technology has always seemed a threat to privacy. When cameras
appeared in the late 19th century, Americans panicked over the
potential hazards they posed to personal space. In response, Samuel
D. Warren and Louis D. Brandeis published "The Right to Privacy" in
the
Harvard Law Review
in 1890.
1
Today, the Internet and the World Wide Web have renewed interest
in privacy. After all, society dubbed the computer sector
"information technology" for a reason: computers and the Internet
that connects them have been designed to store, process, and
exchange information. This capability makes the public rightfully
antsy.
In 21st century America, many others are trying to define and
protect privacy. The Health Insurance Portability and
Accountability Act of 1996 (HIPAA) mandated regulations that govern
administrative simplification standards for health care
information, standardizing all health care data and making many
administrative and financial transactions electronic.
2
When Congress produced this Act, it felt uncomfortable digitizing
all this information. Wouldn't throwing this data into linked
computers endanger an individual's right to privacy?
Well, yes. So Congress mandated that individually identifiable
health data should be private and secure. By June 2003, healthcare
companies must be compliant with HIPAA's rules; however, the
Department of Health and Human Services and the Health Care
Financing Administration are still defining some of these
rules.
Undeniably, this promises a hassle. Digitizing all healthcare
records while ensuring their privacy and security is a monumental
task. In addition, making information digital for ease of
transmission while keeping it private and secure sounds paradoxical
and perhaps illogical, especially when seen in the light of modern
techno-thrillers such as the film
The Web
, in which the main character's identity is stolen by very, very
clever hackers. Rumormongers would have us believe that anyone who
knows information technology, or knows someone who does, can access
any computerized information.
Don't believe the hype. Congress's Act will prove to be a lot of
work, but companies won't toil in vain. Encryption technology has
developed alongside computer technology; digital information may be
more easily accessible than its paper cousin, but it's also more
secure. Hand-held medical-procedure ordering devices, PACS systems,
e-mailed medical correspondence, and payment information can be
secured more safely than their nondigital counterparts. Radiology
films sitting in storage libraries or automobile trunks were never
safe; companies reported losing thousands of films each year. Nor
were film-storage medical management systems secure: few such
storage areas were restricted-access.
With security protection, the single data format mandated by the
Act will pave the way for electronic medical records. Ultimately,
the benefits of HIPAA overshadow the hassle. Anyone would be
hard-pressed to deny the advantages of having all medical
information digital and in a single data format: decreased
film-storage costs, immediate data accessibility, and no accidental
loss of medical information.
In addition, HIPAA compliance facilitates a higher level of
customer satisfaction. Smart healthcare companies will take the
handhold to technology that HIPAA compliance offers and turn it
into a slew of thrilled customers through what
The Journal of Healthcare Information Management
called "consumer empowerment requirements." The journal recommended
that companies provide the following services with their
HIPAA-enabled IT abilities
3
:
* Patient-generated, online medical histories available to
providers, given patient authorization
* Documented patient encounters
* E-mail use and documentation
* Audio and video features
* Prescription refills
* Access to customized medical information to help the patient
understand and make informed health decisions
* Personalized Web page with customized topics for signed-on
patients
* Memory jogger upon sign-on for next appointment, shots,
etc.
* Payment authorization liked to third-party payers
* Physician consultation
* Appointments and referrals
* Timely information sent to primary care provider when patient
sees a consultant
* Consumer ability to "push" physicians current articles on
research, drugs, treatment, and diagnosis
Conclusion
Humankind has been driven to improve itself and its way of life
since the first opposable thumb grasped a writing implement and
jotted a note. Technology--even of that primitive sort--empowers
and enables. Although HIPAA might have seemed daunting initially,
these fears, while not unfounded, are easily assuaged. The
healthcare industry should replace concern with excitement:
HIPAA-induced technology has endless business possibilities.