Applied Radiology

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.