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Everything you need to know about Computed Tomography (CT) & CT Scanning


Liver: Spiral CT of the Liver: Pressing the CT Envelope

Elliot K. Fishman, M.D., FACR
Professor of Radiology and Oncology
Johns Hopkins University School of Medicine

Introduction

The liver has always been the organ that has provided the greatest challenge to the radiologist and referring physician. Whether the imaging modality be CT, MRI, ultrasound or angiography, questions of lesion detection rates (including both false-positive and negative studies) as well as lesion discrimination have always been the focus of heated debate. Lesion detection rates have varied depending on the study protocols used (including various scanners and their capabilities) as well as pathology studied (i.e. metastases from colon cancer vs. hepatoma in a cirrhotic liver). A review of many of the prior studies is far beyond the scope of this article or the time allotted for this discussion. Nevertheless most studies found CT to only be in the detection range of 50-80% for liver tumors. A specific form of CT called CTAP (Computed Tomographic Angiographic Portography) which involved placing a catheter into the SMA, became the gold standard but still was little more than 80% accurate in most studies. Techniques of combining CTA and CTAP were even attempted to try to raise lesion detection levels. A problem with both CTA and CTAP were flow related defects that could result in either false-positive studies or in over-estimation of tumor extent in up to 15% of cases.

Published Reports

The published reports of spiral CT for the detection of liver tumors have been better than any other modality to date. Some of the recent results have included a better than 90% detection rate for lesions with portal phase imaging alone in one series of 21 patients. Addition of arterial phase imaging increased lesion detection by around 9% compared to when portal phase imaging alone was performed. Obviously the success of lesion detection and the role of arterial and portal phase imaging will depend in part on the patient population (i.e. metastatic colon cancer vs. hepatoma) studied as well as the status of the underlying liver parenchyma (i.e. normal liver vs. cirrhosis). Some conclusions from recent published reports include:

 

  • I. "In a subset of 42 patients with proof of tumor burden, 157 proven lesions were found. Consensus readings identified 127 (81%) of these lesions on portal venous phase images, 98 (62%) of these lesions on unenhanced images, and 120 (76%) of these lesions on arterial phase images. Of the 30 lesions not seen on portal venous phase images, 9 were seen on both unenhanced and arterial phase images, 3 were seen on unenhanced images only, and 18 were seen on arterial phase images only." The combination of arterial and portal venous phase images revealed significantly more hepatocellular carcinoma lesions than did the combination of unenhanced and portal phase images."
    Oliver JH III, Baron RL, Federle MP, Rockette HE Jr. Detecting ;hepatocellular carcinoma: Value of unenhanced or arterial phase CT ;imaging or both used in conjunction with conventional portal venous ;phase contrast-enhanced CT imaging. AJR 1996;167:71-77.
  • II.In a series of 103 patients evaluated with dual-phase spiral CT, 119 focal liver lesions were detected. Nine lesions were only seen on arterial phase imaging only and 40 were seen only on portal phase imaging. Eight percent more lesions were seen by adding arterial phase imaging.
    Bonaldi VM, Bret PM, Reinhold C, Atri M. Helical CT of the liver: Value ;of an early hepatic arterial phase. Radiology 1995;197:357-363.
  • III.In a study of 21 patients with portal phase spiral CT and a lesion-by- lesion analysis with surgical and pathologic findings, 91% of lesion greater than 1 cm were detected. If all lesions were analyzed, 81% of lesions were detected.
    Kuszyk BS, Bluemke DA, Urban BA, Choti MA, Hruban RH, Sitzmann JV, ;Fishman EK. Portal-phase contrast-enhanced helical CT for the detection of malignant hepatic tumors: Sensitivity based on comparison with ;intraoperative and pathologic findings. AJR 1996;166:91-95.

Protocols

In terms of study protocol, the question based on published data is when to do portal phase examinations of the liver, when to do arterial phase examinations of the liver and when to do both. In principle, the single best examination technique for the liver would include non-contrast examination, followed by arterial and portal phase imaging (some might even suggest delayed scans following equilibrium as well). However, this is not truly practical in a busy clinical practice, both from time of study perspective as well as from a cost-efficiency basis. Our routine protocol then is to triage patients into various categories and tailor examinations accordingly. Some of the protocols are:

(a) rule out liver metastases in a patient with colon cancer: portal phase imaging

(b) evaluate islet cell tumor of the pancreas and extent of liver involvement: arterial phase imaging

(c) rule out hepatoma in a cirrhotic liver: arterial and portal phase imaging

(d) evaluate patient for potential liver transplant: arterial and portal phase imaging

(e) preoperative evaluation of the liver prior to surgical resection: arterial and portal phase imaging The goal of these protocols is to provide the information needed as efficiently as possible in a real world setting. Specific scanning protocols will vary from scanner to scanner and manufacturer to manufacturer but certain principles are common. The ideal study requires narrow collimation, close interscan spacing but at a mAs that is comparable to non-spiral studies. Recent advances in tube and generator technology have made this possible. Some sample protocols as used on Siemens Somatom Plus-4 (Siemens Medical Systems, Iselin, NJ) include:



Protocol


Rule out liver metastases (colon cancer)


Evaluate vascular tumor (islet cell)


Preoperative evaluation for liver transplant

Preoperative planning study (i.e. liver resection)

Slice Thickness (mm)

5

5

3

3

Scan Time (sec)

0.75

0.75

0.75

0.75

kVp

120

120

120

120

mAs

280

280

120

280

Table Speed (mm/sec)

5

5

3

3

Pitch

1

1

1

1

Recon. Interval

3mm

2mm

2mm

2mm

Contrast

120cc
Omnipaque - 350

120cc
Omnipaque - 350

150cc
Omnipaque - 350

150cc
Omnipaque - 350

Rate Injection

3cc/sec

3cc/sec

3cc/sec

3cc/sec

Actual Scan Delay (from start of injection)

45-50 seconds

25-35 seconds

25-35 seconds

25-35 seconds

Imaging Phase

portal venous

arterial

arterial

arterial

Data Sampling Frequency

5mm

5mm

2-3mm

2-3mm

Dual Phase

No

No

Yes

Yes

3D Needed

No

Usually Not

Yes

Yes

 

Pitfalls

Some of the pitfalls of spiral CT of the liver include:
  • confusion of the normal liver appearance of arterial phase imaging with parenchymal disease
  • inability to diagnose hemangiomas definitively on single phase portal imaging
  • detection of small "indeterminate" lesions (<5mm) that are in fact small incidental cysts
  • flow related defects in the IVC which can simulate thrombus or tumor extension
  • flow related enhancement patterns simulating pathology such as with the presence of biliary catheters, dilated ducts or a distended inflamed gallbladder
  • proper timing of arterial phase for data acquisition
  • proper timing of portal venous phase for data acquisition
  • how to optimize contrast injection schemes

Future Directions

The current success with spiral CT imaging of the liver will be a focal point of further research and technical developments. These include;

(1) continuing studies will be published defining how to optimize lesion detection/definition with spiral CT. The need for single vs. dual phase studies will continue to be defined for specific tumors. The success rate for spiral CT vs. other imaging techniques (especially MRI) will continue to be addressed. (2) technical developments will allow for better on-line real time monitoring of contrast injections building on the initial positive results from Smart-Prep (GE Medical Systems). The CARE program from Siemens Medical Systems may be one step in that direction. (3) 3D imaging of the liver will flourish with automated organ segmentation schemes and optimization of display of vascular maps. (4) blood pool contrast agents may become available and will potentially increase lesion discrimination (cyst vs. malignancy).

Parenchymal Liver Disease

Spiral CT has all of the advantages of dynamic CT in the evaluation of parenchymal liver disease, but all provide some distinct advantages. These advantages center around rapid data acquisition, coupled with the ability to optimize the timing between contrast injection and data acquisition. Just as these advantages can help increase lesion detection when discussing tumors they also can be useful in other applications including;
  • evaluation of cirrhosis
  • portal vein evaluation
  • evaluation of suspected hepatic abscess
  • evaluation of vascular malformations

A full in-depth discussion of all of these entities is impossible but some key points include;

  • Portal vein thrombosis. The use of single and dual phase spiral CT with narrow collimation and close interscan spacing is ideal for evaluation of the portal vein. Properly timed injection acquisition schemes allow the portal venous system to be clearly defined. Flow-related artifacts are rare in these vessels and partial or total thrombosis are therefore easy to detect. Changes in liver parenchymal enhancement with decreased enhancement centrally and normal enhancement peripherally may occur as a transient phenomena. Thrombosis is seen as a filling defect often with extensive collaterals present around the vessels referred to as a cavernous transformation of the portal vein. Extension of clot into the splenic vein and/or the SMV may also been seen. CT can be used to monitor therapy in these patients if thrombolytic agents are used.
  • Hepatic abscesses are usually low attenuation lesions but are best seen following IV contrast injection. Spiral CT with the use of narrow collimation and interscan intervals is ideal for detecting small abscesses.
  • Hepatic artery aneurysms and pseudoaneurysms are best seen with spiral CT in the late arterial phase imaging.

References

1. Oi H, Murakami T, Kim T, Matsushita M, Kashimoto H, Nakamura H. Dynamic MR imaging and early-phase helical CT for detecting small intrahepatic metastases of hepatocellular carcinoma. AJR 1996;166:369-374.

2. Oliver JH III, Baron RL, Federle MP, Rockette HE Jr. Detecting hepatocellular carcinoma: Value of unenhanced or arterial phase CT imaging or both used in conjunction with conventional portal venous phase contrast-enhanced CT imaging. AJR 1996;167:71-77.

3. Kuszyk BS, Bluemke DA, Urban BA, Choti MA, Hruban RH, Sitzmann JV, Fishman EK. Portal-phase contrast-enhanced helical CT for the detection of malignant hepatic tumors: Sensitivity based on comparison with intraoperative and pathologic findings. AJR 1996;166-91-95.

4. Baron RL, Oliver JH III, Dodd GD III, Nalesnik M, Holbert BL, Carr B. Hepatocellular carcinoma: Evaluation with biphasic contrast-enhanced helical CT. Radiology 1996;199:505-511.

5. Semelka RC, Schlund JF, Molina PL, Willms AB, Kahlenberg M, MauroMauro MA, Weeks SM, Gance WG. Malignant liver lesions: Comparison of spiral CT arterial portography and MR imaging for diagnostic accuracy, cost, and effect on patient management. JMRI 1996;1:39-43.

6. Yamashita Y, Mitsuzaki K, Yi T, Ogata I, Nishiharu T, Urata J, Takahashi M. Small hepatocellular carcinoma in patients with chronic;liver damage: Prospective comparison of detection with dynamic MR imaging and helical CT of the whole liver. Radiology 1996;200:79-84.

7. Irie T and Kusano S. Contrast-enhanced spiral CT of the liver: Effect of injection time on time to peak hepatic enhancement. J Comput Assist Tomogr 1996;20(4):633-637.

8. Bonaldi VM, Bret PM, Reinhold C, Atri M. Helical CT of the liver: Value of an early hepatic arterial phase. Radiology 1995;197:357-363.

9. Kanematsu M, Imaeda T, Mizuno S, Yamawaki Y, Sone Y, Iida T, Kato M, Yokoyama R. Value of three-dimensional spiral CT hepatic angiography. AJR 1996;166:585-591.

10. Frederick MG, McElaney BL, Singer A, Park KS, Paulson EK, McGee SG, Nelson RC. Timing of parenchymal enhancement on dual-phase dynamic helical CT of the liver: How long does the hepatic arterial phase predominate? AJR 1996;166:1305-1310.

11. Lin JP and Lu DSK. Early enhancement of tumor thrombus in portal vein on two-phase helical CT. J Comput Assist Tomogr 1996;20(4):653-655.

12. Lupetin AR, Cammisa BA, Beckman I, Dash N, Khoury MB, Kiproff PM, McKenzie RS. Spiral CT during arterial portography. RadioGraphics 1996;16:723-743.

13. Herts BR, Baker ME, Davros WJ, Lorig RJ, Obuchowski N, Shiesly DA, Roelke D. Helical CT of the abdomen: Comparison of image quality between scan times of 0.75 and 1 Sec per revolution. AJR 1996;167:58-60.

14. Silverman PM, Roberts SC, Ducic I, Tefft MC, Olson MC, Cooper C, Zeman RK. Assessment of a technology that permits individualized scan delays on helical hepatic CT: A technique to improve efficiency in use of contrast;material. AJR 1996;167:79-84.

15. Oliver JH III, Baron RL, Federle MP, Rockette HE Jr. Detecting hepatocellular carcinoma: Value of unenhanced or arterial phase CT imaging or both used in conjunction with conventional portal venous phase contrast-enhanced CT imaging. AJR 1996;167:71-77.

© 1999-2019 Elliot K. Fishman, MD, FACR. All rights reserved.