Maximilian F. Reiser, MD

Since he was elected president of the European Society of Radiology in March, Prof. Maximilian F. Reiser has worked hard on expanding ESR's global reach and implementing new training and accreditation initiatives. In this question-and-answer article, he reflects on his first six months in office, as well as his future challenges.

Diagnostic Imaging Europe: What do you consider to be your main achievements so far? 

 I have continued the work of previous presidents in expanding the membership base of the ESR. As of Sept. 28, 2010, we have 50,635 members, and the ESR has developed into an international society. Over the past year or two, particularly rapid growth has come from Central and Eastern Europe, South America, and China, Korea, and Japan.

The European Diploma in Radiology will swing into action by ECR 2011. It has not been a straightforward process because so many different aspects are involved, and we have had to take account of various national sensitivities. But we have had the full support and endorsement of subspecialty societies, which has been valuable. The European Diploma will run alongside national diploma schemes. Mutual recognition may be possible, but will not happen for a while yet. This would help the migration of radiologists between countries.

Strengthening oncologic imaging has been another top priority. We are collaborating with the RSNA for the development of oncologic imaging as a subspecialty, and the plan is to run the same course at both ECR and the RSNA meeting. We are also participating in the EU Action Against Cancer.

 In terms of ESR membership, we are still underrepresented in North America and Mexico. We are trying to reach out to these areas by making new contacts with radiological leaders and have a presence at the 25th Interamerican Congress of Radiology, to be held in Santiago de Chile at the end of October. We'd also like to have closer connections with other societies such as the European Society of MR in Medicine and Biology. We have not had official contact with the ESMRMB in the past, but that is now happening. We also want to make more progress in resource-scarce countries, including those in central Africa.

DIE: What actions are you taking to ensure that radiology continues to prosper in the future?

 We are trying to change how radiology is perceived by the public and by policy decision makers. The public needs to recognize that radiology is a separate clinical discipline in its own right. In addition, we must ensure patients gain a better understanding of radiation risks. To do this, we are running public campaigns on radiation exposure and training schemes for radiologists so they are fully aware of radiation protection and can explain the issue. We also want to show medical students that radiology is a fascinating discipline with a bright future.

At the EU level, the ESR's lobbying efforts are becoming increasingly effective, and they are bringing results. This is shown by the European Parliament's decision first to postpone and now amend the directive on electromagnetic fields, which, in its original form, posed a potential threat to clinical MRI. We have also been involved in European e-health and telecommunications initiatives, including the Cross-Border Healthcare Directive. All legislation should be workable for radiologists, and maintaining the quality of examinations is our main concern.

Lobbying takes a lot of time and resources, but to develop a strategy and monitor the situation closely, we have formed a task force group headed by Prof. Christian J. Herold, ESR's immediate past president. We want to make sure that radiology's voice is heard and we are getting accurate information right out of the horse's mouth!

DIE: What do you regard as the three most important emerging, long-term trends?

 First, I think molecular and functional imaging will become more important-and not just for making a diagnosis. However, we need to express results better and in a more quantitative manner. For instance, it is essential to show not only a stenosis, but also other findings. The liver has specific quantitative characteristics, and they must be displayed. New imaging biomarkers will play a role in this area.

Second, there will be less emphasis on specific organ systems and more stress on diseases like diabetes, atherosclerosis, and rheumatic disorders. This will require a different approach, but we must still have a high level of expertise in specific organs. I think we will need a multidimensional matrix that includes specialization in different organ systems and cross-sectional organ systems according to diseases.

Third, training, integration, and collaboration will be even more significant. For example, we are aware of imaging being used more often in the operating room, and we must be involved in that process. We must do everything we can to preserve the identity and value of radiology.

Articles

Since he was elected president of the European Society of Radiology in March, Prof. Maximilian F. Reiser has worked hard on expanding ESR's global reach and implementing new training and accreditation initiatives.

ESR president unveils his vision of radiology's future

Acute stroke is a medical emergency that is potentially treatable. Because "time is brain," however, the imaging workup must be fast and therapy initiated rapidly.

Images should first be checked to exclude signs of hemorrhage. Practitioners then need to visualize any early ischemic changes, demonstrate the presence of hypoperfusion, locate any underlying vascular pathology, and elucidate the presence of a potential penumbra.

 The presence-or absence-of vessel occlusion is particularly important if the patient is to be treated with recombinant tissue plasminogen activator (rtPA).

A variety of imaging techniques can be used to assess irreversible damage to the infarct core and to evaluate tissue at risk of infarction. These include diffusion- and perfusion-weighted MRI, MR angiography, unenhanced CT, CT angiography, and perfusion-weighted CT (Figures 1 and 2).

MRI is, theoretically, the method of choice for stroke imaging. It can provide precise information about cerebral perfusion, the size and territory of the acute infarction, microangiopathic changes, and the status of cerebral vasculature throughout the entire brain without exposing patients to ionizing radiation. It is not, however, used as the standard modality for acute imaging examinations in the majority of stroke centers. This is due to the limited availability of MR systems, the relatively long time that the scanner would be occupied, noncompliant patients, and various contraindications, such as cardiac pacemakers.

CT typically plays the predominant role in acute stroke imaging. The time needed to complete a CT examination is very short, scanners are more widely available, fewer contraindications must be considered, and patient monitoring is easier. Economic considerations also favor CT.

Concerns over the link between nephrogenic systemic fibrosis and gadolinium-based MR imaging contrast agents

 and the appearance of studies demonstrating no or low incidence of contrast-induced nephropathy following CT stroke imaging with iodinated contrast

 suggest that CT will play an even more important role in stroke imaging in the future.

Information about at-risk brain tissue requires that data on cerebral blood per-fusion be obtained. The most common approach is to acquire a dynamic scan series during contrast injection. This can be achieved with either CT or MRI. The following parameters can then be calculated and mapped:

Cerebral blood flow (CBF): volume of blood flow per unit of brain tissue per minute; normal range in gray matter = 50 to 60 mL/100 g/min.

Cerebral blood volume (CBV): volume of blood per unit of brain tissue; normal range = 4 to 5 mL/100 g.

Mean transit time (MTT): time difference between arterial inflow and venous outflow.

Time to peak (TTP): time between contrast injection and attenuation peak.

It is usually sufficient to calculate either MTT or TTP, not both. The choice will depend on the algorithm used; deconvolution algorithms provide both parameters but need more user interaction and calculation time, while maximum-slope algorithms are less timeconsuming but provide only TTP values.

Regional changes observed in these maps can be used to predict infarction. The penumbra would be expected to show one of the following:

Increased MTT with moderately decreased CBF (>60%), combined with normal or increased CBV (80% to 100% or higher), secondary to autoregulatory mechanisms; or

Increased MTT with markedly reduced CBF (>30%) and moderately reduced CBV (>60%).

Infarcted tissue, on the other hand, should show a significant decrease in both CBF (< 30%) and CBV ( < 40%) together with increased MTT and loss of autoregulatory mechanisms, which show a vasodilatation (increase of CBV) to compensate for decreased bloodflow.

The perfusion CT parameter that most accurately describes tissue at risk of infarction in cases of persistent arterial occlusion is the relative MTT (optimal threshold 145% compared with the healthy hemisphere). The parameter that most accurately describes the infarct core on admission is the absolute CBV (optimal threshold 2 mL/100 g/min).

The main restriction associated with perfusion CT is the limited volume of coverage obtained by sequential scanning. This depends on the width of the CT detector, which typically ranges from 2 to 4 cm.

Scans of pseudo-axial volumes (images oriented orthogonally to the long axis of the head, which is not necessarily parallel to the scanning axis) at the level of the basal ganglia allow vascular territories around the anterior, middle, and posterior cerebral arteries to be assessed in one volume. Smaller, isolated infarctions superior to the basal ganglia will not be visualized.

Advances in CT hardware are now helping to extend the coverage of brain perfusion studies. Toshiba has created a CT system with a much wider detector (15 cm) and 320 detector rows. Other vendors use a toggling table to increase scan coverage. Both of these technologies permit whole-brain perfusion imaging. The high cost of these high-end scanners, however, means that access is currently restricted to a small number of radiology departments.

An alternative to dynamic perfusion CT is the semiquantitative analysis of perfused blood volume (PBV). Developments in postprocessing software systems are enabling 3D PBV to be calculated semiautomatically for the entire brain.

Relevant data are acquired from existing unenhanced CT and CTA images following a modified stroke CT protocol.

 The software registers the volume data set of both scans automatically and performs 3D corrections to obtain identical head positions. The brain parenchyma is enhanced by subtracting unenhanced CT data from the CTA data. A color scale is added to the absolute parenchymal enhancement after subtraction.

An important advantage of this technique is that no additional scanning or injection of contrast agent is required for the PBV calculation. The maps can be constructed from unenhanced CT and CTA data alone. The resulting CT stroke protocol will have a lower radiation dose, require less time for imaging, and involve smaller amounts of contrast.

The major limitation of PBV imaging is that no dynamic information is available. An estimation of at-risk tissue is not feasible without access to CBF and MTT maps. PBV mainly allows visualization of irreversibly infarcted brain parenchyma. Another limitation of the PBV protocol is the restriction of scan length to strictly intracranial CTA volumes. Visualization of the extracranial, supra-aortic vessels is not possible with the current version of the software.

PBV could potentially serve as a surrogate marker for ischemia in a multimodal stroke CT protocol. Dynamic perfusion CT, on the other hand, would identify tissue at risk of infarction. A combination of both methods might increase the detection rate of ischemia and allow better stratification of therapy in the future (Figures 3 and 4).

CT angiography data offernew approach to perfusion

Noninvasive cardiac imaging is gaining widespread acceptance. Both CT and MRI can determine the absence or presence of coronary artery disease accurately and reliably. This is done by either assessing the coronary artery morphology or by offering detailed insight into functional aspects and myocardial perfusion.

Cardiac MRI has also demonstrated its potential in the assessment and differential diagnosis of inflammatory disease, storage disease, and cardiomyopathies. Storage disease is related to metabolic disorders and/or accumulation of excessive metabolic products (e.g., Fabry's disease, amyloidosis, hemochromatosis).

Information from CT and MRI can also help in the assessment of cardiac masses. It is rare for referring physicians to request a CT or MRI examination solely for the assessment of a primary or secondary cardiac neoplasm. Confirmation or exclusion of cardiac thrombi by imaging, however, is of growing interest. Both may be requested after an initial echocardiographic screening.

The parameters needed for cardiac CT and MR imaging in visualizing tumors differ little from those required in other clinical applications, such as imaging of coronary artery disease.1,2

For cardiac CT, the extremely high spatial resolution used to assess coronary pathology is not mandatory in tumor imaging. Using wider collimations (1 to 2 mm) or reconstructing thicker slices (2 to 3 mm) could make it possible to lower the radiation dose substantially for diagnostic assessments. The majority of CT referrals, however, are intended to confirm, locate, differentiate, or simply exclude a potential cardiac mass and to provide detailed information on vessel structures and vascular anatomy near a confirmed mass for therapy planning. The applied scanning protocol consequently usually matches a coronary CT angiography protocol.

Modern cardiac CT can include unenhanced scanning and delayed imaging, as well as first-pass CTA. The former two are used to delineate potential mass calcifications and the latter to assess lesion uptake of contrast in relation to surrounding myocardium.

Addition of one or both of these techniques to the standard protocol will raise patients' overall radiation burden. Exposure can be lowered by using a moderate slice thickness (2 to 3 mm). Delayed scanning is typically initiated five to seven minutes after contrast injection.3 Consideration of possible dose-reducing strategies, including ECG-related dose-saving strategies or even prospective ECG-triggered modes, should be a prerequisite.

The regimens used for contrast application should be revisited when imaging cardiac masses on CT. The latest generation of scanners can cover the heart in just five to 12 seconds. Contrast injection is also tailored to optimize delineation of the coronary artery tree.

The depiction of masses within the right heart is hampered by the use of dual-syringe power injector systems and saline flushing, which leave little or no enhancement in the right atrium and right ventricle. Modern injection systems avoid this limitation by allowing for an individual dilution of contrast agents at arbitrary values. Sufficient contrast for mass delineation within the right atrium and ventricle can be achieved by flushing the initial contrast agent bolus with a contrast agent/saline mix (20% to 30%/80% to 70%) instead of pure saline. Inflow of unenhanced blood from the inferior vena cava still may mimic a cardiac mass within the right atrium. Therefore, patients suspected of having right-sided masses should undergo MRI instead.

Cardiac MRI offers a far wider variety of techniques for the investigation of tumors than does cardiac CT. Cardiac morphology is depicted on MRI with outstanding soft-tissue contrast. MRI can additionally assess tumor function by providing information on perfusion and the distribution of contrast. Proper planning of imaging protocols is strongly recommended.

A substantial part of cardiac mass evaluation on MRI can be performed without gadolinium contrast. Unenhanced images demonstrate excellent soft-tissue contrast and intrinsic flow contrast for blood signal delineation and suppression. Any cardiac MRI examination requested for mass delineation should start with a dedicated and precise morphological overview that covers the entire heart. Fast spin-echo (FSE) techniques, such as HASTE, or single-shot gradient-echo (GRE) techniques, such as steady-state free precession (SSFP), may be applied to reduce examination time.

Initial morphological screening may be followed by more dedicated techniques for soft-issue differentiation, such as T1- and T2-weighted FSE sequences. These single-slice techniques should be restricted to suspicious areas or masses already identified. T1-weighted slice positions need to be repeated after contrast injection to check for contrast uptake and to delineate possible regions of necrosis.

Flow artifacts may mimic cardiac masses even in black-blood prepared FSE. Gradient-echo techniques may provide further information, while cine SSFP techniques may show the flow patterns. The latter technique also provides detailed insight into mass-based functional impairment, such as in- or outflow obstruction, as well as global and regional myocardial function. The use of parallel imaging makes it possible to use multislice cine approaches.4

Plain FSE techniques are helpful in differentiating soft-tissue components of tumors. Contrast-enhanced techniques give further information on tumor vascularization, delineation, and potential invasion of adjacent structures, such as the pericardial space. Additional information can be provided by techniques that are used routinely in the investigation of coronary artery disease, such as myocardial perfusion imaging and delayed-enhancement imaging. The perfusion technique can be applied during contrast injection and does not interfere with any other postcontrast technique.

Dedicated postprocessing demonstrates any mass contrast uptake that is present (Figure 1). Delayed-enhancement GRE sequences give an overview of mass extent.

Cardiac tumors may be classed as primary neoplasms or secondary neoplasms. Primary tumors originate within the heart and are mainly located there. Secondary tumors are most commonly due to metastasis or extension of malignancies located outside of the heart. Primary tumors are far less common than secondary cardiac neoplasms. Autopsy studies have demonstrated a prevalence of 0.0017% to 0.33% for this type of cardiac lesion.5 Secondary tumors are reported to occur 20 to 40 times more often than primary cardiac neoplasms.5,6

Secondary tumors are almost exclusively malignant lesions that affect the heart through metastatic or direct spread, while the majority (over 60%) of primary cardiac neoplasms are benign.5 Neither MRI nor CT will be sufficient for the ultimate diagnosis of a cardiac tumor, though features seen on imaging may support a differential diagnosis.

The patients' age, history, and symptoms; the site of the mass; and specific imaging features will all contribute to the final diagnosis. CT and MRI can help determine whether a mass's location is intracavitary, intramyocardial, pericardial, or paracardial. Differentiation of masses into atrial or ventricular and distinction between left- and right-sided locations may also assist with the final differential diagnosis.7

Cardiac myxomas, for example, may be identified from their typical imaging features and tendency to be found in certain sites. This benign tumor entity accounts for 30% to 35% of all primary cardiac tumors and for approximately 50% of all benign primary cardiac masses.5 Among these tumors, 95% are located within the atria (75% left-sided). They are typically attached to the oval fossa with a stalk (Figures 1A and 2). Myxomas may rarely occur at multiple locations or be part of the Carney complex.8 Prolapsing myxomas may impair ventricular filling. Myxomas exhibiting these typical features can usually be diagnosed on multislice CT or MRI, with the latter modality providing additional information on mass enhancement (Figure 3).9

Papillary fibroelastomas are the second most common benign cardiac tumor and are typically revealed by echocardiography. Most lesions are smaller than 1 cm in diameter and attached to the fast-moving cardiac valves (90%).10 Multislice CT and MRI play a smaller role in the diagnosis of this tumor entity.

Rhabdomyomas and fibromas each account for approximately 5% of all primary cardiac lesions. Their differential diagnosis is of particular interest with regard to treatment options. While rhabdomyomas may present as multiple intramyocardial masses, they tend to regress over time. Fibromas may show progressive growth.11 Rhabdomyomas are the most common cardiac tumor seen in childhood and may be associated with tuberous sclerosis.12,13 The tumors mimic the myocardium on T1-weighted plain FSE. Homogeneous enhancement that exceeds the level of normal myocardial enhancement is seen after contrast injection. Fibromas, on the other hand, may be identified by a typical hypointense presentation on T2-weighted MRI or as coarse calcifications seen on CT (Figure 4).9

Primary malignancies account for 25% to 30% of all primary cardiac masses and include various entities of mesenchymal origin. These lesions represent a clinical dilemma because they are often asymptomatic when small, then produce nonspecific symptoms when they are larger.14 Cardiac angiosarcoma is the second most common primary cardiac tumor (9%). Approximately 75% to 80% of these masses are located within the right atrial wall extending into the lumen.14,15 Large angiosarcomas can be diagnosed easily on CT, though smaller lesions may be obscured by high-density influx artifacts during contrast application.

The masses are typically heterogeneous on T1-weighted and T2-weighted MRI and after contrast injection. Central tumor necrosis with heterogeneous peripheral enhancement is often referred to as a sunray appearanc).16 Another clue is the presence of partially hemorrhagic effusion caused by invasion of the pericardial space.

Rhabdomyosarcoma is the most common malignant childhood cardiac tumor. It mimics normal myocardium on unenhanced T1-weighted MRI but may show a varying level of enhancement, indicating necrosis, after application of contrast.16 Rhabdomyosarcoma is not more or less likely to be found in any particular chamber, unlike angiosarcoma, and may even involve valvular structures.

Other primary cardiac malignancies include malignant fibrous histiocytoma, leiomyosarcoma, lymphoma, and primary cardiac osteosarcoma.5 The latter typically arises at the left atrial roof and may not show any tumor calcification at all.

Secondary involvement of the heart by external primary malignancies is far more common than any primary cardiac tumor.5,6 Hematogenous spread, retrograde lymphatic involvement, direct contiguous involvement (e.g., bronchogenic carcinoma), and transvenous extension (e.g., renal cell carcinoma) can all cause extracardiac malignancies to affect the heart. Presentation will depend on the associated primary malignancy and the nature of cardiac involvement.

Cavitary thrombus is by far the most common mass. This pseudomass is responsible for about 15% of all ischemic strokes. Early identification of thrombi, which may occur in any cardiac chamber, is consequently of paramount importance. Clot development may be due to changes to surface properties or flow dynamics, or changes in blood coagulation. Patients with artificial devices, such as prosthetic valves or pacemakers, with atrial fibrillation or dysfunctional ventricles, are particularly at risk.

Thrombi are typically recognized on cardiac CT as having a hypodense appearance. The signal intensity seen on MRI will vary according to the clot's age and the applied imaging technique (Figure 6). CT studies have shown that atrial and ventricular clots can be identified accurately after a relatively short examination.17 MRI provides more detail on underlying pathologies, while adding information about regional myocardial function and the extent of possible myocardial infarction. The time needed to acquire this information, however, is greater than that associated with cardiac CT.

MRI may not perform as well as a transesophageal echocardiogram when imaging thrombi within atrial appendages.18 FSE sequences can lead to flow artifacts. Contrast-enhanced inversion recovery GRE techniques provide accurate results and fast cardiac coverage and are preferred.16,19 MR perfusion imaging can demonstrate the presence or lack of contrast enhancement if cardiac CT results are inconclusive (Figure 6C). Although echocardiography remains the modality of choice for thrombus screening, even a nongated chest CT may reveal unknown cardiac thrombi.

Caseous calcification of the mitral valve annulus represents another pseudomass. This entity arises in patients with annular calcifications and is found in about 3% of autopsies.20 It is typically seen on CT as an area of high density at the posterior aspect of the mitral valve annulus. This same area is hypointense on all MRI sequences and does not enhance.

State-of-the art cardiac CT and MRI can provide exquisite, adequate delineation of cardiac tumors and pseudomasses. Cardiac MRI is a superior tool for mass differentiation owing to the detail it provides on soft tissue. Assessment of cardiac thrombi, however, can be made reliably and rapidly with CT. Detailed knowledge of possible anatomical variants is mandatory to avoid misinterpretation, given the high spatial resolution of images.

The examination of cardiac masses remains a niche application for both CT and MRI.

DR. WINTERSPERGER and DR. HUBER are associate professors of radiology and cardiac imaging experts in the department of clinical radiology at the Grosshadern Clinic at the University of Munich. DR. BAUNER is a radiology resident, and PROF. REISER is a professor of radiology and chair of the department of clinical radiology at the same institution.

Johnson TR, Nikolaou K, Wintersperger BJ, et al. Dual-source CT cardiac imaging: initial experience. Eur Radiol 2006;16(7):1409-1415.

Nikolaou K, Knez A, Rist C, et al. Accuracy of 64-MDCT in the diagnosis of ischemic heart disease. AJR 2006;187(1):111-117.

Krombach GA, Niendorf T, Gunther RW, et al. Characterization of myocardial viability using MR and CT imaging. Eur Radiol 2007;17(6):1433-1444.

Wintersperger BJ, Sincleair S, Runge VM, et al. Dual breath-hold magnetic resonance cine evaluation of global and regional cardiac function. Eur Radiol 2007;17(1):73-8.

Burke A, Virmani R. Tumors of the heart and great vessels. Atlas of tumor pathology. 3rd series. Fascicle 16. Washington, DC: Armed Forces Institute of Pathology, 1996.

Lam KY, Dickens P, Chan AC. Tumors of the heart. A 20-year experience with a review of 12,485 consecutive autopsies. Arch Pathol Lab Med 1993;117(10):1027-1031.

Krombach GA, Spuentrup E, Buecker A, et al. [Heart tumors: magnetic resonance imaging and multislice spiral CT]. Rofo 2005;177(9):1205-1218. German.

Casey M, Mah C, Merliss AD, et al. Identification of a novel genetic locus for familial cardiac myxomas and Carney complex. Circulation 1998;98(23):2560-2566.

Araoz PA, Mulvagh SL, Tazelaar HD, et al. CT and MR imaging of benign primary cardiac neoplasms with echocardiographic correlation. Radiographics 2000;20(5):1303-1319.

Abu Nassar SG, Parker JC. Incidental papillary endocardial tumor. Its potential significance. Arch Pathol 1971;92(5):370-376.

Freedom RM, Lee KJ, MacDonald C, et al. Selected aspects of cardiac tumors in infancy and childhood. Pediatr Cardiol 2000;21(4):299-316.

Smythe JF, Dyck JD, Smallhorn JF, Freedom RM. Natural history of cardiac rhabdomyoma in infancy and childhood. Am J Cardiol 1990;68(8):1247-1249.

Bosi G, Lintermans JP, Pellegrino PA, et al. The natural history of cardiac rhabdomyoma with and without tuberous sclerosis. Acta Paediatr 1996;85(8):928-931.

Araoz PA, Eklund HE, Welch TJ, Breen JF. CT and MR imaging of primary cardiac malignancies. Radiographics 1999; 19(6):1421-1434

Janigan DT, Husain A, Robinson NA. Cardiac angiosarcomas. A review and a case report. Cancer 1986;57(4):852-859.

Sparrow PJ, Kurian JB, Jones TR, et al. MR imaging of cardiac tumors. Radiographics 2005;25(5):1255-1276.

Kim DH, Choi SI, Choi JA, et al. Various findings of cardiac thrombi on MDCT and MRI. JCAT 2006;30:572-577.

Mohrs OK, Nowak B, Petersen SE, et al. Thrombus detection in the left atrial appendage using contrast-enhanced MRI: a pilot study. AJR 2006;186(1):198-205.

Barkhausen J, Hunold P, Eggebrecht H, et al. Detection and characterization of intracardiac thrombi on MR imaging. AJR 2002;179(6):1539-1544.

Novaro GM, Griffin BP, Hammer DF. Caseous calcification of the mitral annulus: an underappreciated variant. Heart 2004;90(4):388.

CT and MRI give answers in cardiac neoplasms

Renal cell cancer is the most frequently encountered malignant tumor in the kidney. Over 30,000 new cases are diagnosed annually in the U.S. Many cases are now detected incidentally, as a result of the widespread use of multislice CT. MSCT usage is also redefining diagnostic workup of renal lesions and facilitating renal cell cancer staging.

Multiphasic CT acquisitions should be performed after injection of a single contrast bolus to achieve the highest staging accuracy and permit differential diagnosis. Single-phase acquisition after injection of a split contrast agent bolus is also possible. An MSCT protocol for evaluating renal masses should contain unenhanced images of the kidneys, followed by contrast-enhanced scans through the entire abdomen in nephrographic and delayed phases. The nephrographic phase is most effective in tumor localization and staging.

 Low-dose excretory phase scans enable assessment of the renal pelvis and ureters.

 The accuracy of CT for staging renal masses can reach 91%, with most staging errors related to diagnoses of tumoral perinephric extension.

The relative newness of MSCT technology has limited published experience of its protocols used on patients with renal disorders. Various authors have explored the improved temporal and spatial resolution that MSCT has brought to the kidneys and urinary tract. Others, however, caution against the escalation of patients' radiation exposure in this manner.

Increasing use of ultrasound and MSCT for clinical diagnoses has led to an increase in the detection of renal masses.

 The earlier a malignant mass is detected on imaging, the greater the chance of it being an early-stage tumor of low grade. This finding will make the tumor amenable to nephron-sparing surgery as opposed to total nephrectomy.

 Options for treating localized renal cell carcinoma include nephrectomy (open or laparoscopic, total or partial), laparoscopic and percutaneous ablation (for example, radiofrequency ablation, cryotherapy), and minimally invasive catheter techniques such as superselective chemoembolization. Imaging can play an important role in choosing the most appropriate therapeutic strategy.

Prognosis and outcome of renal cell carcinoma are influenced by tumor stage and grade at the time of diagnosis. MSCT can be used both for primary diagnosis and for definitive differentiation of renal masses that are indeterminate or suspicious on intravenous urography (IVU) or ultrasound. CT is more widely available than MRI, scan times are shorter, and costs are lower. MRI, however, offers a wider range of soft-tissue contrasts, radiation-free examination, and imaging across virtually any cross-sectional plane. Patients with impaired renal function and documented allergic reaction to iodinated contrast agents can undergo a complete diagnostic kidney workup on MRI.

MSCT can detect, characterize, and stage renal tumors in one fast, high-resolution examination, assuming that a tailored multiphasic protocol is followed. A narrow detector collimation should be employed to allow for multiplanar reformatting. This could be 4 x 1 mm, 16 x 0.75 mm, or 64 x 0.6 mm, depending on scanner configuration. Scanning should be done in the unenhanced and nephrographic phases and possibly also the delayed phase. Radiation exposure can be kept to a minimum using low-dose techniques for unenhanced and delayed phases. Further reductions are possible if scanners are equipped with dose modulation algorithms. These tailor x-ray tube current to patients' anatomy along the x-, y-, and z-axes.

Delivery of multiple boluses means that patients' kidneys process several phases of contrast in just one CT scan. This is an elegant method of limiting radiation dose, although it decreases contrast between renal arteries, parenchyma, and renal pelvis when compared with multiphasic imaging. Difficulties may also arise with image interpretation if renal function deviates from the expected normal range, or when the two kidneys differ functionally.

State-of-the-art MSCT scanners provide isotropic volume elements (voxels), which make it possible to generate high-quality sagittal and coronal multiplanar reformatted images and to use 3D volume rendering techniques. These displays are an efficient way to handle large volumes of data and facilitate communication with urologists.

We obtain coronal MPR views routinely when assessing renal pathologies on MSCT. We find that volume-rendered views help evaluate renal vasculature, detect extrarenal tumor spread, and visualize the relationship between a tumor and the renal vessels and pelvis. Suspicion of renal vein invasion can be clarified by loading the thin-slice data set into dedicated 3D viewing software. Delayed-phase (excretory phase) scans can be displayed using maximum intensity projection techniques at reconstructed slice thicknesses of 10 to 30 mm. This display does not help find small lesions within the renal pelvis and ureters, because these can be masked by bright contrast within the renal collecting system.

MSCT scanning requires precise timing for contrast bolus injection and image acquisition. We generally use a power injector to deliver 100 to 150 mL nonionic iodinated contrast into a large antecubital vein as a single bolus. The amount of contrast agent depends on patient weight; 0.5 g iodine per kg body weight is usually injected. The delivery rate is typically set to 2 to 5 mL/sec. Radiopaque contrast media then passes through the three renal compartments: blood vessels, interstitium, and tubules. The small size of renal interstitial space means that contrast in the kidney at each time point after bolus injection is primarily a function of renal perfusion and renal excretion.

Protocols concentrating on macroscopic renal arteries will commence scanning shortly after contrast injection. These include examinations to rule out renovascular disorders in patients with arterial hypertension or to check for variants in the renovascular anatomy of living kidney donors. Peak renal enhancement depends on several parameters, including circulation time, heart rate, ejection volume, and circulating blood volume. It often occurs 10 to 25 seconds after injection. It may be necessary, however, to inject a test bolus covering the abdominal aorta at the level of the renal artery takeoffs to determine the most appropriate delay for CT angiography.

The renal cortex enhances strongly 40 to 70 seconds after intravenous contrast administration, whereas enhancement of the renal medulla usually trails behind (corticomedullary phase). Contrast equilibrium between cortex and medulla can be expected 80 to 100 seconds after bolus injection (nephrographic phase).

Bright contrast from excreted contrast shows first in excretory tubules within the renal medulla (excretory phase). The renal calices and pelvis start to fill with excreted contrast after approximately 120 seconds. Contrast in the renal collecting system is, as in IVU, usually strongest three to five minutes after bolus injection, assuming renal excretory function is normal and postrenal transport of excretory products is unimpeded. Ureters may be best visualized after seven to 10 minutes, whereas it may take up to 20 minutes for the bladder to opacify fully. A 250-mL saline flush after acquisition of nephrographic phase images can improve delineation of the ureters.

Contrast-enhanced images should be obtained in the parenchymal and excretory phases for most imaging purposes other than CT angiography. It may be more difficult to recognize parenchymal lesions when contrast is strong between renal cortex and medulla in the early phase of parenchymal enhancement. Preference should thus be given to imaging in the nephrographic phase, with a delay of 80 to 100 seconds. Imaging of lesions arising from, or extending into, renal calices and the pelvis may benefit from strong contrast between soft tissue and excretory products. A delay of three to five minutes may be chosen for this application.

Complete kidney evaluation requires both pre- and postcontrast CT. Exact delineation of calcifications and recognition of small amounts of fat or blood within parenchymal lesions may provide important diagnostic clues that help determine patient management.

Renal cell carcinoma accounts for about 3% of all adult neoplasms.

 Patients with stage T1 or T2 cancer at the time of treatment have an excellent chance of survival. Once malignancy extends beyond the kidney, however, survival depends on whether the tumor progresses by direct extension or at distant metastatic sites. Survival of patients with extensive renal cell carcinoma is closely related to tumor grade and degree of malignancy.

Asymptomatic tumors that are confined within the renal capsule are now discovered more frequently and treated surgically, because of the increased use of ultrasound and abdominal CT. Angiomyolipoma is usually recognized, but differentiation of rarer benign lesions such as adenoma or oncocytoma from renal cell carcinoma is not usually possible (Figure 1).

 Surgery remains the only successful curative treatment of renal adenocarcinoma. This makes early tumor detection and accurate radiological assessment of tumor extent crucial during pretherapy staging.

Both contrast-enhanced CT and multiplanar MRI can demonstrate regional lymph nodes, vascular extension, and distant metastasis.

 MRI has been suggested as the imaging method of choice for cases of vascular tumor extension.

 Other authors have shown that MSCT with MPR and MRI perform equally well in the assessment of tumor thrombus.

 The difference between contrast-enhanced MRI and triphasic MSCT for overall staging accuracy of renal cell cancer has also been questioned.

 Most centers have opted for CT as the standard imaging modality for complete diagnostic workup of renal masses.

Various authors have emphasized the importance of CT during the nephrographic phase. Significantly more renal lesions are detected during this phase than during the corticomedullary phase.

 This is particularly true for lesions that are smaller than 3 cm4 and for those that are hypovascular.

 Combining corticomedullary and nephrographic phase images reveals even more lesions than using nephrographic phase images alone.

 Corticomedullary phase examinations are associated with false-positive findings, however.

 CT in the corticomedullary phase is recommended when information about renal vasculature is desired or when a renal lesion may represent an aneurysm, arteriovenous malformation, or fistula.

Angiomyolipoma may be best characterized on unenhanced CT. Renal cysts, on the other hand, are detected and characterized equally well during the nephrographic and excretory phases of imaging.

 A study of 173 renal lesions (90 malignant) in 96 patients showed that sensitivity for renal lesions was significantly higher in the nephrographic phase of spiral CT than in the corticomedullary phase (97% versus 84%). Specificity and accuracy were higher when both phases were combined, compared with either phase evaluated alone.

 Staging accuracy improved significantly when unenhanced CT images were combined with contrast-enhanced images from the corticomedullary and nephrographic phase. With the three phases combined, staging accuracy was 91%, compared with 82% for unenhanced and corticomedullary phase CT, and 86% for unenhanced and nephrographic phase CT (p< 0.05).

Each additional phase does, of course, increases patients' exposure to radiation. Renal CT should therefore be limited to the minimum number of scans necessary to characterize a renal lesion or stage renal cancer.

Characterization of renal lesions requires good knowledge of differential diagnoses for solid and cystic masses. Malignant differentials include renal lymphoma, metastasis, sarcoma, and transitional cell cancer (Figure 2). Benign lesions that should be recognized are angiomyolipoma (which do not always contain fat on MSCT), oncocytoma, adenoma (considered a precancerous lesion), renal infarction, and dromedary hump, a normal anatomic variant that frequently mimics tumor on ultrasound. Renal abscesses can also have a masslike appearance on MSCT (Figure 1).

Renal cell cancer itself may present as a solid, uniformly enhancing mass, but it can also appear as infiltrating tumor or as a complex cystic lesion. The latter is typically characterized for potential malignancy with the Bosniak classification system.

 Clear cell renal cell carcinoma enhances more strongly than other histologic subtypes, such as chromophobe, collecting duct, or papillary tumors. Corticomedullary phase imaging is superior to excretory phase imaging when differentiating these tumors.

 Renal cell carcinoma may also enhance more strongly than complex high-attenuation renal cysts.

Solid kidney tumors are worked up using the revised TNM system. Primary tumors are classified from T1 to T4:

- T1 tumor is confined to renal parenchyma and measures 0 to 4 cm (T1a, see Figure 3), or 4 to 7 cm (T1b).

- T2 tumor is larger than 7 cm but still confined to the kidney.

- T3 tumor extends beyond kidney but is limited to Gerota's fascia. Extension is into ipsilateral adrenal gland or into perirenal fat (T3a), into renal vein or inferior vena cava below diaphragm (T3b), or into inferior vena cava above the diaphragm (T3c) (Figures 4 and 5).

- T4 is any renal cell cancer extending beyond Gerota's fascia.

Classification of cystic renal lesions into those requiring surgical exploration and those requiring observation remains a difficult task. The value of Bosniak's radiological classification of cystic renal masses has been widely discussed and reviewed in the published literature. Five categories of lesions are distinguished in the revised, extended version of this system:

- Category I lesions are simple benign cysts that demonstrate homogeneous, watery content and a smooth, sharp interface with adjacent renal parenchyma. No wall thickening, calcification, or enhancement is observed.

- Category II lesions show one or two thin septations of no more than 1 mm or fine calcifications within walls or septa. Hyperdense cysts that would otherwise be regarded as category I lesions are also classified as category II. Benign category II lesions measure 3 cm or less in diameter, have one quarter or more of their wall extending outside the kidney, and do not enhance with contrast media. Category II lesions are usually benign but may turn out to be malignant when they are more complex.

- Category IIF describes cystic lesions that are minimally complicated but sufficiently suspicious to require follow-up. This category includes cystic lesions with an increased number of fine septa or increased calcifications that may be thicker or nodular.

- Category III lesions are truly indeterminate cystic masses. A large proportion will turn out to be benign, though the proportion of malignant masses is sufficient to warrant surgical exploration. They are characterized by uniform wall thickening, nodularity, thick or irregular peripheral calcifications, and multilocularity with multiple enhancing septa. Category III also includes hyperdense lesions that do not fit into categories II and IIF.

- Category IV lesions include those with a nonuniform or enhancing thick wall, large or enhancing nodules within the walls, or visibly solid components within a cystic lesion.

The presence of enhancing soft-tissue elements within a cystic lesion is more important than the presence and degree of calcification, when distinguishing between potentially benign and malignant lesions on CT.

 CT-guided cyst puncture, with aspiration and core biopsy, has been suggested as a useful way of avoiding invasive exploration or surgery in patients with Bosniak IIF and III renal lesions. One study, however, found that eight out of 28 malignancies were either indeterminate or misdiagnosed as benign by aspiration and biopsy.

 Renal cell carcinoma may also metastasize along needle tracks after puncture for biopsy or aspiration. Needle puncture of complex renal cysts and solid lesions remains highly controversial.

In summary, MSCT remains the most important imaging modality in preoperative assessment of renal cell cancer. It can detect, classify, and stage renal masses. A multiphasic imaging protocol containing unenhanced, nephrographic, and excretory phases should be used, with radiation dose kept to a minimum. MRI and contrast-enhanced ultrasound offer a viable alternative for patients with known allergies to iodinated contrast agents and can aid evaluation of hypovascularized tumors.

DR. GRASER and DR MUELLER-LISSE are radiologists, and PROF. REISER is department head, all in the department of clinical radiology at Grosshadern Clinic, University of Munich, in Germany. DR. STAEHLER is a urologist at the same institution.

1. Birnbaum BA, Jacobs JE, Ramchandani P. Multiphasic renal CT: comparison of renal mass enhancement during the corticomedullary and nephrographic phases. Radiology 1996; 200(3):753-758.

2. Cohan RH, Sherman LS, Korobkin M, et al. Renal masses: assessment of corticomedullary-phase and nephrographic-phase CT scans. Radiology 1995;196(2):445-451.

3. Kopka L, Fischer U, Zoeller G, et al. Dual-phase helical CT of the kidney: value of the corticomedullary and nephrographic phase for evaluation of renal lesions and preoperative staging of renal cell carcinoma. AJR 1997;169(6):1573-1578.

4. Szolar DH, Kammerhuber F, Altziebler S, et al. Multiphasic helical CT of the kidney: increased conspicuity for detection and characterization of small (< 3-cm) renal masses. Radiology 1997;202(1):211-217.

5. Zeman RK, Zeiberg A, Hayes WS, et al. Helical CT of renal masses: the value of delayed scans. AJR 1996;167(3):771-776.

Maximilian F. Reiser, MD

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