Advancement of imaging technologies has

always presented a challenge to practitioners as

to how best use these new modalities to optimise

patient care. The most significant advancement

in diagnostic imaging technology is the introduc-

tion of hybrid or fusion imaging where anatomy

merges with the function to produce the most

superior diagnostic tool ever made. This is a

technology that can confidently localise

metastatic focus of thyroid cancer, accurately

predict myocardial infarction, perform sensitive

targeting in intensity modulated radiation thera-

py (IMRT), confidently confirm the diagnosis of

Alzheimer disease, and clarify the nature of a

pulmonary nodule. Perhaps it was best said by

The Journal of Nuclear Medicine in May 2001:

“Some believe hybrid technology to be the ‘Eure-

ka' factor that will propel nuclear medicine to the

forefront of imaging in the 21st century.”

Shahran Bonyadlou

IntroductionPrior to the introduction of PET-CT (Positron Emis-sion Tomography - Computed Tomography) andSPECT-CT (Single Photon Emission ComputedTomography - Computed Tomography) to nuclearmedicine, anatomical localisation, poor spatial res-olution, and tissue attenuation have always beenmajor disadvantages in this field, requiring scientif-ic "guestimates". Hybrid imaging systems captureanatomical as well as physiological information inone exam, automatically merging the data to form acomposite image that allows the most accurateinterpretation of both CT and PET studies. Due toinherent high photon influx, the use of X-ray tubetransmission scans in CT provides optimal attenua-tion correction of PET and SPECT images. Addition-ally, attenuation correction using CT scan, in com-parison with radioactive source, reduces the trans-mission scan from 25 minutes to 20 seconds.

The modalities involved in hybrid imaging includePET, SPECT, CT, and MRI (Magnetic Resonance Imag-ing). CT and MRI provide significant anatomicaldata that, when combined with functional data,bestow a synergistic effect, improving detectabilityas well as speed [1].

SPECT-CTThe first commercial hybrid SPECT-CT system(Hawkeye Millennium VG by GE Medical Systems)was introduced in 1998 following major experimen-tation in molecular imaging hybrid systems (Figure1). CT-based attenuation correction of SPECTimages consistently improved overall diagnosticperformance, especially in the field of nuclear cardi-ology [2]. SPECT-CT can provide an excellent alter-native to more expensive PET-CT if affordability ofthe technology is of concern or the institutions donot have access to FDG (PET imaging with fluorine18 fluorodeoxyglucose [FDG]). The synthesis ofSPECT radiopharmaceuticals is relatively lessexpensive. Because SPECT radioisotopes havelonger half-lives than those of isotopes used in PET(hours versus minutes), longer acquisition times arepossible in SPECT, which allows monitoring changesin tissues over time. This further strengthens theability to narrow down the characteristics of a spe-cific disease process, such as in parathyroid adeno-

ma and neuroendocrine tumours. Finally, mostSPECT tracers are lessexpensive, and in certaintumours more accurate thanFDG.

Perhaps, myocardial perfu-sion imaging is an exampleof the clear benefit of hybridimaging. Perfusion imagingis the procedure of choice inthe management of high-riskcardiac patients. Many ofthese patients are morbidlyobese, producing significantattenuation abnormalitiesthat may not be evident evenafter repositioning. The ben-efit of attenuation correctionfrom CT in SPECT-CT is toaccurately detect exactmyocardial activity, eliminat-ing unnecessary coronaryangiography and CABG(Coronary Artery BypassGrafting) procedures. Withthe introduction of 16- andrecently 64-slice CT compo-nents, SPECT-CT provides aspectrum of diagnostic stud-ies including CT coronaryangiography, coronary calci-um scoring and myocardialSPECT perfusion imaging inone stop.

Anatomic localisation is alsoof significant importance,more specifically in bone,thyroid, Prostascint,Oncoscint, octreotide, andMIBG (meta-iodobenzylguanidine) scintigraphies.These studies frequently demonstrate abnormali-ties requiring anatomical localisation by other imag-ing modalities. The thyroid studies have very lowcount rate with extremely poor image resolutionrequiring body markers. Any suspicious metastaticfoci can be easily localised on fused SPECT-CTimages (Figure 2). SPECT-CT has also proven to behighly effective in examining patients with neuroen-docrine tumours.

A major difference between SPECT-CT and PET-CT isthe potential for molecular therapy. At this point,there is no therapeutic PET compound available.However, SPECT radiotracers can, and many havebeen designed to, be target specific (e.g. Somato-statin receptor). First, the presence of a disease isidentified and localisation is achieved with CT. Thishelps identify the exact distribution of the tumour,reducing morbidity from affecting critical organswhen therapy is implemented. Then, thesame radiotracer is labelled with therapeu-tic radio-isotopes (such as yittrium-90 or I-131) to deliver a highly specific radiation totarget the tumour, reducing morbidity ofconventional chemotherapy or radiationtherapy.

PET-CTWith the success of SPECT-CT in 1998,manufacturers began development of PET-CT in 1999 leading to the development ofthe first prototype in collaboration with agroup of researches at the University of

Pittsburgh. The first commercial PET-CT debuted in2000 (Figure 3), and by 2002,the combined PET and PET-CTmarkets generated USD 481.23million in revenue at an annualgrowth rate of 55% according tomarket research firm Frost &Sullivan.

Advantages of this technologyinclude differentiation of benignlesions from malignant lesions,staging of malignant lesions,detection of cancer recurrence,reducing biopsy sampling errors,improving therapy monitoring,such as chemotherapy and radia-tion therapy (Figure 4). Antoch et

al. demonstrated that hybridPET-CT provides a synergisticadvantage in staging of variouscancers. PET-CT correctly staged84% of patients as comparedwith 76% for side-by-side PETplus CT, 63% for CT alone, and64% for PET alone. In this study,the added diagnostic advantageof PET-CT changed the treatmentplan in a substantial number ofpatients [3]. S. Fanti and col-leagues correctly identified thesite of the primary tumour in44% of patients with unknownprimary malignancies [4]. See-man and colleague showed thatthe combination of PET with CTor MRI was found to facilitateaccurate correlation of molecu-lar aspects and metabolic alter-ations with the structural find-ings in patients with known

extensive, non-resectable metastatic gastrointesti-nal malignancies. Perhaps the most established

Figure 1. The first commercial hybrid SPECT-CTsystem, Hawkeye Millennium VG by GE MedicalSystems.

Figure 2. Metastatic disease in the chest in apatient with thyroid carcinoma. Columns A, Band C correspond to CT, SPECT and fusionimages respectively.

Figure 3. The first commercial hybrid PET-CT system,Siemens Biograph.

Figure 4. Pre and post-chemotherapy in a patient with lymphoma.

as published in IHE May 2006IHE MAY FOCUS

Hybrid imaging technology - The so-called“One Stop Shop”

advantage of hybrid imaging is in lymphoma where PET-CT can confirm theabsence of disease after first-line therapy.

ChallengesAlthough the introduction of PET-CT and SPECT-CT scanners represented animportant development in the field of radiology, the hybridisation can be a littlechallenging. These technologies have introduced new challenges in manage-ment, including physician education about new findings, practices and pitfalls,proper siting of these equipments, educating staff, billing, reporting, and final-ly cost effectiveness.

The FutureThere is now a challenge to take this new modality to the extreme with the intro-duction of 64- and later 256-slice CT components to hybrid systems. PhilipsMedical Systems debuted its 64-slice SPECT-CT and PET-CT systems at the 52ndSociety of Nuclear Medicine Annual Meeting in Toronto, Canada (2005).

There are new hybrid systems under development that combine two anatomicalimaging modalities. Dr Rebecca Fahrig and her colleagues from Stanford Uni-versity have developed a truly hybrid X-ray/MR (XMR) system in which the X-raytube and detector lie within the MR scanner. The X-Ray detector is a digital flat-panel detector (FPD), which is immune to magnetic fields. The equipment isused for delicate and sensitive procedures, such as shunt placement in patientswith cirrhosis.

ConclusionIn summary, many clinical studies have indicated that hybrid devices are useful

diagnostic tools that improve diagnostic and therapeutic accuracies that ulti-mately benefit the patient care.

References1. Freudenberg et al. FDG-PET/CT in re-staging of patients with lymphoma. Eur JNuc Med Mol Imaging 2004.2. Masood Y, Liu YH, DePuey G, Taillefer R, Araujo LI, Allen S, Delbeke D, Anstett F,Peretz A, Zito MJ, Tsatkin V, Wackers FJ. Clinical validation of SPECT attenuation cor-rection using X-ray computed tomography-derived attenuation maps: Multicenterclinical trial with angiographic correlation. J Nucl Cardiol. 2005 Nov-Dec;12(6):676-86.3. Antoch G, Saoudi N, Kuehl H et al. Accuracy of whole-body dual-modality fluo-rine -18-2-fluoro-2-deoxy-D-glucose positron emission tomographyand computedtomography (FDG-PET/CT) for tumor staging in solid tumors: comparison with CTand PET. Journal of Clinical Oncology 2004;22:4357-4368. Abstract4. Fanti S, Nanni C, Moretti A et al. FDG PET-CT for the detection of unknown pri-mary cancer. Program and abstracts of the Radiological Society of North America91st Scientific Assembly and Annual Meeting; November 27-December 2, 2005;Chicago, Illinois. Page 225.

The AuthorShahram Bonyadlou, M.D.

Assistant Professor of Radiology

University of Southern California

Keck School of Medicine

Department of Radiology

Division of Nuclear Medicine

bonyadlo@usc.edu